@inproceedings{OskinovaHamannFeldmeier2007, author = {Oskinova, Lida and Hamann, Wolf-Rainer and Feldmeier, Achim}, title = {X-raying clumped stellar winds}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-18133}, year = {2007}, abstract = {X-ray spectroscopy is a sensitive probe of stellar winds. X-rays originate from optically thin shock-heated plasma deep inside the wind and propagate outