TY - JOUR A1 - Oskinova, Lida A1 - Hamann, Wolf-Rainer A1 - Cassinelli, Joseph P. A1 - Brown, John C. A1 - Todt, Helge Tobias T1 - X-ray emission from massive stars with magnetic fields JF - Astronomische Nachrichten = Astronomical notes N2 - We investigate the connections between the magnetic fields and the X-ray emission from massive stars. Our study shows that the X-ray properties of known strongly magnetic stars are diverse: while some comply to the predictions of the magnetically confined wind model, others do not. We conclude that strong, hard, and variable X-ray emission may be a sufficient attribute of magnetic massive stars, but it is not a necessary one. We address the general properties of X-ray emission from "normal" massive stars, especially the long standing mystery about the correlations between the parameters of X-ray emission and fundamental stellar properties. The recent development in stellar structure modeling shows that small-scale surface magnetic fields may be common. We suggest a "hybrid" scenario which could explain the X-ray emission from massive stars by a combination of magnetic mechanisms on the surface and shocks in the stellar wind. The magnetic mechanisms and the wind shocks are triggered by convective motions in sub-photospheric layers. This scenario opens the door for a natural explanation of the well established correlation between bolometric and X-ray luminosities. KW - stars: magnetic fields KW - stars: mass-loss KW - stars: winds, outflows KW - stars: Wolf-Rayet KW - techniques: spectroscopic KW - X-rays: stars Y1 - 2011 U6 - https://doi.org/10.1002/asna.201111602 SN - 0004-6337 VL - 332 IS - 9-10 SP - 988 EP - 993 PB - Wiley-Blackwell CY - Malden ER - TY - JOUR A1 - Burgemeister, S. A1 - Gvaramadze, Visily V. A1 - Stringfellow, G. S. A1 - Kniazev, Alexei Y. A1 - Todt, Helge Tobias A1 - Hamann, Wolf-Rainer T1 - WR 120bb and WR 120bc: a pair of WN9h stars with possibly interacting circumstellar shells JF - Monthly notices of the Royal Astronomical Society N2 - Two optically obscured Wolf-Rayet (WR) stars have been recently discovered by means of their infrared (IR) circumstellar shells, which show signatures of interaction with each other. Following the systematics of the WR star catalogues, these stars obtain the names WR 120bb and WR 120bc. In this paper, we present and analyse new near-IR, J-, H- and K-band spectra using the Potsdam Wolf-Rayet model atmosphere code. For that purpose, the atomic data base of the code has been extended in order to include all significant lines in the near-IR bands. The spectra of both stars are classified as WN9h. As their spectra are very similar the parameters that we obtained by the spectral analyses hardly differ. Despite their late spectral subtype, we found relatively high stellar temperatures of 63 kK. The wind composition is dominated by helium, while hydrogen is depleted to 25 per cent by mass. Because of their location in the Scutum-Centaurus Arm, WR 120bb and WR 120bc appear highly reddened, A(Ks) approximate to 2 mag. We adopt a common distance of 5.8 kpc to both stars, which complies with the typical absolute K-band magnitude for the WN9h subtype of -6.5 mag, is consistent with their observed extinction based on comparison with other massive stars in the region, and allows for the possibility that their shells are interacting with each other. This leads to luminosities of log(L/L-circle dot) = 5.66 and 5.54 for WR 120bb and WR 120bc, with large uncertainties due to the adopted distance. The values of the luminosities of WR 120bb and WR 120bc imply that the immediate precursors of both stars were red supergiants (RSG). This implies in turn that the circumstellar shells associated with WR 120bb and WR 120bc were formed by interaction between the WR wind and the dense material shed during the preceding RSG phase. KW - line: identification KW - circumstellar matter KW - stars: fundamental parameters KW - stars: massive KW - stars: Wolf-Rayet Y1 - 2013 U6 - https://doi.org/10.1093/mnras/sts588 SN - 0035-8711 SN - 1365-2966 VL - 429 IS - 4 SP - 3305 EP - 3315 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Shenar, Tomer A1 - Hainich, Rainer A1 - Todt, Helge Tobias A1 - Sander, Andreas Alexander Christoph A1 - Hamann, Wolf-Rainer A1 - Moffat, Anthony F. J. A1 - Eldridge, J. J. A1 - Pablo, H. A1 - Oskinova, Lida A1 - Richardson, N. D. T1 - Wolf-Rayet stars in the Small Magellanic Cloud II. Analysis of the binaries JF - American mineralogist : an international journal of earth and planetary materials N2 - Context. Massive Wolf-Rayet (WR) stars are evolved massive stars (M-i greater than or similar to 20 M-circle dot) characterized by strong mass-loss. Hypothetically, they can form either as single stars or as mass donors in close binaries. About 40% of all known WR stars are confirmed binaries, raising the question as to the impact of binarity on the WR population. Studying WR binaries is crucial in this context, and furthermore enable one to reliably derive the elusive masses of their components, making them indispensable for the study of massive stars. Aims. By performing a spectral analysis of all multiple WR systems in the Small Magellanic Cloud (SMC), we obtain the full set of stellar parameters for each individual component. Mass-luminosity relations are tested, and the importance of the binary evolution channel is assessed. Methods. The spectral analysis is performed with the PotsdamWolf-Rayet (PoWR) model atmosphere code by superimposing model spectra that correspond to each component. Evolutionary channels are constrained using the Binary Population and Spectral Synthesis (BPASS) evolution tool. Results. Significant hydrogen mass fractions (0.1 < X-H < 0.4) are detected in all WN components. A comparison with mass-luminosity relations and evolutionary tracks implies that the majority of the WR stars in our sample are not chemically homogeneous. The WR component in the binary AB6 is found to be very luminous (log L approximate to 6.3 [L-circle dot]) given its orbital mass (approximate to 10 M-circle dot), presumably because of observational contamination by a third component. Evolutionary paths derived for our objects suggest that Roche lobe overflow had occurred in most systems, affecting their evolution. However, the implied initial masses (greater than or similar to 60 M-circle dot) are large enough for the primaries to have entered the WR phase, regardless of binary interaction. Conclusions. Together with the results for the putatively single SMC WR stars, our study suggests that the binary evolution channel does not dominate the formation of WR stars at SMC metallicity. KW - stars: massive KW - stars: Wolf-Rayet KW - stars: evolution KW - binaries: close KW - binaries: symbiotic KW - Magellanic Clouds Y1 - 2016 U6 - https://doi.org/10.1051/0004-6361/201527916 SN - 1432-0746 VL - 591 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Hainich, Rainer A1 - Pasemann, Diana A1 - Todt, Helge Tobias A1 - Shenar, Tomer A1 - Sander, Andreas Alexander Christoph A1 - Hamann, Wolf-Rainer T1 - Wolf-Rayet stars in the Small Magellanic Cloud I. Analysis of the single WN stars JF - Astronomy and astrophysics : an international weekly journal N2 - Context. Wolf-Rayet (WR) stars have a severe impact on their environments owing to their strong ionizing radiation fields and powerful stellar winds. Since these winds are considered to be driven by radiation pressure, it is theoretically expected that the degree of the wind mass-loss depends on the initial metallicity of WR stars. Aims. Following our comprehensive studies of WR stars in the Milky Way, M31, and the LMC, we derive stellar parameters and mass-loss rates for all seven putatively single WN stars known in the SMC. Based on these data, we discuss the impact of a low-metallicity environment on the mass loss and evolution of WR stars. Methods. The quantitative analysis of the WN stars is performed with the Potsdam Wolf-Rayet (PoWR) model atmosphere code. The physical properties of our program stars are obtained from fitting synthetic spectra to multi-band observations. Results. In all SMC WN stars, a considerable surface hydrogen abundance is detectable. The majority of these objects have stellar temperatures exceeding 75 kK, while their luminosities range from 10(5.5) to 10(6.1) L-circle dot. The WN stars in the SMC exhibit on average lower mass-loss rates and weaker winds than their counterparts in the Milky Way, M31, and the LMC. Conclusions. By comparing the mass-loss rates derived for WN stars in different Local Group galaxies, we conclude that a clear dependence of the wind mass-loss on the initial metallicity is evident, supporting the current paradigm that WR winds are driven by radiation. A metallicity effect on the evolution of massive stars is obvious from the HRD positions of the SMC WN stars at high temperatures and high luminosities. Standard evolution tracks are not able to reproduce these parameters and the observed surface hydrogen abundances. Homogeneous evolution might provide a better explanation for their evolutionary past. KW - stars: Wolf-Rayet KW - Magellanic Clouds KW - stars: early-type KW - stars: atmospheres KW - stars: winds, outflows KW - stars: mass-loss Y1 - 2015 U6 - https://doi.org/10.1051/0004-6361/201526241 SN - 1432-0746 VL - 581 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Todt, Helge Tobias A1 - Hamann, Wolf-Rainer T1 - Wolf-Rayet central stars of planetary nebulae JF - Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.–5. June 2015 N2 - A significant number of the central stars of planetary nebulae (CSPNe) are hydrogen-deficient, showing a chemical composition of helium, carbon, and oxygen. Most of them exhibit Wolf-Rayet-like emission line spectra, similar to those of the massive WC Pop I stars, and are therefore classified as of spectral type [WC]. In the last years, CSPNe of other Wolf-Rayet spectral subtypes have been identified, namely PB 8, which is of spectral type [WN/C], and IC 4663 and Abell 48, which are of spectral type [WN]. We review spectral analyses of Wolf-Rayet type central stars of different evolutionary stages and discuss the results in the context of stellar evolution. Especially we consider the question of a common evolutionary channel for [WC] stars. The constraints on the formation of [WN] or [WC/N] subtype stars will also be addressed. Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-88147 SP - 253 EP - 258 ER - TY - JOUR A1 - Steffen, M. A1 - Hubrig, Swetlana A1 - Todt, Helge Tobias A1 - Schoeller, M. A1 - Hamann, Wolf-Rainer A1 - Sandin, Christer A1 - Schönberner, Detlef T1 - Weak magnetic fields in central stars of planetary nebulae? JF - Astronomy and astrophysics : an international weekly journal N2 - Context. It is not yet clear whether magnetic fields play an essential role in shaping planetary nebulae (PNe), or whether stellar rotation alone and/or a close binary companion, stellar or substellar, can account for the variety of the observed nebular morphologies. Aims. In a quest for empirical evidence verifying or disproving the role of magnetic fields in shaping planetary nebulae, we follow up on previous attempts to measure the magnetic field in a representative sample of PN central stars. Methods. We obtained low-resolution polarimetric spectra with FORS2 installed on the Antu telescope of the VLT for a sample of 12 bright central stars of PNe with different morphologies, including two round nebulae, seven elliptical nebulae, and three bipolar nebulae. Two targets are Wolf-Rayet type central stars. Results. For the majority of the observed central stars, we do not find any significant evidence for the existence of surface magnetic fields. However, our measurements may indicate the presence of weak mean longitudinal magnetic fields of the order of 100 Gauss in the central star of the young elliptical planetary nebula IC 418 as well as in the Wolf-Rayet type central star of the bipolar nebula Hen 2-113 and the weak emission line central star of the elliptical nebula Hen 2-131. A clear detection of a 250 G mean longitudinal field is achieved for the A-type companion of the central star of NGC 1514. Some of the central stars show a moderate night-to-night spectrum variability, which may be the signature of a variable stellar wind and/or rotational modulation due to magnetic features. Conclusions. Since our analysis indicates only weak fields, if any, in a few targets of our sample, we conclude that strong magnetic fields of the order of kG are not widespread among PNe central stars. Nevertheless, simple estimates based on a theoretical model of magnetized wind bubbles suggest that even weak magnetic fields below the current detection limit of the order of 100 G may well be sufficient to contribute to the shaping of the surrounding nebulae throughout their evolution. Our current sample is too small to draw conclusions about a correlation between nebular morphology and the presence of stellar magnetic fields. KW - planetary nebulae: general KW - stars: magnetic field KW - stars: AGB and post-AGB KW - binaries: close KW - techniques: polarimetric Y1 - 2014 U6 - https://doi.org/10.1051/0004-6361/201423842 SN - 0004-6361 SN - 1432-0746 VL - 570 PB - EDP Sciences CY - Les Ulis ER - TY - GEN A1 - Todt, Helge Tobias A1 - Peña, Miriam A1 - Zühlke, Julia A1 - Oskinova, Lida A1 - Hamann, Wolf-Rainer A1 - Gräfener, Götz T1 - Weak emission line central stars of planetary nebulae T2 - Planetary Nebulae: an Eye to the Future N2 - To understand the evolution and morphology of planetary nebulae, a detailed knowledge of their central stars is required. Central stars that exhibit emission lines in their spectra, indicating stellar mass-loss allow to study the evolution of planetary nebulae in action. Emission line central stars constitute about 10 % of all central stars. Half of them are practically hydrogen-free Wolf-Rayet type central stars of the carbon sequence, [WC], that show strong emission lines of carbon and oxygen in their spectra. In this contribution we address the weak emission-lines central stars (wels). These stars are poorly analyzed and their hydrogen content is mostly unknown. We obtained optical spectra, that include the important Balmer lines of hydrogen, for four weak emission line central stars. We present the results of our analysis, provide spectral classification and discuss possible explanations for their formation and evolution. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 462 KW - stars: AGB and post-AGB KW - stars: Wolf-Rayet KW - stars: abundances Y1 - 2018 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-413775 ER - TY - JOUR A1 - Hainich, Rainer A1 - Ruehling, Ute A1 - Todt, Helge Tobias A1 - Oskinova, Lida A1 - Liermann, A. A1 - Graefener, G. A1 - Foellmi, C. A1 - Schnurr, O. A1 - Hamann, Wolf-Rainer T1 - The Wolf-Rayet stars in the Large Magellanic Cloud - A comprehensive analysis of the WN class JF - Astronomy and astrophysics : an international weekly journal N2 - Context. Massive stars, although being important building blocks of galaxies, are still not fully understood. This especially holds true for Wolf-Rayet (WR) stars with their strong mass loss, whose spectral analysis requires adequate model atmospheres. Aims. Following our comprehensive studies of the WR stars in the Milky Way, we now present spectroscopic analyses of almost all known WN stars in the LMC. Methods. For the quantitative analysis of the wind-dominated emission-line spectra, we employ the Potsdam Wolf-Rayet (PoWR) model atmosphere code. By fitting synthetic spectra to the observed spectral energy distribution and the available spectra (ultraviolet and optical), we obtain the physical properties of 107 stars. Results. We present the fundamental stellar and wind parameters for an almost complete sample of WN stars in the LMC. Among those stars that are putatively single, two different groups can be clearly distinguished. While 12% of our sample are more luminous than 10(6) L-circle dot and contain a significant amount of hydrogen, 88% of the WN stars, with little or no hydrogen, populate the luminosity range between log (L/L-circle dot) = 5.3 ... 5.8. Conclusions. While the few extremely luminous stars (log (L/L-circle dot) > 6), if indeed single stars, descended directly from the main sequence at very high initial masses, the bulk of WN stars have gone through the red-supergiant phase. According to their luminosities in the range of log (L/L-circle dot) = 5.3 ... 5.8, these stars originate from initial masses between 20 and 40 M-circle dot. This mass range is similar to the one found in the Galaxy, i.e. the expected metallicity dependence of the evolution is not seen. Current stellar evolution tracks, even when accounting for rotationally induced mixing, still partly fail to reproduce the observed ranges of luminosities and initial masses. Moreover, stellar radii are generally larger and effective temperatures correspondingly lower than predicted from stellar evolution models, probably due to subphotospheric inflation. KW - stars: Wolf-Rayet KW - Magellanic Clouds KW - stars: early-type KW - stars: atmospheres KW - stars: winds, outflows KW - stars: mass-loss Y1 - 2014 U6 - https://doi.org/10.1051/0004-6361/201322696 SN - 0004-6361 SN - 1432-0746 VL - 565 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Sander, Andreas Alexander Christoph A1 - Todt, Helge Tobias A1 - Hainich, Rainer A1 - Hamann, Wolf-Rainer T1 - The Wolf-Rayet stars in M31 I. Analysis of the late-type WN stars JF - Astronomy and astrophysics : an international weekly journal N2 - Context. Comprehensive studies of Wolf-Rayet stars were performed in the past for the Galactic and the LMC population. The results revealed significant differences, but also unexpected similarities between the WR populations of these different galaxies. Analyzing the WR stars in M 31 will extend our understanding of these objects in different galactic environments. Aims. The present study aims at the late-type WN stars in M 31. The stellar and wind parameters will tell about the formation of WR stars in other galaxies with different metallicity and star formation histories. The obtained parameters will provide constraints to the evolution of massive stars in the environment of M 31. Methods. We used the latest version of the Potsdam Wolf-Rayet model atmosphere code to analyze the stars via fitting optical spectra and photometric data. To account for the relatively low temperatures of the late WN I 0 and WN I I subtypes, our WN models have been extended into this temperature regime. Results. Stellar and atmospheric parameters are derived for all known late-type WN stars in M 31 with available spectra. All of these stars still have hydrogen in their outer envelopes, some of them up to 50% by mass. The stars are located on the cool side of the zero age main sequence in the Hertzsprung-Russell diagram, while their luminosities range from 105 to 1064). It is remarkable that no star exceeds 106 L. Conclusions. If formed via single-star evolution, the late-type WN stars in M 31 stem from an initial mass range between 20 and 60 M-circle dot. From the very late-type WN9-11 stars, only one star is located in the S Doradus instability strip. We do not find any late-type WN stars with the high luminosities known in the Milky Way. KW - stars: massive KW - stars: evolution KW - stars: mass-loss KW - stars: Wolf-Rayet KW - stars: atmospheres KW - stars: winds KW - outflows Y1 - 2014 U6 - https://doi.org/10.1051/0004-6361/201323240 SN - 0004-6361 SN - 1432-0746 VL - 563 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Shenar, Tomer A1 - Sablowski, D. P. A1 - Hainich, Rainer A1 - Todt, Helge Tobias A1 - Moffat, Anthony F. J. A1 - Oskinova, Lida A1 - Ramachandran, Varsha A1 - Sana, Hugues A1 - Sander, Andreas Alexander Christoph A1 - Schnurr, O. A1 - St-Louis, N. A1 - Vanbeveren, D. A1 - Gotberg, Y. A1 - Hamann, Wolf-Rainer T1 - The Wolf-Rayet binaries of the nitrogen sequence in the Large Magellanic Cloud Spectroscopy, orbital analysis, formation, and evolution JF - Astronomy and astrophysics : an international weekly journal N2 - Context. Massive Wolf-Rayet (WR) stars dominate the radiative and mechanical energy budget of galaxies and probe a critical phase in the evolution of massive stars prior to core collapse. It is not known whether core He-burning WR stars (classical WR; cWR) form predominantly through wind stripping (w-WR) or binary stripping (b-WR). Whereas spectroscopy of WR binaries has so-far largely been avoided because of its complexity, our study focuses on the 44 WR binaries and binary candidates of the Large Magellanic Cloud (LMC; metallicity Z approximate to 0.5 Z(circle dot)), which were identified on the basis of radial velocity variations, composite spectra, or high X-ray luminosities. Aims. Relying on a diverse spectroscopic database, we aim to derive the physical and orbital parameters of our targets, confronting evolution models of evolved massive stars at subsolar metallicity and constraining the impact of binary interaction in forming these stars. Methods. Spectroscopy was performed using the Potsdam Wolf-Rayet (PoWR) code and cross-correlation techniques. Disentanglement was performed using the code Spectangular or the shift-and-add algorithm. Evolutionary status was interpreted using the Binary Population and Spectral Synthesis (BPASS) code, exploring binary interaction and chemically homogeneous evolution. Results. Among our sample, 28/44 objects show composite spectra and are analyzed as such. An additional five targets show periodically moving WR primaries but no detected companions (SB1); two (BAT99 99 and 112) are potential WR + compact-object candidates owing to their high X-ray luminosities. We cannot confirm the binary nature of the remaining 11 candidates. About two-thirds of the WN components in binaries are identified as cWR, and one-third as hydrogen-burning WR stars. We establish metallicity-dependent mass-loss recipes, which broadly agree with those recently derived for single WN stars, and in which so-called WN3/O3 stars are clear outliers. We estimate that 45 +/- 30% of the cWR stars in our sample have interacted with a companion via mass transfer. However, only approximate to 12 +/- 7% of the cWR stars in our sample naively appear to have formed purely owing to stripping via a companion (12% b-WR). Assuming that apparently single WR stars truly formed as single stars, this comprises approximate to 4% of the whole LMC WN population, which is about ten times less than expected. No obvious differences in the properties of single and binary WN stars, whose luminosities extend down to log L approximate to 5.2 [L-circle dot], are apparent. With the exception of a few systems (BAT99 19, 49, and 103), the equatorial rotational velocities of the OB-type companions are moderate (v(eq) less than or similar to 250 km s(-1)) and challenge standard formalisms of angular-momentum accretion. For most objects, chemically homogeneous evolution can be rejected for the secondary, but not for the WR progenitor. Conclusions. No obvious dichotomy in the locations of apparently single and binary WN stars on the Hertzsprung-Russell diagram is apparent. According to commonly used stellar evolution models (BPASS, Geneva), most apparently single WN stars could not have formed as single stars, implying that they were stripped by an undetected companion. Otherwise, it must follow that pre-WR mass-loss/mixing (e.g., during the red supergiant phase) are strongly underestimated in standard stellar evolution models. KW - stars: massive KW - stars: Wolf-Rayet KW - Magellanic Clouds KW - binaries: close KW - binaries: spectroscopic KW - stars: evolution Y1 - 2019 U6 - https://doi.org/10.1051/0004-6361/201935684 SN - 0004-6361 SN - 1432-0746 VL - 627 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Shenar, Tomer A1 - Richardson, N. D. A1 - Sablowski, Daniel P. A1 - Hainich, Rainer A1 - Sana, H. A1 - Moffat, A. F. J. A1 - Todt, Helge Tobias A1 - Hamann, Wolf-Rainer A1 - Oskinova, Lida A1 - Sander, Andreas Alexander Christoph A1 - Tramper, Frank A1 - Langer, Norbert A1 - Bonanos, Alceste Z. A1 - de Mink, Selma E. A1 - Gräfener, G. A1 - Crowther, Paul A1 - Vink, J. S. A1 - Almeida, Leonardo A. A1 - de Koter, A. A1 - Barbá, Rodolfo A1 - Herrero, A. A1 - Ulaczyk, Krzysztof T1 - The tarantula massive binary monitoring BT - II. First SB2 orbital and spectroscopic analysis for the Wolf-Rayet binary R145 JF - Astronomy and astrophysics : an international weekly journal N2 - We present the first SB2 orbital solution and disentanglement of the massive Wolf-Rayet binary R145 (P = 159 d) located in the Large Magellanic Cloud. The primary was claimed to have a stellar mass greater than 300 M-circle dot, making it a candidate for being the most massive star known to date. While the primary is a known late-type, H-rich Wolf-Rayet star (WN6h), the secondary has so far not been unambiguously detected. Using moderate-resolution spectra, we are able to derive accurate radial velocities for both components. By performing simultaneous orbital and polarimetric analyses, we derive the complete set of orbital parameters, including the inclination. The spectra are disentangled and spectroscopically analyzed, and an analysis of the wind-wind collision zone is conducted. The disentangled spectra and our models are consistent with a WN6h type for the primary and suggest that the secondary is an O3.5 If*/WN7 type star. We derive a high eccentricity of e = 0 : 78 and minimum masses of M-1 sin(3) i approximate to M-2 sin(3) i = 13 +/- 2 M-circle dot, with q = M-2/M-1 = 1.01 +/- 0.07. An analysis of emission excess stemming from a wind-wind collision yields an inclination similar to that obtained from polarimetry (i = 39 +/- 6 degrees). Our analysis thus implies M-1 = 53(-20)(+40) and M2 = 54(-20)(+40) M-circle dot, excluding M-1 > 300 M-circle dot. A detailed comparison with evolution tracks calculated for single and binary stars together with the high eccentricity suggests that the components of the system underwent quasi-homogeneous evolution and avoided mass-transfer. This scenario would suggest current masses of approximate to 80 M-circle dot and initial masses of M-i,M-1 approximate to 10(5) and M-i,M-2 approximate to 90 M-circle dot, consistent with the upper limits of our derived orbital masses, and would imply an age of approximate to 2.2 Myr. KW - binaries: spectroscopic KW - stars: Wolf-Rayet KW - stars: massive KW - Magellanic Clouds KW - stars: individual: R 145 KW - stars: atmospheres Y1 - 2017 U6 - https://doi.org/10.1051/0004-6361/201629621 SN - 1432-0746 VL - 598 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Shenar, Tomer A1 - Hainich, Rainer A1 - Todt, Helge Tobias A1 - Moffat, Anthony F. J. A1 - Sander, Andreas Alexander Christoph A1 - Oskinova, Lida A1 - Ramachandran, Varsha A1 - Munoz, M. A1 - Pablo, H. A1 - Sana, Hugues A1 - Hamann, Wolf-Rainer T1 - The shortest-period Wolf-Rayet binary in the small magellanic cloud BT - Part of a high-order multiple system Spectral and orbital analysis of SMC AB 6 JF - Astronomy and astrophysics : an international weekly journal N2 - Context. SMC AB6 is the shortest-period (P = 6.5 d) Wolf-Rayet (WR) binary in the Small Magellanic Cloud. This binary is therefore a key system in the study of binary interaction and formation of WR stars at low metallicity. The WR component in AB6 was previously found to be very luminous (log L = 6.3 [L-circle dot]) compared to its reported orbital mass (approximate to 8 M-circle dot), placing it significantly above the Eddington limit. Aims. Through spectroscopy and orbital analysis of newly acquired optical data taken with the Ultraviolet and Visual Echelle Spectrograph (UVES), we aim to understand the peculiar results reported for this system and explore its evolutionary history. Methods. We measured radial velocities via cross-correlation and performed a spectral analysis using the Potsdam Wolf-Rayet model atmosphere code. The evolution of the system was analyzed using the Binary Population and Spectral Synthesis evolution code. Results. AB6 contains at least four stars. The 6.5 d period WR binary comprises the WR primary (WN3:h, star A) and a rather rapidly rotating (v(eq) = 265 km s(-1)) early O-type companion (O5.5 V, star B). Static N III and N IV emission lines and absorption signatures in He lines suggest the presence of an early-type emission line star (O5.5 I(f), star C). Finally, narrow absorption lines portraying a long-term radial velocity variation show the existence of a fourth star (O7.5 V, star D). Star D appears to form a second 140 d period binary together with a fifth stellar member, which is a B-type dwarf or a black hole. It is not clear that these additional components are bound to the WR binary. We derive a mass ratio of M-O/M-WR = 2.2 +/- 0.1. The WR star is found to be less luminous than previously thought (log L = 5.9 [L-circle dot]) and, adopting M-O = 41 M-circle dot for star B, more massive (M-WR = 18 M-circle dot). Correspondingly, the WR star does not exceed the Eddington limit. We derive the initial masses of M-i,M-WR = 60 M-circle dot and M-i,M-O = 40 M-circle dot and an age of 3.9 Myr for the system. The WR binary likely experienced nonconservative mass transfer in the past supported by the relatively rapid rotation of star B. Conclusions. Our study shows that AB6 is a multiple - probably quintuple - system. This finding resolves the previously reported puzzle of the WR primary exceeding the Eddington limit and suggests that the WR star exchanged mass with its companion in the past. KW - stars: massive KW - binaries: spectroscopic KW - stars: Wolf-Rayet KW - Magellanic Clouds KW - stars: individual: SMC AB 6 KW - stars: atmospheres Y1 - 2018 U6 - https://doi.org/10.1051/0004-6361/201833006 SN - 1432-0746 SN - 0004-6361 VL - 616 PB - EDP Sciences CY - Les Ulis ER - TY - GEN A1 - Sandin, Christer A1 - Steffen, Matthias A1 - Jacob, Ralf A1 - Schönberner, Detlef A1 - Rühling, Ute A1 - Hamann, Wolf-Rainer A1 - Todt, Helge Tobias T1 - The role of heat conduction to the formation of [WC]-type planetary nebulae T2 - Proceedings of the International Astronomical Union N2 - X-ray observations of young Planetary Nebulæ (PNe) have revealed diffuse emission in extended regions around both H-rich and H-deficient central stars. In order to also repro-duce physical properties of H-deficient objects, we have, at first, extended our time-dependent radiation-hydrodynamic models with heat conduction for such conditions. Here we present some of the important physical concepts, which determine how and when a hot wind-blown bubble forms. In this study we have had to consider the, largely unknown, evolution of the CSPN, the slow (AGB) wind, the fast hot-CSPN wind, and the chemical composition. The main conclusion of our work is that heat conduction is needed to explain X-ray properties of wind-blown bubbles also in H-deficient objects. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 582 KW - conduction KW - hydrodynamics KW - planetary nebulae: general Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-413702 SN - 1866-8372 IS - 582 SP - 494 EP - 495 ER - TY - JOUR A1 - Reindl, Nicole A1 - Rauch, Thomas A1 - Parthasarathy, M. A1 - Werner, K. A1 - Kruk, J. W. A1 - Hamann, Wolf-Rainer A1 - Sander, Andreas Alexander Christoph A1 - Todt, Helge Tobias T1 - The rapid evolution of the exciting star of the Stingray nebula JF - Astronomy and astrophysics : an international weekly journal N2 - Context. SAO 244567, the exciting star of the Stingray nebula, is rapidly evolving. Previous analyses suggested that it has heated up from an effective temperature of about 21 kK in 1971 to over 50 kK in the 1990s. Canonical post-asymptotic giant branch evolution suggests a relatively high mass while previous analyses indicate a low-mass star. Aims. A comprehensive model-atmosphere analysis of UV and optical spectra taken during 1988-2006 should reveal the detailed temporal evolution of its atmospheric parameters and provide explanations for the unusually fast evolution. Methods. Fitting line profiles from static and expanding non-LTE model atmospheres to the observed spectra allowed us to study the temporal change of effective temperature, surface gravity, mass-loss rate, and terminal wind velocity. In addition, we determined the chemical composition of the atmosphere. Results. We find that the central star has steadily increased its effective temperature from 38 kK in 1988 to a peak value of 60 kK in 2002. During the same time, the star was contracting, as concluded from an increase in surface gravity from log g = 4.8 to 6.0 and a drop in luminosity. Simultaneously, the mass-loss rate declined from log(M/M-circle dot yr(-1)) = -9.0 to -11.6 and the terminal wind velocity increased from v(infinity) = 1800 km s(-1) to 2800 km s(-1). Since around 2002, the star stopped heating and has cooled down again to 55 kK by 2006. It has a largely solar surface composition with the exception of slightly subsolar carbon, phosphorus, and sulfur. The results are discussed by considering different evolutionary scenarios. Conclusions. The position of SAO 244567 in the log T-eff-log g plane places the star in the region of sdO stars. By comparison with stellar-evolution calculations, we confirm that SAO 244567 must be a low-mass star (M < 0.55 M-circle dot). However, the slow evolution of the respective stellar evolutionary models is in strong contrast to the observed fast evolution and the young planetary nebula with a kinematical age of only about 1000 years. We speculate that the star could be a late He-shell flash object. Alternatively, it could be the outcome of close-binary evolution. Then SAD 244567 would be a low-mass (0.354 M-circle dot) helium pre-white dwarf after the common-envelope phase, during which the planetary nebula was ejected. KW - stars: abundances KW - stars: evolution KW - stars: AGB and post-AGB KW - stars: individual: SAO 244567 KW - stars: fundamental parameters KW - planetary nebulae: individual: Stingray nebula (Henize 3-1357) Y1 - 2014 U6 - https://doi.org/10.1051/0004-6361/201323189 SN - 0004-6361 SN - 1432-0746 VL - 565 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Leto, Paolo A1 - Trigilio, C. A1 - Oskinova, Lida A1 - Ignace, R. A1 - Buemi, C. S. A1 - Umana, G. A1 - Cavallaro, Francesco A1 - Ingallinera, A. A1 - Bufano, F. A1 - Phillips, N. M. A1 - Agliozzo, Claudia A1 - Cerrigone, L. A1 - Todt, Helge Tobias A1 - Riggi, S. A1 - Leone, Francesco T1 - The polarization mode of the auroral radio emission from the early-type star HD 142301 JF - Monthly notices of the Royal Astronomical Society N2 - We report the detection of the auroral radio emission from the early-type magnetic star HD142301. New VLA observations of HD142301 detected highly polarized amplified emission occurring at fixed stellar orientations. The coherent emission mechanism responsible for the stellar auroral radio emission amplifies the radiation within a narrow beam, making the star where this phenomenon occurs similar to a radio lighthouse. The elementary emission process responsible for the auroral radiation mainly amplifies one of the two magneto-ionic modes of the electromagnetic wave. This explains why the auroral pulses are highly circularly polarized. The auroral radio emission of HD142301 is characterized by a reversal of the sense of polarization as the star rotates. The effective magnetic field curve of HD142301 is also available making it possible to correlate the transition from the left to the right-hand circular polarization sense ( and vice versa) of the auroral pulses with the known orientation of the stellar magnetic field. The results presented in this letter have implications for the estimation of the dominant magneto-ionic mode amplified within the HD142301 magnetosphere. KW - masers KW - polarization KW - stars: early-type KW - stars: individual: HD142301 KW - stars: magnetic field KW - radio continuum: stars Y1 - 2018 U6 - https://doi.org/10.1093/mnrasl/sly179 SN - 0035-8711 SN - 1365-2966 VL - 482 IS - 1 SP - L4 EP - L8 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Guerrero, Martin A. A1 - Fang, Xuan A1 - Miller Bertolami, Marcelo Miguel A1 - Ramos-Larios, Gerardo A1 - Todt, Helge Tobias A1 - Alarie, Alexandre A1 - Sabin, Laurence A1 - Miranda, Luis F. A1 - Morisset, Christophe A1 - Kehrig, Carolina A1 - Zavala, Saul A. T1 - The inside-out planetary nebula around a born-again star JF - Nature Astronomy N2 - Planetary nebulae are ionized clouds of gas formed by the hydrogen-rich envelopes of low- and intermediate-mass stars ejected at late evolutionary stages. The strong UV flux from their central stars causes a highly stratified ionization structure, with species of higher ionization potential closer to the star. Here, we report on the exceptional case of HuBi 1, a double-shell planetary nebula whose inner shell presents emission from low-ionization species close to the star and emission from high-ionization species farther away. Spectral analysis demonstrates that the inner shell of HuBi 1 is excited by shocks, whereas its outer shell is recombining. The anomalous excitation of these shells can be traced to its low-temperature [WC10] central star whose optical brightness has declined continuously by 10 magnitudes in a period of 46 years. Evolutionary models reveal that this star is the descendant of a low-mass star (≃1.1 M⊙) that has experienced a ‘born-again’ event1 whose ejecta shock-excite the inner shell. HuBi 1 represents the missing link in the formation of metal-rich central stars of planetary nebulae from low-mass progenitors, offering unique insight regarding the future evolution of the born-again Sakurai’s object2. Coming from a solar-mass progenitor, HuBi 1 represents a potential end-state for our Sun. KW - Astronomy and astrophysics KW - Astronomy and planetary science KW - Interstellar medium KW - Stars KW - Stellar evolution Y1 - 2018 U6 - https://doi.org/10.1038/s41550-018-0551-8 SN - 2397-3366 VL - 2 IS - 10 SP - 784 EP - 789 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Shenar, Tomer A1 - Hamann, Wolf-Rainer A1 - Todt, Helge Tobias T1 - The impact of rotation on the line profiles of Wolf-Rayet stars JF - Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.–5. June 2015 N2 - The distribution of angular momentum in massive stars is a critical component of their evolution, yet not much is known on the rotation velocities of Wolf-Rayet stars. There are various indications that rapidly rotating Wolf-Rayet stars should exist. Unfortunately, due to their expanding atmospheres, rotational velocities of Wolf-Rayet stars are very difficult to measure. In this work, we model the effects of rotation on the atmospheres of Wolf-Rayet stars by implementing a 3D integration scheme in the PoWR code. We further investigate whether the peculiar spectra of five Wolf-Rayet stars may imply rapid rotation, infer the corresponding rotation parameters, and discuss the implications of our results. We find that rotation helps to reproduce the unique spectra analyzed here. However, if rotation is indeed involved, the inferred rotational velocities at the stellar surface are large (∼ 200 km/s), and the implied co-rotation radii (∼ 10R∗) suggest the existence of very strong photospheric magnetic fields (∼ 20 kG). Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-88008 SP - 193 EP - 196 ER - TY - JOUR A1 - Shenar, Tomer A1 - Hamann, Wolf-Rainer A1 - Todt, Helge Tobias T1 - The impact of rotation on the line profiles of Wolf-Rayet stars JF - Astronomy and astrophysics : an international weekly journal N2 - Context. Massive Wolf-Rayet stars are recognized today to be in a very common, but short, evolutionary phase of massive stars. While our understanding of Wolf-Rayet stars has increased dramatically over the past decades, it remains unclear whether rapid rotators are among them. There are various indications that rapidly rotating Wolf-Rayet stars should exist. Unfortunately, due to their expanding atmospheres, rotational velocities of Wolf-Rayet stars are very difficult to measure. However, recently observed spectra of several Wolf-Rayet stars reveal peculiarly broad and round emission lines. Could these spectra imply rapid rotation? Aims. In this work, we model the effects of rotation on the atmospheres of Wolf-Rayet stars. We further investigate whether the peculiar spectra of five Wolf-Rayet stars may be explained with the help of stellar rotation, infer appropriate rotation parameters, and discuss the implications of our results. Methods. We make use of the Potsdam Wolf-Rayet (PoWR) non-LTE model atmosphere code. Since the observed spectra of WolfRayet stars are mainly formed in their expanding atmospheres, rotation must be accounted for with a 3D integration scheme of the formal integral. For this purpose, we assume a rotational velocity field consisting of an inner co-rotating domain and an outer domain, where the angular momentum is conserved. Results. We find that rotation can reproduce the unique spectra analyzed here. However, the inferred rotational velocities at the stellar surface are large (similar to 200 km s(-1)), and the inferred co-rotation radii (similar to 10R.) suggest the existence of very strong photospheric magnetic fields (similar to 20 kG). KW - stars: Wolf-Rayet KW - Magellanic Clouds KW - stars: magnetic field KW - stars: massive KW - gamma-ray burst: general KW - stars: rotation Y1 - 2014 U6 - https://doi.org/10.1051/0004-6361/201322496 SN - 0004-6361 SN - 1432-0746 VL - 562 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Hamann, Wolf-Rainer A1 - Gräfener, G. A1 - Liermann, A. A1 - Hainich, Rainer A1 - Sander, Andreas Alexander Christoph A1 - Shenar, Tomer A1 - Ramachandran, Varsha A1 - Todt, Helge Tobias A1 - Oskinova, Lida T1 - The Galactic WN stars revisited BT - Impact of Gaia distances on fundamental stellar parameters JF - Astronomy and astrophysics : an international weekly journal N2 - Comprehensive spectral analyses of the Galactic Wolf-Rayet stars of the nitrogen sequence (i.e. the WN subclass) have been performed in a previous paper. However, the distances of these objects were poorly known. Distances have a direct impact on the "absolute" parameters, such as luminosities and mass-loss rates. The recent Gaia Data Release (DR2) of trigonometric parallaxes includes nearly all WN stars of our Galactic sample. In the present paper, we apply the new distances to the previously analyzed Galactic WN stars and rescale the results accordingly. On this basis, we present a revised catalog of 55 Galactic WN stars with their stellar and wind parameters. The correlations between mass-loss rate and luminosity show a large scatter, for the hydrogen-free WN stars as well as for those with detectable hydrogen. The slopes of the log L - log M correlations are shallower than found previously. The empirical Hertzsprung-Russell diagram (HRD) still shows the previously established dichotomy between the hydrogen-free early WN subtypes that are located on the hot side of the zero-age main sequence (ZAMS), and the late WN subtypes, which show hydrogen and reside mostly at cooler temperatures than the ZAMS (with few exceptions). However, with the new distances, the distribution of stellar luminosities became more continuous than obtained previously. The hydrogen-showing stars of late WN subtype are still found to be typically more luminous than the hydrogen-free early subtypes, but there is a range of luminosities where both subclasses overlap. The empirical HRD of the Galactic single WN stars is compared with recent evolutionary tracks. Neither these single-star evolutionary models nor binary scenarios can provide a fully satisfactory explanation for the parameters of these objects and their location in the HRD. KW - stars: mass-loss KW - stars: winds, outflows KW - stars: Wolf-Rayet KW - stars: atmospheres KW - stars: evolution KW - stars: distances Y1 - 2019 U6 - https://doi.org/10.1051/0004-6361/201834850 SN - 1432-0746 VL - 625 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Sander, A. A1 - Hamann, Wolf-Rainer A1 - Todt, Helge Tobias T1 - The Galactic WC stars Stellar parameters from spectral analyses indicate a new evolutionary sequence JF - Astronomy and astrophysics : an international weekly journal N2 - Context. The life cycles of massive stars from the main sequence to their explosion as supernovae or gamma ray bursts are not yet fully clear, and the empirical results from spectral analyses are partly in conflict with current evolutionary models. The spectral analysis of Wolf-Rayet stars requires the detailed modeling of expanding stellar atmospheres in non-LTE. The Galactic WN stars have been comprehensively analyzed with such models of the latest stage of sophistication, while a similarly comprehensive study of the Galactic WC sample remains undone. Aims. We aim to establish the stellar parameters and mass-loss rates of the Galactic WC stars. These data provide the empirical basis of studies of (i) the role of WC stars in the evolution of massive stars, (ii) the wind-driving mechanisms, and (iii) the feedback of WC stars as input to models of the chemical and dynamical evolution of galaxies. Methods. We analyze the nearly complete sample of un-obscured Galactic WC stars, using optical spectra as well as ultraviolet spectra when available. The observations are fitted with theoretical spectra, using the Potsdam Wolf-Rayet (PoWR) model atmosphere code. A large grid of line-blanked models has been established for the range of WC subtypes WC4 - WC8, and smaller grids for the WC9 parameter domain. Both WO stars and WN/WC transit types are also analyzed using special models. Results. Stellar and atmospheric parameters are derived for more than 50 Galactic WC and two WO stars, covering almost the whole Galactic WC population as far as the stars are single, and un-obscured in the visual. In the Hertzsprung-Russell diagram, the WC stars reside between the hydrogen and the helium zero-age main sequences, having luminosities L from 10(4.9) to 10(5.6) L-circle dot. The mass-loss rates scale very tightly with L-0.8. The two WO stars in our sample turn out to be outstandingly hot (approximate to 200 kK) and do not fit into the WC scheme. Conclusions. By comparing the empirical WC positions in the Hertzsprung-Russell diagram with evolutionary models, and from recent supernova statistics, we conclude that WC stars have evolved from initial masses between 20 solar masses and 45 M-circle dot. In contrast to previous assumptions, it seems that WC stars in general do not descend from the most massive stars. Only the WO stars might stem from progenitors that have been initially more massive than 45 M-circle dot. KW - stars: massive KW - stars: mass-loss KW - stars: Wolf-Rayet KW - stars: evolution KW - stars: atmospheres KW - stars: winds, outflows Y1 - 2012 U6 - https://doi.org/10.1051/0004-6361/201117830 SN - 0004-6361 VL - 540 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Sander, Andreas Alexander Christoph A1 - Hamann, Wolf-Rainer A1 - Todt, Helge Tobias A1 - Hainich, Rainer A1 - Shenar, Tomer A1 - Ramachandran, Varsha A1 - Oskinova, Lida T1 - The Galactic WC and WO stars BT - The impact of revised distances from Gaia DR2 and their role as massive black hole progenitors JF - Astronomy and astrophysics : an international weekly journal N2 - Wolf-Rayet stars of the carbon sequence (WC stars) are an important cornerstone in the late evolution of massive stars before their core collapse. As core-helium burning, hydrogen-free objects with huge mass-loss, they are likely the last observable stage before collapse and thus promising progenitor candidates for type Ib/c supernovae. Their strong mass-loss furthermore provides challenges and constraints to the theory of radiatively driven winds. Thus, the determination of the WC star parameters is of major importance for several astrophysical fields. With Gaia DR2, for the first time parallaxes for a large sample of Galactic WC stars are available, removing major uncertainties inherent to earlier studies. In this work, we re-examine a previously studied sample of WC stars to derive key properties of the Galactic WC population. All quantities depending on the distance are updated, while the underlying spectral analyzes remain untouched. Contrasting earlier assumptions, our study yields that WC stars of the same subtype can significantly vary in absolute magnitude. With Gaia DR2, the picture of the Galactic WC population becomes more complex: We obtain luminosities ranging from log L/L-circle dot = 4.9-6.0 with one outlier (WR 119) having log L/L-circle dot = 4.7. This indicates that the WC stars are likely formed from a broader initial mass range than previously assumed. We obtain mass-loss rates ranging between log(M) over dot = -5.1 and -4.1, with (M) over dot proportional to L-0.68 and a linear scaling of the modified wind momentum with luminosity. We discuss the implications for stellar evolution, including unsolved issues regarding the need of envelope inflation to address the WR radius problem, and the open questions in regard to the connection of WR stars with Gamma-ray bursts. WC and WO stars are progenitors of massive black holes, collapsing either silently or in a supernova that most-likely has to be preceded by a WO stage. KW - stars: evolution KW - stars: mass-loss KW - stars: Wolf-Rayet KW - stars: massive KW - stars: distances KW - Galaxy: stellar content Y1 - 2019 U6 - https://doi.org/10.1051/0004-6361/201833712 SN - 1432-0746 VL - 621 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Gruner, David A1 - Hainich, Rainer A1 - Sander, Andreas Alexander Christoph A1 - Shenar, Tomer A1 - Todt, Helge Tobias A1 - Oskinova, Lida A1 - Ramachandran, Varsha A1 - Ayres, T. A1 - Hamann, Wolf-Rainer T1 - The extreme O-type spectroscopic binary HD 93129A A quantitative, multiwavelength analysis JF - Astronomy and astrophysics : an international weekly journal N2 - Context. HD 93129A was classified as the earliest O-type star in the Galaxy (O2 If*) and is considered as the prototype of its spectral class. However, interferometry shows that this object is a binary system, while recent observations even suggest a triple configuration. None of the previous spectral analyses of this object accounted for its multiplicity. With new high-resolution UV and optical spectra, we have the possibility to reanalyze this key object, taking its binary nature into account for the first time. Aims. We aim to derive the fundamental parameters and the evolutionary status of HD 93129A, identifying the contributions of both components to the composite spectrum Results. Despite the similar spectral types of the two components, we are able to find signatures from each of the components in the combined spectrum, which allows us to estimate the parameters of both stars. We derive log(L/L-circle dot) = 6.15, T-eff = 52 kK, and log (M)over dot = -4.7[M-circle dot yr(-1)] for the primary Aa, and log(L/L-circle dot) = 5.58, T-eff = 45 kK, and log (M)over dot = -5.8 [M(circle dot)yr(-1)] for the secondary Ab. Conclusions. Even when accounting for the binary nature, the primary of HD 93129A is found to be one of the hottest and most luminous O stars in our Galaxy. Based on the theoretical decomposition of the spectra, we assign spectral types O2 If* and O3 III(f*) to components Aa and Ab, respectively. While we achieve a good fit for a wide spectral range, specific spectral features are not fully reproduced. The data are not sufficient to identify contributions from a hypothetical third component in the system. KW - stars: individual: HD 93129A KW - stars: atmospheres KW - stars: fundamental parameters KW - stars: early-typeP Y1 - 2018 U6 - https://doi.org/10.1051/0004-6361/201833178 SN - 1432-0746 VL - 621 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Leto, Paolo A1 - Trigilio, C. A1 - Oskinova, Lida A1 - Ignace, R. A1 - Buemi, C. S. A1 - Umana, G. A1 - Ingallinera, A. A1 - Todt, Helge Tobias A1 - Leone, F. T1 - The detection of variable radio emission from the fast rotating magnetic hot B-star HR 7355 and evidence for its X-ray aurorae JF - Monthly notices of the Royal Astronomical Society N2 - In this paper, we investigate the multiwavelength properties of the magnetic early B-type star HR 7355. We present its radio light curves at several frequencies, taken with the Jansky Very Large Array, and X-ray spectra, taken with the XMM-Newton X-ray telescope. Modelling of the radio light curves for the Stokes I and V provides a quantitative analysis of the HR 7355 magnetosphere. A comparison between HR 7355 and a similar analysis for the Ap star CU Vir allows us to study how the different physical parameters of the two stars affect the structure of the respective magnetospheres where the non-thermal electrons originate. Our model includes a cold thermal plasma component that accumulates at high magnetic latitudes that influences the radio regime, but does not give rise to X-ray emission. Instead, the thermal X-ray emission arises from shocks generated by wind stream collisions close to the magnetic equatorial plane. The analysis of the X-ray spectrum of HR 7355 also suggests the presence of a non-thermal radiation. Comparison between the spectral index of the power-law X-ray energy distribution with the non-thermal electron energy distribution indicates that the non-thermal X-ray component could be the auroral signature of the non-thermal electrons that impact the stellar surface, the same non-thermal electrons that are responsible for the observed radio emission. On the basis of our analysis, we suggest a novel model that simultaneously explains the X-ray and the radio features of HR 7355 and is likely relevant for magnetospheres of other magnetic early-type stars. KW - stars: chemically peculiar KW - stars: early-type KW - stars: individual: HR 7355 KW - stars: magnetic field KW - radio continuum: stars KW - X-rays: stars Y1 - 2017 U6 - https://doi.org/10.1093/mnras/stx267 SN - 0035-8711 SN - 1365-2966 VL - 467 SP - 2820 EP - 2833 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Hajduk, Marcin A1 - Todt, Helge Tobias A1 - Hamann, Wolf-Rainer A1 - Borek, Karolina A1 - van Hoof, Peter A. M. A1 - Zijlstra, Albert A. T1 - The cooling-down central star of the planetary nebula SwSt 1 BT - a late thermal pulse in a massive post-AGB star? JF - Monthly notices of the Royal Astronomical Society N2 - SwSt 1 (PN G001.5-06.7) is a bright and compact planetary nebula containing a late [WC]-type central star. Previous studies suggested that the nebular and stellar lines are slowly changing with time. We studied new and archival optical and ultraviolet spectra of the object. The [O III] 4959 and 5007 angstrom to H beta line flux ratios decreased between about 1976 and 1997/2015. The stellar spectrum also shows changes between these epochs. We modelled the stellar and nebular spectra observed at different epochs. The analyses indicate a drop of the stellar temperature from about 42 kK to 40.5 kK between 1976 and 1993. We do not detect significant changes between 1993 and 2015. The observations show that the star performed a loop in the H-R diagram. This is possible when a shell source is activated during its post-AGB evolution. We infer that a late thermal pulse (LTP) experienced by a massive post-AGB star can explain the evolution of the central star. Such a star does not expand significantly as the result of the LTP and does not became a born-again red giant. However, the released energy can remove the tiny H envelope of the star. KW - stars: AGB and post-AGB KW - stars: atmospheres KW - stars: evolution KW - planetary KW - nebulae: general KW - planetary nebulae: individual: SwSt1 Y1 - 2020 U6 - https://doi.org/10.1093/mnras/staa2274 SN - 0035-8711 SN - 1365-2966 VL - 498 IS - 1 SP - 1205 EP - 1220 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Todt, Helge Tobias A1 - Pena, Maria A. A1 - Hamann, Wolf-Rainer A1 - Gräfener, Götz T1 - The central star of the planetary nebula PB8 : a Wolf-Rayet-type wind of an unusual WN/WC chemical composition N2 - A considerable fraction of the central stars of planetary nebulae (CSPNe) are hydrogen-deficient. As a rule, these CSPNe exhibit a chemical composition of helium, carbon, and oxygen with the majority showing Wolf-Rayet-like emission line spectra. These stars are classified as CSPNe of a spectral type [WC]. We perform a spectral analysis of CSPN PB 8 with the Potsdam Wolf-Rayet (PoWR) models for expanding atmospheres. The source PB8 displays wind-broadened emission lines from strong mass loss. Most strikingly, we find that its surface composition is hydrogen-deficient, but not carbon-rich. With mass fractions of 55% helium, 40% hydrogen, 1.3% carbon, 2% nitrogen, and 1.3% oxygen, it differs greatly from the 30-50% of carbon which are typically seen in [WC]-type central stars. The atmospheric mixture in PB8 has an analogy in the WN/WC transition type among the massive Wolf-Rayet stars. Therefore we suggest to introduce a new spectral type [WN/WC] for CSPNe, with PB8 as its first member. The central star of PB8 has a relatively low temperature of T-* = 52 kK, as expected for central stars in their early evolutionary stages. Its surrounding nebula is less than 3000 years old, i.e. relatively young. Existing calculations for the post-AGB evolution can produce hydrogen-deficient stars of the [WC] type, but do not predict the composition found in PB8. We discuss various scenarios that might explain the origin of this unique object. Y1 - 2010 UR - http://www.aanda.org/ U6 - https://doi.org/10.1051/0004-6361/200912183 SN - 0004-6361 ER - TY - JOUR A1 - Toala, Jesús Alberto A1 - Guerrero, Martín A. A1 - Todt, Helge Tobias A1 - Hamann, Wolf-Rainer A1 - Chu, Y.-H. A1 - Gruendl, R. A. A1 - Schönberner, Detlef A1 - Oskinova, Lida A1 - Marquez-Lugo, R. A. A1 - Fang, X. A1 - Ramos-Larios, Gerardo T1 - The born-again Planetary nebula A78: an X-RAY twin of A30 JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We present the XMM-Newton discovery of X-ray emission from the planetary nebula (PN) A78, the second born-again PN detected in X-rays apart from A30. These two PNe share similar spectral and morphological characteristics: they harbor diffuse soft X-ray emission associated with the interaction between the H-poor ejecta and the current fast stellar wind and a point-like source at the position of the central star (CSPN). We present the spectral analysis of the CSPN, using for the first time an NLTE code for expanding atmospheres that takes line blanketing into account for the UV and optical spectra. The wind abundances are used for the X-ray spectral analysis of the CSPN and the diffuse emission. The X-ray emission from the CSPN in A78 can be modeled by a single C VI emission line, while the X-ray emission from its diffuse component is better described by an optically thin plasma emission model with a temperature of kT = 0.088 keV (T approximate to 1.0 x 10(6) K). We estimate X-ray luminosities in the 0.2-2.0 keV energy band of L-X,L-CSPN =(1.2 +/- 0.3) x 10(31) erg s(-1) and L-X,L-DIFF =(9.2 +/- 2.3) x 10(30) erg s(-1) for the CSPN and diffuse components, respectively. KW - planetary nebulae: general KW - planetary nebulae: individual (A78) KW - stars: winds, outflows KW - X-rays: ISM Y1 - 2015 U6 - https://doi.org/10.1088/0004-637X/799/1/67 SN - 0004-637X SN - 1538-4357 VL - 799 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Ramachandran, Varsha A1 - Hamann, Wolf-Rainer A1 - Oskinova, Lida A1 - Gallagher, J. S. A1 - Hainich, Rainer A1 - Shenar, Tomer A1 - Sander, Andreas Alexander Christoph A1 - Todt, Helge Tobias A1 - Fulmer, Leah M. T1 - Testing massive star evolution, star formation history, and feedback at low metallicity BT - Spectroscopic analysis of OB stars in the SMC Wing JF - Astronomy and astrophysics : an international weekly journal N2 - Stars that start their lives with spectral types O and early B are the progenitors of core-collapse supernovae, long gamma-ray bursts, neutron stars, and black holes. These massive stars are the primary sources of stellar feedback in star-forming galaxies. At low metallicities, the properties of massive stars and their evolution are not yet fully explored. Here we report a spectroscopic study of 320 massive stars of spectral types O (23 stars) and B (297 stars) in the Wing of the Small Magellanic Cloud (SMC). The spectra, which we obtained with the ESO Very Large Telescope, were analyzed using state-of-the-art stellar atmosphere models, and the stellar parameters were determined. We find that the stellar winds of our sample stars are generally much weaker than theoretically expected. The stellar rotation rates show broad, tentatively bimodal distributions. The upper Hertzsprung-Russell diagram (HRD) is well populated by the stars of our sample from a specific field in the SMC Wing. A few very luminous O stars are found close to the main sequence, while all other, slightly evolved stars obey a strict luminosity limit. Considering additional massive stars in evolved stages, with published parameters and located all over the SMC, essentially confirms this picture. The comparison with single-star evolutionary tracks suggests a dichotomy in the fate of massive stars in the SMC. Only stars with an initial mass below similar to 30 M-circle dot seem to evolve from the main sequence to the cool side of the HRD to become a red supergiant and to explode as type II-P supernova. In contrast, stars with initially more than similar to 30 M-circle dot appear to stay always hot and might evolve quasi chemically homogeneously, finally collapsing to relatively massive black holes. However, we find no indication that chemical mixing is correlated with rapid rotation. We measured the key parameters of stellar feedback and established the links between the rates of star formation and supernovae. Our study demonstrates that in metal-poor environments stellar feedback is dominated by core-collapse supernovae in combination with winds and ionizing radiation supplied by a few of the most massive stars. We found indications of the stochastic mode of massive star formation, where the resulting stellar population is fully capable of producing large-scale structures such as the supergiant shell SMC-SGS 1 in the Wing. The low level of feedback in metal-poor stellar populations allows star formation episodes to persist over long timescales. KW - stars: evolution KW - stars: massive KW - stars: mass-loss KW - Magellanic Clouds KW - Hertzsprung-Russell and C-M diagrams KW - techniques: spectroscopic Y1 - 2019 U6 - https://doi.org/10.1051/0004-6361/201935365 SN - 1432-0746 VL - 625 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Ramachandran, Varsha A1 - Hamann, Wolf-Rainer A1 - Hainich, Rainer A1 - Oskinova, Lida A1 - Shenar, Tomer A1 - Sander, Andreas Alexander Christoph A1 - Todt, Helge Tobias A1 - Gallagher, John S. T1 - Stellar population of the superbubble N206 in the LMC II. Parameters of the OB and WR stars, and the total massive star feedback JF - Astronomy and astrophysics : an international weekly journal N2 - Context. Clusters or associations of early-type stars are often associated with a "superbubble" of hot gas. The formation of such superbubbles is caused by the feedback from massive stars. The complex N206 in the Large Magellanic Cloud (LMC) exhibits a superbubble and a rich massive star population. Aims. Our goal is to perform quantitative spectral analyses of all massive stars associated with the N206 superbubble in order to determine their stellar and wind parameters. We compare the superbubble energy budget to the stellar energy input and discuss the star formation history of the region. Results. We present the stellar and wind parameters of the OB stars and the two Wolf-Rayet (WR) binaries in the N206 complex. Twelve percent of the sample show Oe/Be type emission lines, although most of them appear to rotate far below critical. We found eight runaway stars based on their radial velocity. The wind-momentum luminosity relation of our OB sample is consistent with the expectations. The Hertzsprung-Russell diagram (HRD) of the OB stars reveals a large age spread (1-30 Myr), suggesting different episodes of star formation in the complex. The youngest stars are concentrated in the inner part of the complex, while the older OB stars are scattered over outer regions. We derived the present day mass function for the entire N206 complex as well as for the cluster NGC2018. The total ionizing photon flux produced by all massive stars in the N206 complex is Q(0) approximate to 5 x 10(50) s(-1), and the mechanical luminosity of their stellar winds amounts to L-mec = 1.7 x 10(38) erg s(-1). Three very massive Of stars are found to dominate the feedback among 164 OB stars in the sample. The two WR winds alone release about as much mechanical luminosity as the whole OB star sample. The cumulative mechanical feedback from all massive stellar winds is comparable to the combined mechanical energy of the supernova explosions that likely occurred in the complex. Accounting also for the WR wind and supernovae, the mechanical input over the last five Myr is approximate to 2.3 x 10(52) erg. Conclusions. The N206 complex in the LMC has undergone star formation episodes since more than 30 Myr ago. From the spectral analyses of its massive star population, we derive a current star formation rate of 2.2 x 10(-3) M-circle dot yr(-1). From the combined input of mechanical energy from all stellar winds, only a minor fraction is emitted in the form of X-rays. The corresponding input accumulated over a long time also exceeds the current energy content of the complex by more than a factor of five. The morphology of the complex suggests a leakage of hot gas from the superbubble. KW - stars: massive KW - Magellanic Clouds KW - stars: winds, outflows KW - Hertzsprung-Russell and C-M diagrams KW - techniques: spectroscopic KW - ISM: bubbles Y1 - 2018 U6 - https://doi.org/10.1051/0004-6361/201832816 SN - 1432-0746 VL - 615 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Ramachandran, Varsha A1 - Hainich, Rainer A1 - Hamann, Wolf-Rainer A1 - Oskinova, Lida A1 - Shenar, T. A1 - Sander, Andreas Alexander Christoph A1 - Todt, Helge Tobias A1 - Gallagher, John S. T1 - Stellar population of the superbubble N206 in the LMC I. Analysis of the Of-type stars JF - Astronomy and astrophysics : an international weekly journal N2 - Context. Massive stars severely influence their environment by their strong ionizing radiation and by the momentum and kinetic energy input provided by their stellar winds and supernovae. Quantitative analyses of massive stars are required to understand how their feedback creates and shapes large scale structures of the interstellar medium. The giant H II region N206 in the Large Magellanic Cloud contains an OB association that powers a superbubble filled with hot X-ray emitting gas, serving as an ideal laboratory in this context. Aims. We aim to estimate stellar and wind parameters of all OB stars in N206 by means of quantitative spectroscopic analyses. In this first paper, we focus on the nine Of-type stars located in this region. We determine their ionizing flux and wind mechanical energy. The analysis of nitrogen abundances in our sample probes rotational mixing. Methods. We obtained optical spectra with the multi-object spectrograph FLAMES at the ESO-VLT. When possible, the optical spectroscopy was complemented by UV spectra from the HST, IUE, and FUSE archives. Detailed spectral classifications are presented for our sample Of-type stars. For the quantitative spectroscopic analysis we used the Potsdam Wolf-Rayet model atmosphere code. We determined the physical parameters and nitrogen abundances of our sample stars by fitting synthetic spectra to the observations. Results. The stellar and wind parameters of nine Of-type stars, which are largely derived from spectral analysis are used to construct wind momentum luminosity relationship. We find that our sample follows a relation close to the theoretical prediction, assuming clumped winds. The most massive star in the N206 association is an Of supergiant that has a very high mass-loss rate. Two objects in our sample reveal composite spectra, showing that the Of primaries have companions of late O subtype. All stars in our sample have an evolutionary age of less than 4 million yr, with the O2-type star being the youngest. All these stars show a systematic discrepancy between evolutionary and spectroscopic masses. All stars in our sample are nitrogen enriched. Nitrogen enrichment shows a clear correlation with increasing projected rotational velocities. Conclusions. The mechanical energy input from the Of stars alone is comparable to the energy stored in the N206 superbubble as measured from the observed X-ray and H alpha emission. KW - stars: early-type KW - Magellanic Clouds KW - stars: atmospheres KW - stars: winds, outflows KW - stars: mass-loss KW - stars: massive Y1 - 2017 U6 - https://doi.org/10.1051/0004-6361/201731093 SN - 1432-0746 SN - 0004-6361 VL - 609 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Löbling, Lisa A1 - Rauch, Thomas A1 - Bertolami Miller, Marcelo Miguel A1 - Todt, Helge Tobias A1 - Friederich, F. A1 - Ziegler, M. A1 - Werner, Klaus A1 - Kruk, J. W. T1 - Spectral analysis of the hybrid PG 1159-type central stars of the planetary nebulae Abell 43 and NGC7094 JF - Monthly notices of the Royal Astronomical Society N2 - Stellar post asymptotic giant branch (post-AGB) evolution can be completely altered by a final thermal pulse (FTP) which may occur when the star is still leaving the AGB (AFTP), at the departure from the AGB at still constant luminosity (late TP, LTP) or after the entry to the white-dwarf cooling sequence (very late TP, VLTP). Then convection mixes the Herich material with the H-rich envelope. According to stellar evolution models the result is a star with a surface composition of H approximate to 20 per cent by mass (AFTP), approximate to 1 per cent (LTP), or (almost) no H (VLTP). Since FTP stars exhibit intershell material at their surface, spectral analyses establish constraints for AGB nucleosynthesis and stellar evolution. We performed a spectral analysis of the so-called hybrid PG 1159-type central stars (CS) of the planetary nebulae Abell 43 and NGC7094 by means of non-local thermodynamical equilibrium models. We confirm the previously determined effective temperatures of T-eff = 115 000 +/- 5 000K and determine surface gravities of log (g /(cm s(-2))) = 5.6 +/- 0.1 for both. From a comparison with AFTP evolutionary tracks, we derive stellar masses of 0.57(-0.04)(+0.07)M(circle dot) and determine the abundances of H, He, and metals up to Xe. Both CS are likely AFTP stars with a surface H mass fraction of 0.25 +/- 0.03 and 0.15 +/- 0.03, respectively, and an Fe deficiency indicating subsolar initial metallicities. The light metals show typical PG 1159-type abundances and the elemental composition is in good agreement with predictions from AFTP evolutionary models. However, the expansion ages do not agree with evolution time-scales expected from the AFTP scenario and alternatives should be explored. KW - stars: abundances KW - stars: AGB and post-AGB KW - stars: atmospheres KW - stars: evolution KW - stars: individual: WD1751+106 KW - stars: individual: WD2134+125 Y1 - 2019 U6 - https://doi.org/10.1093/mnras/stz1994 SN - 0035-8711 SN - 1365-2966 VL - 489 IS - 1 SP - 1054 EP - 1071 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Guerrero, Martín A. A1 - Ruiz, N. A1 - Hamann, Wolf-Rainer A1 - Chu, Y.-H. A1 - Todt, Helge Tobias A1 - Schönberner, Detlef A1 - Oskinova, Lida A1 - Gründl, R. A. A1 - Steffen, M. A1 - Blair, William P. A1 - Toala, Jesús Alberto T1 - Rebirth of X-Ray emission from the born-again planetary Nebula A30 JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - The planetary nebula A30 is believed to have undergone a very late thermal pulse resulting in the ejection of knots of hydrogen-poor material. Using multi-epoch Hubble Space Telescope images, we have detected the angular expansion of these knots and derived an age of 850(-150)(+280) yr. To investigate the spectral and spatial properties of the soft X-ray emission detected by ROSAT, we have obtained Chandra and XMM-Newton deep observations of A30. The X-ray emission from A30 can be separated into two components: a point source at the central star and diffuse emission associated with the hydrogen-poor knots and the cloverleaf structure inside the nebular shell. To help us assess the role of the current stellar wind in powering this X-ray emission, we have determined the stellar parameters and wind properties of the central star of A30 using a non-LTE model fit to its optical and UV spectra. The spatial distribution and spectral properties of the diffuse X-ray emission are highly suggestive that it is generated by the post-born-again and present fast stellar winds interacting with the hydrogen-poor ejecta of the born-again event. This emission can be attributed to shock-heated plasma, as the hydrogen-poor knots are ablated by the stellar winds, under which circumstances the efficient mass loading of the present fast stellar wind raises its density and damps its velocity to produce the observed diffuse soft X-rays. Charge transfer reactions between the ions of the stellar winds and material of the born-again ejecta have also been considered as a possible mechanism for the production of diffuse X-ray emission, and upper limits on the expected X-ray production by this mechanism have been derived. The origin of the X-ray emission from the central star of A30 is puzzling: shocks in the present fast stellar wind and photospheric emission can be ruled out, while the development of a new, compact hot bubble confining the fast stellar wind seems implausible. KW - planetary nebulae: general KW - planetary nebulae: individual (A30) KW - stars: winds, outflows KW - X-rays: ISM Y1 - 2012 U6 - https://doi.org/10.1088/0004-637X/755/2/129 SN - 0004-637X VL - 755 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Hainich, Rainer A1 - Ramachandran, Varsha A1 - Shenar, Tomer A1 - Sander, Andreas Alexander Christoph A1 - Todt, Helge Tobias A1 - Gruner, David A1 - Oskinova, Lida A1 - Hamann, Wolf-Rainer T1 - PoWR grids of non-LTE model atmospheres for OB-type stars of various metallicities JF - Astronomy and astrophysics : an international weekly journal N2 - The study of massive stars in different metallicity environments is a central topic of current stellar research. The spectral analysis of massive stars requires adequate model atmospheres. The computation of such models is difficult and time-consuming. Therefore, spectral analyses are greatly facilitated if they can refer to existing grids of models. Here we provide grids of model atmospheres for OB-type stars at metallicities corresponding to the Small and Large Magellanic Clouds, as well as to solar metallicity. In total, the grids comprise 785 individual models. The models were calculated using the state-of-the-art Potsdam Wolf-Rayet (PoWR) model atmosphere code. The parameter domain of the grids was set up using stellar evolution tracks. For all these models, we provide normalized and flux-calibrated spectra, spectral energy distributions, feedback parameters such as ionizing photons, Zanstra temperatures, and photometric magnitudes. The atmospheric structures (the density and temperature stratification) are available as well. All these data are publicly accessible through the PoWR website. KW - stars: massive KW - stars: early-type KW - stars: atmospheres KW - stars: winds KW - outflows KW - stars: mass-loss KW - radiative transfer Y1 - 2019 U6 - https://doi.org/10.1051/0004-6361/201833787 SN - 1432-0746 VL - 621 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Todt, Helge Tobias A1 - Sander, Angelika A1 - Hainich, Rainer A1 - Hamann, Wolf-Rainer A1 - Quade, Markus A1 - Shenar, Tomer T1 - Potsdam Wolf-Rayet model atmosphere grids for WN stars JF - Astronomy and astrophysics : an international weekly journal N2 - We present new grids of Potsdam Wolf-Rayet (PoWR) model atmospheres for Wolf-Rayet stars of the nitrogen sequence (WN stars). The models have been calculated with the latest version of the PoWR stellar atmosphere code for spherical stellar winds. The WN model atmospheres include the non-LTE solutions of the statistical equations for complex model atoms, as well as the radiative transfer equation in the co-moving frame. Iron-line blanketing is treated with the help of the superlevel approach, while wind inhomogeneities are taken into account via optically thin clumps. Three of our model grids are appropriate for Galactic metallicity. The hydrogen mass fraction of these grids is 50%, 20%, and 0%, thus also covering the hydrogen-rich late-type WR stars that have been discovered in recent years. Three grids are adequate for LMC WN stars and have hydrogen fractions of 40%, 20%, and 0%. Recently, additional grids with SMC metallicity and with 60%, 40%, 20%, and 0% hydrogen have been added. We provide contour plots of the equivalent widths of spectral lines that are usually used for classification and diagnostics. KW - stars: evolution KW - stars: mass-loss KW - stars: winds, outflows KW - stars: Wolf-Rayet KW - stars: atmospheres KW - stars: massive Y1 - 2015 U6 - https://doi.org/10.1051/0004-6361/201526253 SN - 0004-6361 SN - 1432-0746 VL - 579 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Rubio, Gabriel A1 - Toalá, Jesús Alberto A1 - Todt, Helge Tobias A1 - Sabin, Laurence A1 - Santamaría, Edgar A1 - Ramos-Larios, Gerardo A1 - Martín Guerrero, José David T1 - Planetary nebulae with Wolf-Rayet-type central stars - IV. NGC 1501 and its mixing layer JF - Monthly notices of the Royal Astronomical Society N2 - Theory predicts that the temperature of the X-ray-emitting gas (similar to 10(6) K) detected from planetary nebulae (PNe) is a consequence of mixing or thermal conduction when in contact with the ionized outer rim (similar to 10(4) K). Gas at intermediate temperatures (similar to 10(5) K) can be used to study the physics of the production of X-ray-emitting gas, via C iv, N v, and O vi ions. Here, we model the stellar atmosphere of the CSPN of NGC 1501 to demonstrate that even this hot H-deficient [WO4]-type star cannot produce these emission lines by photoionization. We use the detection of the C iv lines to assess the physical properties of the mixing region in this PNe in comparison with its X-ray-emitting gas, rendering NGC 1501 only the second PNe with such characterization. We extend our predictions to the hottest [WO1] and cooler [WC5] spectral types and demonstrate that most energetic photons are absorbed in the dense winds of [WR] CSPN and highly ionized species can be used to study the physics behind the production of hot bubbles in PNe. We found that the UV observations of NGC 2452, NGC 6751, and NGC 6905 are consistent with the presence mixing layers and hot bubbles, providing excellent candidates for future X-ray observations. KW - stars: evolution KW - stars: individual: WD0402+607 KW - stars: winds KW - outflows; KW - stars: Wolf-Rayet KW - planetary nebulae: general KW - planetary nebulae KW - individual: NGC1501 Y1 - 2022 U6 - https://doi.org/10.1093/mnras/stac3011 SN - 0035-8711 SN - 1365-2966 VL - 517 IS - 4 SP - 5166 EP - 5179 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Oskinova, Lida A1 - Steinke, M. A1 - Hamann, Wolf-Rainer A1 - Sander, A. A1 - Todt, Helge Tobias A1 - Liermann, Adriane T1 - One of the most massive stars in the Galaxy may have formed in isolation JF - Monthly notices of the Royal Astronomical Society N2 - Very massive stars, 100 times heavier than the sun, are rare. It is not yet known whether such stars can form in isolation or only in star clusters. The answer to this question is of fundamental importance. The central region of our Galaxy is ideal for investigating very massive stars and clusters located in the same environment. We used archival infrared images to investigate the surroundings of apparently isolated massive stars presently known in the Galactic Centre (GC). We find that two such isolated massive stars display bow shocks and hence may be 'runaways' from their birthplace. Thus, some isolated massive stars in the GC region might have been born in star clusters known in this region. However, no bow shock is detected around the isolated star WR 102ka (Peony nebula star), which is one of the most massive and luminous stars in the Galaxy. This star is located at the centre of an associated circumstellar nebula. To study whether a star cluster may be 'hidden' in the surroundings of WR 102ka, to obtain new and better spectra of this star, and to measure its radial velocity, we obtained observations with the integral-field spectrograph SINFONI at the ESO's Very Large Telescope. Our observations confirm that WR 102ka is one of the most massive stars in the Galaxy and reveal that this star is not associated with a star cluster. We suggest that WR 102ka has been born in relative isolation, outside of any massive star cluster. KW - stars: individual: WR 102ka KW - Galaxy: centre KW - infrared: stars Y1 - 2013 U6 - https://doi.org/10.1093/mnras/stt1817 SN - 0035-8711 SN - 1365-2966 VL - 436 IS - 4 SP - 3357 EP - 3365 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Oskinova, Lida A1 - Todt, Helge Tobias A1 - Huenemoerder, David P. A1 - Hubrig, Swetlana A1 - Ignace, Richard A1 - Hamann, Wolf-Rainer A1 - Balona, Luis T1 - On X-ray pulsations in beta Cephei-type variables JF - Astronomy and astrophysics : an international weekly journal N2 - Context. beta Cep-type variables are early B-type stars that are characterized by oscillations observable in their optical light curves. At least one beta Cep-variable also shows periodic variability in X-rays. Aims. Here we study the X-ray light curves in a sample of beta Cep-variables to investigate how common X-ray pulsations are for this type of stars. Methods. We searched the Chandra and XMM-Newton X-ray archives and selected stars that were observed by these telescopes for at least three optical pulsational periods. We retrieved and analyzed the X-ray data for kappa Sco, beta Cru, and alpha Vir. The X-ray light curves of these objects were studied to test for their variability and periodicity. Results. While there is a weak indication for X-ray variability in beta Cru, we find no statistically significant evidence of X-ray pulsations in any of our sample stars. This might be due either to the insufficient data quality or to the physical lack of modulations. New, more sensitive observations should settle this question. KW - stars: massive KW - stars: variables: general KW - X-rays: stars Y1 - 2015 U6 - https://doi.org/10.1051/0004-6361/201525908 SN - 0004-6361 SN - 1432-0746 VL - 577 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Hünemörder, David P. A1 - Oskinova, Lida A1 - Ignace, Richard A1 - Waldron, Wayne L. A1 - Todt, Helge Tobias A1 - Hamaguchi, Kenji A1 - Kitamoto, Shunji T1 - On the weak-wind problem in massive stars X-ray spectra reveal a massive hot wind in mu columbaea JF - The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters N2 - mu Columbae is a prototypical weak-wind O star for which we have obtained a high-resolution X-ray spectrum with the Chandra LETG/ACIS instrument and a low-resolution spectrum with Suzaku. This allows us, for the first time, to investigate the role of X-rays on the wind structure in a bona fide weak-wind system and to determine whether there actually is a massive hot wind. The X-ray emission measure indicates that the outflow is an order of magnitude greater than that derived from UV lines and is commensurate with the nominal wind-luminosity relationship for O stars. Therefore, the "weak-wind problem"-identified from cool wind UV/optical spectra-is largely resolved by accounting for the hot wind seen in X-rays. From X-ray line profiles, Doppler shifts, and relative strengths, we find that this weak-wind star is typical of other late O dwarfs. The X-ray spectra do not suggest a magnetically confined plasma-the spectrum is soft and lines are broadened; Suzaku spectra confirm the lack of emission above 2 keV. Nor do the relative line shifts and widths suggest any wind decoupling by ions. The He-like triplets indicate that the bulk of the X-ray emission is formed rather close to the star, within five stellar radii. Our results challenge the idea that some OB stars are "weak-wind" stars that deviate from the standard wind-luminosity relationship. The wind is not weak, but it is hot and its bulk is only detectable in X-rays. KW - stars: early-type KW - stars: individual (mu Col) KW - stars: mass-loss KW - X-rays: stars Y1 - 2012 U6 - https://doi.org/10.1088/2041-8205/756/2/L34 SN - 2041-8205 VL - 756 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Sander, Andreas Alexander Christoph A1 - Shenar, Tomer A1 - Hainich, Rainer A1 - Gimenez-Garcia, Ana A1 - Todt, Helge Tobias A1 - Hamann, Wolf-Rainer T1 - On the consistent treatment of the quasi-hydrostatic layers in hot star atmospheres JF - Astronomy and astrophysics : an international weekly journal N2 - Context. Spectroscopic analysis remains the most common method to derive masses of massive stars, the most fundamental stellar parameter. While binary orbits and stellar pulsations can provide much sharper constraints on the stellar mass, these methods are only rarely applicable to massive stars. Unfortunately, spectroscopic masses of massive stars heavily depend on the detailed physics of model atmospheres. Aims. We demonstrate the impact of a consistent treatment of the radiative pressure on inferred gravities and spectroscopic masses of massive stars. Specifically, we investigate the contribution of line and continuum transitions to the photospheric radiative pressure. We further explore the effect of model parameters, e.g., abundances, on the deduced spectroscopic mass. Lastly, we compare our results with the plane-parallel TLUSTY code, commonly used for the analysis of massive stars with photospheric spectra. Methods. We calculate a small set of O-star models with the Potsdam Wolf-Rayet (PoWR) code using different approaches for the quasi-hydrostatic part. These models allow us to quantify the effect of accounting for the radiative pressure consistently. We further use PoWR models to show how the Doppler widths of line profiles and abundances of elements such as iron affect the radiative pressure, and, as a consequence, the derived spectroscopic masses. Results. Our study implies that errors on the order of a factor of two in the inferred spectroscopic mass are to be expected when neglecting the contribution of line and continuum transitions to the radiative acceleration in the photosphere. Usage of implausible microturbulent velocities, or the neglect of important opacity sources such as Fe, may result in errors of approximately 50% in the spectroscopic mass. A comparison with TLUSTY model atmospheres reveals a very good agreement with PoWR at the limit of low mass-loss rates. KW - stars: early-type KW - stars: mass-loss KW - stars: winds, outflows KW - stars: atmospheres KW - stars: fundamental parameters KW - stars: massive Y1 - 2015 U6 - https://doi.org/10.1051/0004-6361/201425356 SN - 0004-6361 SN - 1432-0746 VL - 577 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Oskinova, Lida A1 - Huenemoerder, D. P. A1 - Hamann, Wolf-Rainer A1 - Shenar, Tomer A1 - Sander, Andreas Alexander Christoph A1 - Ignace, R. A1 - Todt, Helge Tobias A1 - Hainich, Rainer T1 - On the Binary Nature of Massive Blue Hypergiants: High-resolution X-Ray Spectroscopy Suggests That Cyg OB2 12 is a Colliding Wind Binary JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - The blue hypergiant Cyg OB2 12 (B3Ia(+)) is a representative member of the class of very massive stars in a poorly understood evolutionary stage. We obtained its high-resolution X-ray spectrum using the Chandra observatory. PoWR model atmospheres were calculated to provide realistic wind opacities and to establish the wind density structure. We find that collisional de-excitation is the dominant mechanism depopulating the metastable upper levels of the forbidden lines of the He-like ions Si XIV and Mg XII. Comparison between the model and observations reveals that X-ray emission is produced in a dense plasma, which could reside only at the photosphere or in a colliding wind zone between binary components. The observed X-ray spectra are well-fitted by thermal plasma models, with average temperatures in excess of 10 MK. The wind speed in Cyg OB2 12 is not high enough to power such high temperatures, but the collision of two winds in a binary system can be sufficient. We used archival data to investigate the X-ray properties of other blue hypergiants. In general, stars of this class are not detected as X-ray sources. We suggest that our new Chandra observations of Cyg OB2 12 can be best explained if Cyg OB2 12 is a colliding wind binary possessing a late O-type companion. This makes Cyg OB2 12 only the second binary system among the 16 known Galactic hypergiants. This low binary fraction indicates that the blue hypergiants are likely products of massive binary evolution during which they either accreted a significant amount of mass or already merged with their companions. KW - stars: individual (Cyg OB2 12) KW - stars: massive KW - stars: mass-loss KW - stars: winds, outflows KW - supergiants KW - X-rays: stars Y1 - 2017 U6 - https://doi.org/10.3847/1538-4357/aa7e79 SN - 0004-637X SN - 1538-4357 VL - 845 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Toala, Jesús Alberto A1 - Oskinova, Lida A1 - Hamann, Wolf-Rainer A1 - Ignace, Richard A1 - Sander, Andreas Alexander Christoph A1 - Shenar, Tomer A1 - Todt, Helge Tobias A1 - Chu, Y. -H. A1 - Guerrero, Martin A. A1 - Hainich, Rainer A1 - Torrejon, Jose Miguel T1 - On the Apparent Absence of Wolf-Rayet plus Neutron Star Systems BT - the Curious Case of WR124 JF - The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters N2 - Among the different types of massive stars in advanced evolutionary stages is the enigmatic WN8h type. There are only a few Wolf-Rayet (WR) stars with this spectral type in our Galaxy. It has long been suggested that WN8h-type stars are the products of binary evolution that may harbor neutron stars (NS). One of the most intriguing WN8h stars is the runaway WR 124 surrounded by its magnificent nebula M1-67. We test the presence of an accreting NS companion in WR 124 using similar to 100 ks long observations by the Chandra X-ray observatory. The hard X-ray emission from WR 124 with a luminosity of L-X similar to 10(31) erg s(-1) is marginally detected. We use the non-local thermodynamic equilibrium stellar atmosphere code PoWR to estimate the WR wind opacity to the X-rays. The wind of a WN8-type star is effectively opaque for X-rays, hence the low X-ray luminosity of WR 124 does not rule out the presence of an embedded compact object. We suggest that, in general, high-opacity WR winds could prevent X-ray detections of embedded NS, and be an explanation for the apparent lack of WR+NS systems. KW - circumstellar matter KW - ISM: jets and outflows KW - stars: massive KW - stars: evolution KW - stars: neutron KW - stars: Wolf-Rayet Y1 - 2018 U6 - https://doi.org/10.3847/2041-8213/aaf39d SN - 2041-8205 SN - 2041-8213 VL - 869 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Reindl, Nicole A1 - Rauch, Thomas A1 - Werner, Klaus A1 - Kruk, J. W. A1 - Todt, Helge Tobias T1 - On helium-dominated stellar evolution: the mysterious role of the O(He)-type stars JF - Astronomy and astrophysics : an international weekly journal N2 - Context. About a quarter of all post-asymptotic giant branch (AGB) stars are hydrogen-deficient. Stellar evolutionary models explain the carbon-dominated H-deficient stars by a (very) late thermal pulse scenario where the hydrogen-rich envelope is mixed with the helium-rich intershell layer. Depending on the particular time at which the final flash occurs, the entire hydrogen envelope may be burned. In contrast, helium-dominated post-AGB stars and their evolution are not yet understood. Aims. A small group of very hot, helium-dominated stars is formed by O(He)-type stars. A precise analysis of their photospheric abundances will establish constraints to their evolution. Methods. We performed a detailed spectral analysis of ultraviolet and optical spectra of four O(He) stars by means of state-of-the-art non-LTE model-atmosphere techniques. Results. We determined effective temperatures, surface gravities, and the abundances of H, He, C, N, O, F, Ne, Si, P, S, Ar, and Fe. By deriving upper limits for the mass-loss rates of the O(He) stars, we found that they do not exhibit enhanced mass-loss. The comparison with evolutionary models shows that the status of the O(He) stars remains uncertain. Their abundances match predictions of a double helium white dwarf (WD) merger scenario, suggesting that they might be the progeny of the compact and of the luminous helium-rich sdO-type stars. The existence of planetary nebulae that do not show helium enrichment around every other O(He) star precludes a merger origin for these stars. These stars must have formed in a different way, for instance via enhanced mass-loss during their post-AGB evolution or a merger within a common-envelope (CE) of a CO-WD and a red giant or AGB star. Conclusions. A helium-dominated stellar evolutionary sequence exists that may be fed by different types of mergers or CE scenarios. It appears likely that all these pass through the O(He) phase just before they become WDs. KW - stars: AGB and post-AGB KW - stars: evolution KW - stars: fundamental parameters KW - stars: abundances Y1 - 2014 U6 - https://doi.org/10.1051/0004-6361/201423498 SN - 0004-6361 SN - 1432-0746 VL - 566 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Danehkar, A. A1 - Todt, Helge Tobias A1 - Ercolano, B. A1 - Kniazev, A. Y. T1 - Observations and three-dimensional photoionization modelling of the Wolf-Rayet planetary nebula Abell 48(star) JF - Monthly notices of the Royal Astronomical Society N2 - Recent observations reveal that the central star of the planetary nebula Abell 48 exhibits spectral features similar to massive nitrogen-sequence Wolf-Rayet stars. This raises a pertinent question, whether it is still a planetary nebula or rather a ring nebula of a massive star. In this study, we have constructed a three-dimensional photoionization model of Abell 48, constrained by our new optical integral field spectroscopy. An analysis of the spatially resolved velocity distributions allowed us to constrain the geometry of Abell 48. We used the collisionally excited lines to obtain the nebular physical conditions and ionic abundances of nitrogen, oxygen, neon, sulphur and argon, relative to hydrogen. We also determined helium temperatures and ionic abundances of helium and carbon from the optical recombination lines. We obtained a good fit to the observations for most of the emission-line fluxes in our photoionization model. The ionic abundances deduced from our model are in decent agreement with those derived by the empirical analysis. However, we notice obvious discrepancies between helium temperatures derived from the model and the empirical analysis, as overestimated by our model. This could be due to the presence of a small fraction of cold metal-rich structures, which were not included in our model. It is found that the observed nebular line fluxes were best reproduced by using a hydrogen-deficient expanding model atmosphere as the ionizing source with an effective temperature of T-eff = 70 kK and a stellar luminosity of L-star = 5500 L-circle dot, which corresponds to a relatively low-mass progenitor star (similar to 3 M-circle dot) rather than a massive Pop I star. KW - stars: Wolf-Rayet KW - ISM: abundances KW - planetary nebulae: individual: Abell 48 Y1 - 2014 U6 - https://doi.org/10.1093/mnras/stu203 SN - 0035-8711 SN - 1365-2966 VL - 439 IS - 4 SP - 3605 EP - 3615 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Hainich, Rainer A1 - Oskinova, Lida A1 - Shenar, Tomer A1 - Marchant Campos, Pablo A1 - Eldridge, J. J. A1 - Sander, Andreas Alexander Christoph A1 - Hamann, Wolf-Rainer A1 - Langer, Norbert A1 - Todt, Helge Tobias T1 - Observational properties of massive black hole binary progenitors JF - Astronomy and astrophysics : an international weekly journal N2 - Context: The first directly detected gravitational waves (GW 150914) were emitted by two coalescing black holes (BHs) with masses of ≈ 36 M⊙ and ≈ 29 M⊙. Several scenarios have been proposed to put this detection into an astrophysical context. The evolution of an isolated massive binary system is among commonly considered models. Aims: Various groups have performed detailed binary-evolution calculations that lead to BH merger events. However, the question remains open as to whether binary systems with the predicted properties really exist. The aim of this paper is to help observers to close this gap by providing spectral characteristics of massive binary BH progenitors during a phase where at least one of the companions is still non-degenerate. Methods: Stellar evolution models predict fundamental stellar parameters. Using these as input for our stellar atmosphere code (Potsdam Wolf-Rayet), we compute a set of models for selected evolutionary stages of massive merging BH progenitors at different metallicities. Results: The synthetic spectra obtained from our atmosphere calculations reveal that progenitors of massive BH merger events start their lives as O2-3V stars that evolve to early-type blue supergiants before they undergo core-collapse during the Wolf-Rayet phase. When the primary has collapsed, the remaining system will appear as a wind-fed high-mass X-ray binary. Based on our atmosphere models, we provide feedback parameters, broad band magnitudes, and spectral templates that should help to identify such binaries in the future. Conclusions: While the predicted parameter space for massive BH binary progenitors is partly realized in nature, none of the known massive binaries match our synthetic spectra of massive BH binary progenitors exactly. Comparisons of empirically determined mass-loss rates with those assumed by evolution calculations reveal significant differences. The consideration of the empirical mass-loss rates in evolution calculations will possibly entail a shift of the maximum in the predicted binary-BH merger rate to higher metallicities, that is, more candidates should be expected in our cosmic neighborhood than previously assumed. KW - gravitational waves KW - binaries: close KW - stars: early-type KW - stars: atmospheres KW - stars: winds KW - outflows KW - stars: mass-loss Y1 - 2018 U6 - https://doi.org/10.1051/0004-6361/201731449 SN - 1432-0746 VL - 609 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Hubrig, Swetlana A1 - Schöller, Markus A1 - Kholtygin, Alexander F. A1 - Tsumura, Hiroki A1 - Hoshino, Akio A1 - Kitamoto, Shunji A1 - Oskinova, Lida A1 - Ignace, Richard A1 - Todt, Helge Tobias A1 - Ilyin, Ilya T1 - New multiwavelength observations of the Of?p star CPD-28 degrees 2561 JF - Monthly notices of the Royal Astronomical Society N2 - A rather strong mean longitudinal magnetic field of the order of a few hundred gauss was detected a few years ago in the Of?p star CPD -28 degrees 2561 using FORS2 (FOcal Reducer low dispersion Spectrograph 2) low-resolution spectropolarimetric observations. In this work, we present additional low-resolution spectropolarimetric observations obtained during several weeks in 2013 December using FORS 2 mounted at the 8-m Antu telescope of the Very Large Telescope (VLT). These observations cover a little less than half of the stellar rotation period of 73.41 d mentioned in the literature. The behaviour of the mean longitudinal magnetic field is consistent with the assumption of a single-wave variation during the stellar rotation cycle, indicating a dominant dipolar contribution to the magnetic field topology. The estimated polar strength of the surface dipole B-d is larger than 1.15 kG. Further, we compared the behaviour of the line profiles of various elements at different rotation phases associated with different magnetic field strengths. The strongest contribution of the emission component is observed at the phases when the magnetic field shows a negative or positive extremum. The comparison of the spectral behaviour of CPD -28 degrees 2561 with that of another Of?p star, HD 148937 of similar spectral type, reveals remarkable differences in the degree of variability between both stars. Finally, we present new X-ray observations obtained with the Suzaku X-ray Observatory. We report that the star is X-ray bright with log L-X/L-bol approximate to -5.7. The low-resolution X-ray spectra reveal the presence of a plasma heated up to 24 MK. We associate the 24 MK plasma in CPD -28 degrees 2561 with the presence of a kG strong magnetic field capable to confine stellar wind. KW - stars: atmospheres KW - stars: individual: CPD-28 degrees 2561 KW - stars: magnetic field KW - stars: mass-loss KW - stars: variables: general KW - X-rays: stars Y1 - 2015 U6 - https://doi.org/10.1093/mnras/stu2516 SN - 0035-8711 SN - 1365-2966 VL - 447 IS - 2 SP - 1885 EP - 1894 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Toalá, Jesús Alberto A1 - Bowman, Dominic A1 - Van Reeth, Timothy A1 - Todt, Helge Tobias A1 - Dsilva, Karan A1 - Shenar, Tomer A1 - Koenigsberger, Gloria Suzanne A1 - Estrada-Dorado, Sandino A1 - Oskinova, Lida A1 - Hamann, Wolf-Rainer T1 - Multiple variability time-scales of the early nitrogen-rich Wolf-Rayet star WR 7 JF - Monthly notices of the Royal Astronomical Society N2 - We present the analysis of the optical variability of the early, nitrogen-rich Wolf-Rayet (WR) star WR 7. The analysis of multisector Transiting Exoplanet Survey Satellite (TESS) light curves and high-resolution spectroscopic observations confirm multiperiodic variability that is modulated on time-scales of years. We detect a dominant period of 2.6433 +/- 0.0005 d in the TESS sectors 33 and 34 light curves in addition to the previously reported high-frequency features from sector 7. We discuss the plausible mechanisms that may be responsible for such variability in WR 7, including pulsations, binarity, co-rotating interaction regions (CIRs), and clumpy winds. Given the lack of strong evidence for the presence of a stellar or compact companion, we suggest that WR 7 may pulsate in quasi-coherent modes in addition to wind variability likely caused by CIRs on top of stochastic low-frequency variability. WR 7 is certainly a worthy target for future monitoring in both spectroscopy and photometry to sample both the short (less than or similar to 1 d) and long (greater than or similar to 1000 d) variability time-scales. KW - stars: atmospheres KW - stars: evolution KW - stars: individual: WR 7 KW - stars: winds KW - outflows KW - stars: Wolft-Rayet Y1 - 2022 U6 - https://doi.org/10.1093/mnras/stac1455 SN - 0035-8711 SN - 1365-2966 VL - 514 IS - 2 SP - 2269 EP - 2277 PB - Oxford University Press CY - Oxford ER - TY - JOUR A1 - Kubátová, Brankica A1 - Hamann, Wolf-Rainer A1 - Todt, Helge Tobias A1 - Sander, A. A1 - Steinke, M. A1 - Hainich, Rainer A1 - Shenar, Tomer T1 - Macroclumping in WR 136 JF - Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.–5. June 2015 N2 - Macroclumping proved to resolve the discordance between different mass-loss rate diagnostics for O-type stars, in particular between Hα and the P v resonance lines. In this paper, we report first results from a corresponding investigation for WR stars. We apply our detailed 3-D Monte Carlo (MC) line formation code to the P v resonance doublet and show, for the Galactic WNL star WR136, that macroclumping is require to bring this line in accordance with the mass-loss rate derived from the emission-line spectrum. Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-87823 SP - 125 EP - 128 ER - TY - THES A1 - Todt, Helge Tobias T1 - Hydrogen-deficient central stars of planetary nebulae T1 - Wasserstoffarme Zentralsterne Planetarischer Nebel N2 - Central stars of planetary nebulae are low-mass stars on the brink of their final evolution towards white dwarfs. Because of their surface temperature of above 25,000 K their UV radiation ionizes the surrounding material, which was ejected in an earlier phase of their evolution. Such fluorescent circumstellar gas is called a "Planetary Nebula". About one-tenth of the Galactic central stars are hydrogen-deficient. Generally, the surface of these central stars is a mixture of helium, carbon, and oxygen resulting from partial helium burning. Moreover, most of them have a strong stellar wind, similar to massive Pop-I Wolf-Rayet stars, and are in analogy classified as [WC]. The brackets distinguish the special type from the massive WC stars. Qualitative spectral analyses of [WC] stars lead to the assumption of an evolutionary sequence from the cooler, so-called late-type [WCL] stars to the very hot, early-type [WCE] stars. Quantitative analyses of the winds of [WC] stars became possible by means of computer programs that solve the radiative transfer in the co-moving frame, together with the statistical equilibrium equations for the population numbers. First analyses employing models without iron-line blanketing resulted in systematically different abundances for [WCL] and [WCE] stars. While the mass ratio of He:C is roughly 40:50 for [WCL] stars, it is 60:30 in average for [WCE] stars. The postulated evolution from [WCL] to [WCE] however could only lead to an increase of carbon, since heavier elements are built up by nuclear fusion. In the present work, improved models are used to re-analyze the [WCE] stars and to confirm their He:C abundance ratio. Refined models, calculated with the Potsdam WR model atmosphere code (PoWR), account now for line-blanketing due to iron group elements, small scale wind inhomogeneities, and complex model atoms for He, C, O, H, P, N, and Ne. Referring to stellar evolutionary models for the hydrogen-deficient [WC] stars, Ne and N abundances are of particular interest. Only one out of three different evolutionary channels, the VLTP scenario, leads to a Ne and N overabundance of a few percent by mass. A VLTP, a very late thermal pulse, is a rapid increase of the energy production of the helium-burning shell, while hydrogen burning has already ceased. Subsequently, the hydrogen envelope is mixed with deeper layers and completely burnt in the presence of C, He, and O. This results in the formation of N and Ne. A sample of eleven [WCE] stars has been analyzed. For three of them, PB 6, NGC 5189, and [S71d]3, a N overabundance of 1.5% has been found, while for three other [WCE] stars such high abundances of N can be excluded. In the case of NGC 5189, strong spectral lines of Ne can be reproduced qualitatively by our models. At present, the Ne mass fraction can only be roughly estimated from the Ne emission lines and seems to be in the order of a few percent by mass. Furthermore, using a diagnostic He-C line pair, the He:C abundance ratio of 60:30 for [WCE] stars is confirmed. Within the framework of the analysis, a new class of hydrogen-deficient central stars has been discovered, with PB 8 as its first member. Its atmospheric mixture resembles rather that of the massive WNL stars than of the [WC] stars. The determined mass fractions H:He:C:N:O are 40:55:1.3:2:1.3. As the wind of PB 8 contains significant amounts of O and C, in contrast to WN stars, a classification as [WN/WC] is suggested. N2 - Zentralsterne Planetarischer Nebel sind massearme Sterne kurz vor ihrer finalen Entwicklung zu Weißen Zwergen. Aufgrund ihrer Oberflächentemperatur von über 25 000 K sind sie in der Lage, durch Abstrahlung von UV-Licht das sie umgebende Material, welches in einer vorigen Phase ihrer Entwicklung abgestoßen wurde, zu ionisieren. Das solchermaßen zum Leuchten angeregte Gas bezeichnet man als Planetarischen Nebel. Etwa ein Zehntel der galaktischen Zentralsterne sind wasserstoffarm. Im Allgemeinen besteht die Oberfläche dieser Zentralsterne aus einer Mischung der Elemente Helium, Kohlenstoff und Sauerstoff, welche z.T. durch Heliumbrennen erzeugt wurden. Die meisten dieser Sterne haben darüberhinaus einen starken Sternwind, ähnlich den massereichen Pop-I-Wolf-Rayet-Sternen und werden in Analogie zu diesen als [WC] klassifiziert, wobei die eckigen Klammern der Unterscheidung von den massereichen WC-Sternen dienen. Qualitative Analysen der Spektren von [WC]-Sternen lassen eine Entwicklungssequenz dieser Sterne von kühleren sogenannten late-type [WC]-Sternen (kurz [WCL]) zu sehr heißen, early-type [WC]-Sternen (kurz [WCE]) vermuten. Mithilfe von Computerprogrammen, die den Strahlungstransport im mitbewegten Beobachtersystem zusammen mit den statistischen Gleichungen der Besetzungszahlen der Ionen im Sternwind rechnen können, wurden quantitative Untersuchungen der Winde von [WC]-Sternen möglich. Erste Analysen mit Modellen ohne Eisenlinien ergaben dabei systematisch unterschiedliche Häufigkeiten für [WCL]- und [WCE]-Sterne. Während sich für [WCL]-Sterne ein Verhältnis der Massenanteile von He:C von etwas 40:50 ergab, fand man für die [WCE]-Sterne ein mittleres Verhältnis von 60:30 für die He:C-Massenanteile. Dabei sollte die Entwicklung von [WCL] nach [WCE] innerhalb einer sehr kurzen Zeit durch Aufheizung infolge der Kontraktion der Hülle erfolgen und nicht mit einer wesentlichen Abnahme der Kohlenstoffhäufigkeit bei gleichzeitiger Zunahme der Heliumhäufigkeit an der Oberfläche einhergehen. Im Rahmen der vorgelegten Arbeit wird untersucht, ob sich mittels verbesserter Modelle für die Atmosphären von [WC]-Sternen das He:C-Häufigkeitsverhältnis der [WCE]-Sterne bestätigt. Elaboriertere Modelle, welche vom Potsdamer WR-Modelatmosphären-Code (PoWR) berechnet werden können, berücksichtigen Line-Blanketing aufgrund von Elementen der Eisengruppe, kleinskalige Windinhomogenitäten und die Elemente He, C, O, H, P, N und Ne. Unter Bezug auf Sternentwicklungsmodelle, die die Ursache der Wasserstoffunterhäufigkeit von [WC]-Sternen erklären, sind insbesondere die Neon- und Stickstoff-Häufigkeiten interessant. Von den drei möglichen Entwicklungskanälen für [WC]-Sterne führt lediglich das VLTP-Szenario zu einer Stickstoff-Überhäufigkeit von einigen Prozent bezogen auf die Masse. Bei einem VLTP, einem very late thermal pulse, handelt es sich um einen plötzlichen, starken Anstieg der Energieproduktion in der helium-brennenden Schale, während das Wasserstoffbrennen bereits zum Erliegen gekommen ist. Infolge eines VLTPs wird sämtlicher Wasserstoff kurz nach dem thermischen Puls in tiefere Schichten gemischt und in Anwesenheit von C, He und O verbrannt. Infolgedessen wird N und auch Ne erzeugt. Bei der Analyse von elf [WCE]-Sternen wurden für drei von ihnen, PB 6, NGC 5189 und [S71d]3, Stickststoffmassenanteile von 1,5 % bestimmt, während für drei andere Sterne solche hohen Stickstoffhäufigkeiten ausgeschlossen werden können. Für NGC 5189 gelang außerdem die qualitative Reproduktion der beobachteten, starken Ne-Spektrallinien mittels unserer Modelle. Zur Zeit lässt sich aus der Stärke der Ne-Emissionslinien der Ne-Massenanteil leider nur abschätzen, er scheint aber im Bereich einiger Prozent zu liegen. Mittels eines diagnostischen He-C-Linienpaares konnte das He:C-Massenverhältnis von 60:30 für [WCE]-Sterne bestätigt werden. Als Ergebnis der Analyse von PB 8 postulieren wir eine neue Klasse von wasserstoffarmen Zentralsternen, die in ihrer Elementzusammensetzung eher an massereiche WNL-Sterne als an [WC]-Sterne erinnern. Die ermittelten Massenanteile H:He:C:N:O betragen 40:55:1.3:2:1.3, der Wind von PB 8 enthält daher im Unterschied zu WN-Sternen signifikante Mengen von O und C. Es wird daher eine Klassifizierung als [WN/WC] vorgeschlagen. KW - Zentralsterne KW - Planetarische Nebel KW - Sternatmosphären KW - Wolf-Rayet-Sterne KW - Post-AGB-Sterne KW - central stars KW - planetary nebulae KW - stellar atmospheres KW - Wolf-Rayet stars KW - post-AGB stars Y1 - 2009 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-41047 ER - TY - JOUR A1 - Sander, A. A1 - Hamann, Wolf-Rainer A1 - Hainich, Rainer A1 - Shenar, Tomer A1 - Todt, Helge Tobias T1 - Hydrodynamic modeling of massive star atmospheres JF - Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.–5. June 2015 N2 - In the last decades, stellar atmosphere codes have become a key tool in understanding massive stars, including precise calculations of stellar and wind parameters, such as temperature, massloss rate, and terminal wind velocity. Nevertheless, for these models the hydrodynamic equation is not solved in the wind. Motivated by the results of the CAK theory, the models typically use a beta velocity law, which however turns out not to be adequate for stars with very strong winds, and treat the mass-loss rate as a free parameter. In a new branch of the Potsdam Wolf-Rayet model atmosphere (PoWR) code, we solve the hydrodynamic equation consistently throughout the stellar atmosphere. The PoWR code performs the calculation of the radiative force without approximations (e.g. Sobolev). We show the impact of hydrodynamically consistent modelling on OB and WR stars in comparison to conventional models and discuss the obtained velocity fields and their impact on the observed spectral lines. Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-87857 SP - 139 EP - 142 ER - TY - JOUR A1 - Toalá, Jesús Alberto A1 - Ramos-Larios, Gerardo A1 - Guerrero, Martin A. A1 - Todt, Helge Tobias T1 - Hidden IR structures in NGC40 BT - Signpost of an ancient born-again event JF - Monthly notices of the Royal Astronomical Society N2 - We present the analysis of infrared (IR) observations of the planetary nebula NGC40 together with spectral analysis of its [WC]-type central starHD826. Spitzer IRS observations were used to produce spectral maps centred at polycyclic aromatic hydrocarbons (PAH) bands and ionic transitions to compare their spatial distribution. The ionic lines show a clumpy distribution of material around the main cavity of NGC40, with the emission from [Ar II] being the most extended, whilst the PAHs show a rather smooth spatial distribution. Analysis of ratio maps shows the presence of a toroidal structure mainly seen in PAH emission, but also detected in a Herschel PACS 70 mu m image. We argue that the toroidal structure absorbs the UV flux from HD826, preventing the nebula to exhibit lines of high-excitation levels as suggested by previous authors. We discuss the origin of this structure and the results from the spectral analysis of HD826 under the scenario of a late thermal pulse. KW - stars: carbon KW - stars: evolution KW - stars: winds, outflows KW - ISM: molecules KW - planetary nebulae: individual: NGC40 KW - infrared: ISM Y1 - 2019 U6 - https://doi.org/10.1093/mnras/stz624 SN - 0035-8711 SN - 1365-2966 VL - 485 IS - 3 SP - 3360 EP - 3369 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Fang, X. A1 - Guerrero, Martín A. A1 - Marquez-Lugo, R. A. A1 - Toala, Jesús Alberto A1 - Arthur, S. J. A1 - Chu, Y.-H. A1 - Blair, William P. A1 - Gruendl, R. A. A1 - Hamann, Wolf-Rainer A1 - Oskinova, Lida A1 - Todt, Helge Tobias T1 - Expansion of hydrogen-poor knots in the born-again planetary nebulae A30 and A78 JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We analyze the expansion of hydrogen-poor knots and filaments in the born-again planetary nebulae A30 and A78 based on Hubble Space Telescope ( HST) images obtained almost 20 yr apart. The proper motion of these features generally increases with distance to the central star, but the fractional expansion decreases, i.e., the expansion is not homologous. As a result, there is not a unique expansion age, which is estimated to be 610-950 yr for A30 and 600-1140 yr for A78. The knots and filaments have experienced complex dynamical processes: the current fast stellar wind is mass loaded by the material ablated from the inner knots; the ablated material is then swept up until it shocks the inner edges of the outer, hydrogen-rich nebula. The angular expansion of the outer filaments shows a clear dependence on position angle, indicating that the interaction of the stellar wind with the innermost knots channels the wind along preferred directions. The apparent angular expansion of the innermost knots seems to be dominated by the rocket effect of evaporating gas and by the propagation of the ionization front inside them. Radiation-hydrodynamical simulations show that a single ejection of material followed by a rapid onset of the stellar wind and ionizing flux can reproduce the variety of clumps and filaments at different distances from the central star found in A30 and A78. KW - ISM: kinematics and dynamics KW - planetary nebulae: individual (A30 and A78) Y1 - 2014 U6 - https://doi.org/10.1088/0004-637X/797/2/100 SN - 0004-637X SN - 1538-4357 VL - 797 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Oskinova, Lida A1 - Todt, Helge Tobias A1 - Ignace, Richard A1 - Brown, John C. A1 - Cassinelli, Joseph P. A1 - Hamann, Wolf-Rainer T1 - Early magnetic B-type stars X-ray emission and wind properties JF - Monthly notices of the Royal Astronomical Society N2 - We present a comprehensive study of X-ray emission by, and wind properties of, massive magnetic early B-type stars. Dedicated XMM-Newton observations were obtained for three early-type B-type stars, xi(1) CMa, V2052 Oph and zeta Cas, with recently discovered magnetic fields. We report the first detection of X-ray emission from V2052 Oph and zeta Cas. The latter is one the softest X-ray sources among the early-type stars, while the former is one of the X-ray faintest. The observations show that the X-ray spectra of our programme stars are quite soft with the bulk of X-ray emitting material having a temperature of about 1 MK. We compile the complete sample of early B-type stars with detected magnetic fields to date and existing X-ray measurements, in order to study whether the X-ray emission can be used as a general proxy for stellar magnetism. We find that the X-ray properties of early massive B-type magnetic stars are diverse, and that hard and strong X-ray emission does not necessarily correlate with the presence of a magnetic field, corroborating similar conclusions reached earlier for the classical chemically peculiar magnetic Bp-Ap stars. We analyse the ultraviolet (UV) spectra of five non-supergiant B stars with magnetic fields (tau Sco, beta Cep, xi(1) CMa, V2052 Oph and zeta Cas) by means of non-local thermodynamic equilibrium (non-LTE) iron-blanketed model atmospheres. The latter are calculated with the Potsdam Wolf-Rayet (PoWR) code, which treats the photosphere as well as the wind, and also accounts for X-rays. With the exception of t Sco, this is the first analysis of these stars by means of stellar wind models. Our models accurately fit the stellar photospheric spectra in the optical and the UV. The parameters of X-ray emission, temperature and flux are included in the model in accordance with observations. We confirm the earlier findings that the filling factors of X-ray emitting material are very high. Our analysis reveals that the magnetic early-type B stars studied here have weak winds with velocities not significantly exceeding upsilon(esc). The mass-loss rates inferred from the analysis of UV lines are significantly lower than predicted by hydrodynamically consistent models. We find that, although the X-rays strongly affect the ionization structure of the wind, this effect is not sufficient in reducing the total radiative acceleration. When the X-rays are accounted for at the intensity and temperatures observed, there is still sufficient radiative acceleration to drive a stronger mass-loss than we empirically infer from the UV spectral lines. KW - techniques: spectroscopic KW - stars: magnetic field KW - stars: massive KW - stars: mass loss KW - X-rays: stars Y1 - 2011 U6 - https://doi.org/10.1111/j.1365-2966.2011.19143.x SN - 0035-8711 VL - 416 IS - 2 SP - 1456 EP - 1474 PB - Wiley-Blackwell CY - Hoboken ER - TY - JOUR A1 - Oskinova, Lida A1 - Naze, Yael A1 - Todt, Helge Tobias A1 - Huenemoerder, David P. A1 - Ignace, Richard A1 - Hubrig, Swetlana A1 - Hamann, Wolf-Rainer T1 - Discovery of X-ray pulsations from a massive star JF - Nature Communications N2 - X-ray emission from stars much more massive than the Sun was discovered only 35 years ago. Such stars drive fast stellar winds where shocks can develop, and it is commonly assumed that the X-rays emerge from the shock-heated plasma. Many massive stars additionally pulsate. However, hitherto it was neither theoretically predicted nor observed that these pulsations would affect their X-ray emission. All X-ray pulsars known so far are associated with degenerate objects, either neutron stars or white dwarfs. Here we report the discovery of pulsating X-rays from a non-degenerate object, the massive B-type star xi(1) CMa. This star is a variable of beta Cep-type and has a strong magnetic field. Our observations with the X-ray Multi-Mirror (XMM-Newton) telescope reveal X-ray pulsations with the same period as the fundamental stellar oscillations. This discovery challenges our understanding of stellar winds from massive stars, their X-ray emission and their magnetism. Y1 - 2014 U6 - https://doi.org/10.1038/ncomms5024 SN - 2041-1723 VL - 5 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Gvaramadze, V. V. A1 - Kniazev, A. Y. A1 - Miroshnichenko, A. S. A1 - Berdnikov, Leonid N. A1 - Langer, N. A1 - Stringfellow, G. S. A1 - Todt, Helge Tobias A1 - Hamann, Wolf-Rainer A1 - Grebel, E. K. A1 - Buckley, D. A1 - Crause, L. A1 - Crawford, S. A1 - Gulbis, A. A1 - Hettlage, C. A1 - Hooper, E. A1 - Husser, T. -O. A1 - Kotze, P. A1 - Loaring, N. A1 - Nordsieck, K. H. A1 - O'Donoghue, D. A1 - Pickering, T. A1 - Potter, S. A1 - Colmenero, E. Romero A1 - Vaisanen, P. A1 - Williams, T. A1 - Wolf, M. A1 - Reichart, D. E. A1 - Ivarsen, K. M. A1 - Haislip, J. B. A1 - Nysewander, M. C. A1 - LaCluyze, A. P. T1 - Discovery of two new Galactic candidate luminous blue variables with Wide-field Infrared Survey Explorer JF - Monthly notices of the Royal Astronomical Society N2 - We report the discovery of two new Galactic candidate luminous blue variable (LBV) stars via detection of circular shells (typical of confirmed and candidate LBVs) and follow-up spectroscopy of their central stars. The shells were detected at 22 mu m in the archival data of the Mid-Infrared All Sky Survey carried out with the Wide-field Infrared Survey Explorer (WISE). Follow-up optical spectroscopy of the central stars of the shells conducted with the renewed Southern African Large Telescope (SALT) showed that their spectra are very similar to those of the well-known LBVs P Cygni and AG Car, and the recently discovered candidate LBV MN112, which implies the LBV classification for these stars as well. The LBV classification of both stars is supported by detection of their significant photometric variability: one of them brightened in the R and I bands by 0.68 +/- 0.10 and 0.61 +/- 0.04 mag, respectively, during the last 1318 years, while the second one (known as Hen 3-1383) varies its B, V, R, I and Ks brightnesses by similar or equal to 0.50.9 mag on time-scales from 10 d to decades. We also found significant changes in the spectrum of Hen 3-1383 on a time-scale of similar or equal to 3 months, which provides additional support for the LBV classification of this star. Further spectrophotometric monitoring of both stars is required to firmly prove their LBV status. We discuss a connection between the location of massive stars in the field and their fast rotation, and suggest that the LBV activity of the newly discovered candidate LBVs might be directly related to their possible runaway status. KW - line: identification KW - circumstellar matter KW - stars: emission-line, Be KW - stars: evolution KW - stars: individual: Hen 3-1383 KW - stars: massive Y1 - 2012 U6 - https://doi.org/10.1111/j.1365-2966.2012.20556.x SN - 0035-8711 VL - 421 IS - 4 SP - 3325 EP - 3337 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Sander, Andreas Alexander Christoph A1 - Hamann, Wolf-Rainer A1 - Todt, Helge Tobias A1 - Hainich, Rainer A1 - Shenar, Tomer T1 - Coupling hydrodynamics with comoving frame radiative transfer I. A unified approach for OB and WR stars JF - Astronomy and astrophysics : an international weekly journal N2 - Context. For more than two decades, stellar atmosphere codes have been used to derive the stellar and wind parameters of massive stars. Although they have become a powerful tool and sufficiently reproduce the observed spectral appearance, they can hardly be used for more than measuring parameters. One major obstacle is their inconsistency between the calculated radiation field and the wind stratification due to the usage of prescribed mass-loss rates and wind-velocity fields. Aims. We present the concepts for a new generation of hydrodynamically consistent non-local thermodynamical equilibrium (nonLTE) stellar atmosphere models that allow for detailed studies of radiation-driven stellar winds. As a first demonstration, this new kind of model is applied to a massive O star. Methods. Based on earlier works, the PoWR code has been extended with the option to consistently solve the hydrodynamic equation together with the statistical equations and the radiative transfer in order to obtain a hydrodynamically consistent atmosphere stratification. In these models, the whole velocity field is iteratively updated together with an adjustment of the mass-loss rate. Results. The concepts for obtaining hydrodynamically consistent models using a comoving-frame radiative transfer are outlined. To provide a useful benchmark, we present a demonstration model, which was motivated to describe the well-studied O4 supergiant zeta Pup. The obtained stellar and wind parameters are within the current range of literature values. Conclusions. For the first time, the PoWR code has been used to obtain a hydrodynamically consistent model for a massive O star. This has been achieved by a profound revision of earlier concepts used for Wolf-Rayet stars. The velocity field is shaped by various elements contributing to the radiative acceleration, especially in the outer wind. The results further indicate that for more dense winds deviations from a standard beta-law occur. KW - stars: mass-loss KW - stars: winds, outflows KW - stars: early-type KW - stars: atmospheres KW - stars: fundamental parameters KW - stars: massive Y1 - 2017 U6 - https://doi.org/10.1051/0004-6361/201730642 SN - 1432-0746 VL - 603 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Sander, Andreas Alexander Christoph A1 - Fürst, F. A1 - Kretschmar, P. A1 - Oskinova, Lida A1 - Todt, Helge Tobias A1 - Hainich, Rainer A1 - Shenar, Tomer A1 - Hamann, Wolf-Rainer T1 - Coupling hydrodynamics with comoving frame radiative transfer BT - Stellar wind stratification in the high-mass X-ray binary Vela X-1 JF - Astronomy and astrophysics : an international weekly journal N2 - Aims. To gain a realistic picture of the donor star in Vela X-1, we constructed a hydrodynamically consistent atmosphere model describing the wind stratification while properly reproducing the observed donor spectrum. To investigate how X-ray illumination affects the stellar wind, we calculated additional models for different X-ray luminosity regimes. Methods. We used the recently updated version of the Potsdam Wolf-Rayet code to consistently solve the hydrodynamic equation together with the statistical equations and the radiative transfer. Results. The wind flow in Vela X-1 is driven by ions from various elements, with Fe III and S III leading in the outer wind. The model-predicted mass-loss rate is in line with earlier empirical studies. The mass-loss rate is almost unaffected by the presence of the accreting NS in the wind. The terminal wind velocity is confirmed at u(infinity) approximate to 600 km s(-1). On the other hand, the wind velocity in the inner region where the NS is located is only approximate to 100 km s(-1), which is not expected on the basis of a standard beta-velocity law. In models with an enhanced level of X-rays, the velocity field in the outer wind can be altered. If the X-ray flux is too high, the acceleration breaks down because the ionization increases. Conclusions. Accounting for radiation hydrodynamics, our Vela X-1 donor atmosphere model reveals a low wind speed at the NS location, and it provides quantitative information on wind driving in this important HMXB. KW - stars: mass-loss KW - stars: winds, outflows KW - stars: early-type KW - stars: atmospheres KW - stars: massive KW - X-rays: binaries Y1 - 2018 U6 - https://doi.org/10.1051/0004-6361/201731575 SN - 1432-0746 VL - 610 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Plat, A. A1 - Charlot, S. A1 - Bruzual, G. A1 - Feltre, A. A1 - Vidal-Garcia, A. A1 - Morisset, C. A1 - Chevallard, Jacopo A1 - Todt, Helge Tobias T1 - Constraints on the production and escape of ionizing radiation from the emission-line spectra of metal-poor star-forming galaxies JF - Monthly notices of the Royal Astronomical Society N2 - We explore the production and escape of ionizing photons in young galaxies by investigating the ultraviolet and optical emission-line properties of models of ionization-bounded and density-bounded H II regions, active-galactic-nucleus (AGN) narrow-line regions, and radiative shocks computed all using the same physically consistent description of element abundances and depletion on to dust grains down to very low metallicities. We compare these models with a reference sample of metal-poor star-forming galaxies and Lyman-continuum (LyC) leakers at various redshifts, which allows the simultaneous exploration of more spectral diagnostics than typically available at once for individual subsamples. We confirm that current single- and binary-star population synthesis models do not produce hard-enough radiation to account for the high-ionization emission of the most metal-poor galaxies. Introducing either an AGN or radiative-shock component brings models into agreement with observations. A published model including X-ray binaries is an attractive alternative to reproduce the observed rise in He II lambda 4686/H beta ratio with decreasing oxygen abundance in metal-poor star-forming galaxies, but not the high observed He II lambda 4686/H beta ratios of galaxies with large EW(H beta). A source of harder ionizing radiation appears to be required in these extreme objects, such as an AGN or radiative-shock component, perhaps linked to an initial-mass-function bias towards massive stars at low metallicity. This would also account for the surprisingly high [O I]/[O III] ratios of confirmed LyC leakers relative to ionization-bounded models. We find no simple by-eye diagnostic of the nature of ionizing sources and the escape of LyC photon, which require proper simultaneous fits of several lines to be discriminated against. KW - galaxies: general KW - galaxies: high-redshift KW - galaxies: ISM Y1 - 2019 U6 - https://doi.org/10.1093/mnras/stz2616 SN - 0035-8711 SN - 1365-2966 VL - 490 IS - 1 SP - 978 EP - 1009 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Shenar, Tomer A1 - Oskinova, Lida A1 - Jaervinen, S. P. A1 - Luckas, P. A1 - Hainich, Rainer A1 - Todt, Helge Tobias A1 - Hubrig, Swetlana A1 - Sander, Andreas Alexander Christoph A1 - Ilyin, Ilya A1 - Hamann, Wolf-Rainer T1 - Constraining the weak-wind problem BT - an XMM-HST campaign for the magnetic 09.7 V star HD 54879 JF - Contributions Of The Astronomical Observatory Skalnate Pleso N2 - Mass-loss rates of massive, late type main sequence stars are much weaker than currently predicted, but their true values are very difficult to measure. We suggest that confined stellar winds of magnetic stars can be exploited to constrain the true mass-loss rates M of massive main sequence stars. We acquired UV, X-ray, and optical amateur data of HD 54879 (09.7 V), one of a few O-type stars with a detected atmospheric magnetic field (B-d greater than or similar to 2 kG). We analyze these data with the Potsdam Wolf-Rayet (PoWR) and XSPEC codes. We can roughly estimate the mass-loss rate the star would have in the absence of a magnetic field as log M-B=0 approximate to -9.0 M-circle dot yr(-1). Since the wind is partially trapped within the Alfven radius rA greater than or similar to 12 R-*,, the true mass-loss rate of HD 54879 is log M less than or similar to -10.2 M-circle dot yr(-1). Moreover, we find that the microturbulent, macroturbulent, and projected rotational velocities are lower than previously suggested (< 4 km s(-1)). An initial mass of 16 M-circle dot and an age of 5 Myr are inferred. We derive a mean X-ray emitting temperature of log T-x = 6.7 K and an X-ray luminosity of log L-x = 32 erg s(-1). The latter implies a significant X-ray excess (log L-x/L-Bol approximate to - 6.0), most likely stemming from collisions at the magnetic equator. A tentative period of P approximate to 5 yr is derived from variability of the Ha line. Our study confirms that strongly magnetized stars lose little or no mass, and supplies important constraints on the weak-wind problem of massive main sequence stars. KW - stars: massive KW - stars: magnetic field KW - stars: mass-loss Y1 - 2017 U6 - https://doi.org/10.1051/0004-6361/201731291 SN - 1335-1842 SN - 1336-0337 VL - 48 IS - 1 SP - 139 EP - 143 PB - Astronomický Ústav SAV CY - Tatranská Lomnica ER - TY - CHAP A1 - Todt, Helge Tobias A1 - Hamann, Wolf-Rainer A1 - Gräfener, G. T1 - Clumping in [WC]-type Central Stars from electron-scattering line wings N2 - While there is strong evidence for clumping in the winds of massive hot stars, very little is known about clumping in the winds from Central Stars. We have checked [WC]-type CSPN winds for clumping by inspecting the electron-scattering line wings. At least for three stars we found indications for wind inhomogeneities. Y1 - 2007 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-17711 ER - TY - JOUR A1 - Reindl, Nicole A1 - Rauch, Thomas A1 - Miller Bertolami, Marcelo Miguel A1 - Todt, Helge Tobias A1 - Werner, K. T1 - Breaking news from the HST BT - the central star of the Stingray Nebula is now returning towards the AGB JF - Monthly notices of the Royal Astronomical Society N2 - SAO 244567 is a rare example of a star that allows us to witness stellar evolution in real time. Between 1971 and 1990, it changed from a B-type star into the hot central star of the Stingray Nebula. This observed rapid heating has been a mystery for decades, since it is in strong contradiction with the low mass of the star and canonical post-asymptotic giant branch (AGB) evolution. We speculated that SAO 244567 might have suffered from a late thermal pulse (LTP) and obtained new observations with Hubble Space Telescope (HST)/COS to follow the evolution of the surface properties of SAO 244567 and to verify the LTP hypothesis. Our non-LTE spectral analysis reveals that the star cooled significantly since 2002 and that its envelope is now expanding. Therefore, we conclude that SAO 244567 is currently on its way back towards the AGB, which strongly supports the LTP hypothesis. A comparison with state-of-the-art LTP evolutionary calculations shows that these models cannot fully reproduce the evolution of all surface parameters simultaneously, pointing out possible shortcomings of stellar evolution models. Thereby, SAO 244567 keeps on challenging stellar evolution theory and we highly encourage further investigations. KW - stars: AGB and post-AGB KW - stars: atmospheres KW - stars: evolution Y1 - 2016 U6 - https://doi.org/10.1093/mnrasl/slw175 SN - 0035-8711 SN - 1365-2966 VL - 464 SP - L51 EP - L55 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Steinke, Martin A1 - Oskinova, Lida A1 - Hamann, Wolf-Rainer A1 - Sander, Andreas Alexander Christoph A1 - Liermann, A. A1 - Todt, Helge Tobias T1 - Analysis of the WN star WR102c, its WR nebula, and the associated cluster of massive stars in the Sickle Nebula JF - Tectonophysics : international journal of geotectonics and the geology and physics of the interior of the earth N2 - Context. The massive Wolf-Rayet type star WR102c is located near the Quintuplet Cluster, one of the three massive star clusters in the Galactic centre region. Previous studies indicated that WR102c may have a dusty circumstellar nebula and is among the main ionising sources of the Sickle Nebula associated with the Quintuplet Cluster. Aims. The goals of our study are to derive the stellar parameters of WR102c from the analysis of its spectrum and to investigate its stellar and nebular environment. Methods. We obtained observations with the ESO VLT integral field spectrograph SINFONI in the K-band, extracted the stellar spectra, and analysed them by means of stellar atmosphere models. Results. Our new analysis supersedes the results previously reported for WR102c. We significantly decrease its bolometric luminosity and hydrogen content. We detect four early OB type stars close to WR102c. These stars have radial velocities similar to that of WR102c. We suggest that together with WR102c these stars belong to a distinct star cluster with a total mass of similar to 1000 M-circle dot. We identify a new WR nebula around WR102c in the SINFONI map of the di ff use Br gamma emission and in the HST Pa ff images. The Br gamma line at di ff erent locations is not significantly broadened and similar to the width of nebular emission elsewhere in the H i i region around WR102c. Conclusions. The massive star WR102c located in the Galactic centre region resides in a star cluster containing additional early-type stars. The stellar parameters of WR102c are typical for hydrogen-free WN6 stars. We identify a nebula surrounding WR102c that has a morphology similar to other nebulae around hydrogen-free WR stars, and propose that the formation of this nebula is linked to interaction of the fast stellar wind with the matter ejected at a previous evolutionary stage of WR102c. KW - stars: early-type KW - stars: individual: WR 102c KW - stars: Wolf KW - Rayet KW - Galaxy: center KW - HII regions KW - infrared: stars Y1 - 2016 U6 - https://doi.org/10.1051/0004-6361/201527692 SN - 1432-0746 VL - 588 PB - EDP Sciences CY - Les Ulis ER - TY - GEN A1 - Guerrero, Martín A. A1 - Chu, You-Hua A1 - Hamann, Wolf-Rainer A1 - Oskinova, Lida A1 - Schönberner, Detlef A1 - Todt, Helge Tobias A1 - Steffen, Matthias A1 - Ruiz, Nieves A1 - Gruendl, Robert A. A1 - Blair, William P. T1 - Ablation and wind mass-loading in the born-again planetary nebula A 30 T2 - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - We present XMM-Newton and Chandra observations of the born-again planetary nebula A 30. These X-ray observations reveal a bright unresolved source at the position of the central star whose X-ray luminosity exceeds by far the model expectations for photospheric emission and for shocks within the stellar wind. We suggest that a “born-again hot bubble” may be responsible for this X-ray emission. Diffuse X-ray emission associated with the petal-like features and one of the H-poor knots seen in the optical is also found. The weakened emission of carbon lines in the spectrum of the diffuse emission can be interpreted as the dilution of stellar wind by mass-loading or as the detection of material ejected during a very late thermal pulse. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 563 KW - planetary nebulae: individual (A 30) KW - X-rays: ISM Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-412341 SN - 1866-8372 IS - 563 ER - TY - JOUR A1 - Todt, Helge Tobias A1 - Kniazev, A. Y. A1 - Gvaramadze, V. V. A1 - Hamann, Wolf-Rainer A1 - Buckley, D. A1 - Crause, L. A1 - Crawford, S. M. A1 - Gulbis, A. A. S. A1 - Hettlage, C. A1 - Hooper, E. A1 - Husser, T. -O. A1 - Kotze, P. A1 - Loaring, N. A1 - Nordsieck, K. H. A1 - O'Donoghue, D. A1 - Pickering, T. A1 - Potter, S. A1 - Romero-Colmenero, E. A1 - Vaisanen, P. A1 - Williams, T. A1 - Wolf, M. T1 - Abell 48-a rare WN-type central star of a planetary nebula JF - Monthly notices of the Royal Astronomical Society N2 - A considerable fraction of the central stars of planetary nebulae (CSPNe) are hydrogen-deficient. Almost all of these H-deficient central stars (CSs) display spectra with strong carbon and helium lines. Most of them exhibit emission-line spectra resembling those of massive WC stars. Therefore these stars are classed as CSPNe of spectral type [WC]. Recently, quantitative spectral analysis of two emission-line CSs, PB 8 and IC 4663, revealed that these stars do not belong to the [WC] class. Instead PB 8 has been classified as [WN/WC] type and IC 4663 as [WN] type. In this work we report the spectroscopic identification of another rare [WN] star, the CS of Abell 48. We performed a spectral analysis of Abell 48 with the Potsdam Wolf-Rayet (PoWR) models for expanding atmospheres. We find that the expanding atmosphere of Abell 48 is mainly composed of helium (85 per cent by mass), hydrogen (10 per cent) and nitrogen (5 per cent). The residual hydrogen and the enhanced nitrogen abundance make this object different from the other [WN] star IC 4663. We discuss the possible origin of this atmospheric composition. KW - stars: abundances KW - stars: AGB and post-AGB KW - stars: mass-loss KW - stars: Wolf-Rayet KW - planetary nebulae: general KW - planetary nebulae: individual: PN G029.0+00.4 Y1 - 2013 U6 - https://doi.org/10.1093/mnras/stt056 SN - 0035-8711 SN - 1365-2966 VL - 430 IS - 3 SP - 2302 EP - 2312 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Evans, C. J. A1 - Hainich, Rainer A1 - Oskinova, Lida A1 - Gallagher, J. S. A1 - Chu, Y.-H. A1 - Gruendl, R. A. A1 - Hamann, Wolf-Rainer A1 - Henault-Brunet, V. A1 - Todt, Helge Tobias T1 - A rare early-type star revealed in the wing of the small megellanic cloud JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - Sk 183 is the visually brightest star in the N90 nebula, a young star-forming region in the Wing of the Small Magellanic Cloud (SMC). We present new optical spectroscopy from the Very Large Telescope which reveals Sk 183 to be one of the most massive O-type stars in the SMC. Classified as an O3-type dwarf on the basis of its nitrogen spectrum, the star also displays broadened He I absorption, which suggests a later type. We propose that Sk 183 has a composite spectrum and that it is similar to another star in the SMC, MPG 324. This brings the number of rare O2- and O3-type stars known in the whole of the SMC to a mere four. We estimate physical parameters for Sk 183 from analysis of its spectrum. For a single-star model, we estimate an effective temperature of 46 +/- 2 kK, a low mass-loss rate of similar to 10(-7) M-circle dot yr(-1), and a spectroscopic mass of 46(-8)(+ 9) M-circle dot (for an adopted distance modulus of 18.7 mag to the young population in the SMC Wing). An illustrative binary model requires a slightly hotter temperature (similar to 47.5 kK) for the primary component. In either scenario, Sk 183 is the earliest-type star known in N90 and will therefore be the dominant source of hydrogen-ionizing photons. This suggests Sk 183 is the primary influence on the star formation along the inner edge of the nebula. KW - open clusters and associations: individual (NGC 602) KW - stars: early-type KW - stars: fundamental parameters KW - stars: individual (Sanduleak 183) Y1 - 2012 U6 - https://doi.org/10.1088/0004-637X/753/2/173 SN - 0004-637X VL - 753 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Shenar, Tomer A1 - Oskinova, Lida A1 - Hamann, Wolf-Rainer A1 - Corcoran, Michael F. A1 - Moffat, Anthony F. J. A1 - Pablo, Herbert A1 - Richardson, Noel D. A1 - Waldron, Wayne L. A1 - Huenemoerder, David P. A1 - Maiz Apellaniz, Jesus A1 - Nichols, Joy S. A1 - Todt, Helge Tobias A1 - Naze, Yael A1 - Hoffman, Jennifer L. A1 - Pollock, Andy M. T. A1 - Negueruela, Ignacio T1 - A coordinated X-Ray and optical campaign of the nearest massive eclipsing binary, delta ORIONIS Aa. IV. A multiwavelength, non-lte spectroscopic analysis JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - Eclipsing systems of massive stars allow one to explore the properties of their components in great detail. We perform a multi-wavelength, non-LTE analysis of the three components of the massive multiple system delta Ori A, focusing on the fundamental stellar properties, stellar winds, and X-ray characteristics of the system. The primary's distance-independent parameters turn out to be characteristic for its spectral type (O9.5 II), but usage of the Hipparcos parallax yields surprisingly low values for the mass, radius, and luminosity. Consistent values follow only if delta Ori lies at about twice the Hipparcos distance, in the vicinity of the sigma-Orionis cluster. The primary and tertiary dominate the spectrum and leave the secondary only marginally detectable. We estimate the V-band magnitude difference between primary and secondary to be Delta V approximate to 2.(m)8. The inferred parameters suggest that the secondary is an early B-type dwarf (approximate to B1 V), while the tertiary is an early B-type subgiant (approximate to B0 IV). We find evidence for rapid turbulent velocities (similar to 200 km s(-1)) and wind inhomogeneities, partially optically thick, in the primary's wind. The bulk of the X-ray emission likely emerges from the primary's stellar wind (logL(X)/L-Bol approximate to -6.85), initiating close to the stellar surface at R-0 similar to 1.1 R-*. Accounting for clumping, the mass-loss rate of the primary is found to be log (M) over dot approximate to -6.4 (M-circle dot yr(-1))., which agrees with hydrodynamic predictions, and provides a consistent picture along the X-ray, UV, optical, and radio spectral domains. KW - binaries: close KW - binaries: eclipsing KW - stars: early-type KW - stars: individual ([HD 36486]delta Ori A) KW - X-rays: stars Y1 - 2015 U6 - https://doi.org/10.1088/0004-637X/809/2/135 SN - 0004-637X SN - 1538-4357 VL - 809 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Leto, Paolo A1 - Trigilio, C. A1 - Oskinova, Lida A1 - Ignace, R. A1 - Buemi, C. S. A1 - Umana, G. A1 - Ingallinera, A. A1 - Leone, Francesco A1 - Phillips, N. M. A1 - Agliozzo, Claudia A1 - Todt, Helge Tobias A1 - Cerrigone, L. T1 - A combined multiwavelength VLA/ALMA/Chandra study unveils the complex magnetosphere of the B-type star HR5907 JF - Monthly notices of the Royal Astronomical Society N2 - We present new radio/millimeter measurements of the hot magnetic star HR5907 obtained with the VLA and ALMA interferometers. We find that HR5907 is the most radio luminous early type star in the cm-mm band among those presently known. Its multi-wavelength radio light curves are strongly variable with an amplitude that increases with radio frequency. The radio emission can be explained by the populations of the non-thermal electrons accelerated in the current sheets on the outer border of the magnetosphere of this fast-rotating magnetic star. We classify HR5907 as another member of the growing class of strongly magnetic fast-rotating hot stars where the gyro-synchrotron emission mechanism efficiently operates in their magnetospheres. The new radio observations of HR5907 are combined with archival X-ray data to study the physical condition of its magnetosphere. The X-ray spectra of HR5907 show tentative evidence for the presence of non-thermal spectral component. We suggest that non-thermal X-rays originate a stellar X-ray aurora due to streams of non-thermal electrons impacting on the stellar surface. Taking advantage of the relation between the spectral indices of the X-ray power-law spectrum and the non-thermal electron energy distributions, we perform 3-D modelling of the radio emission for HR5907. The wavelength-dependent radio light curves probe magnetospheric layers at different heights above the stellar surface. A detailed comparison between simulated and observed radio light curves leads us to conclude that the stellar magnetic field of HR 5907 is likely non-dipolar, providing further indirect evidence of the complex magnetic field topology of HR5907. KW - stars: chemically peculiar KW - stars: early-type KW - stars: individual: HR 5907 KW - stars: magnetic field KW - radio continuum: stars KW - X-rays: stars Y1 - 2018 U6 - https://doi.org/10.1093/mnras/sty244 SN - 0035-8711 SN - 1365-2966 VL - 476 IS - 1 SP - 562 EP - 579 PB - Oxford Univ. Press CY - Oxford ER -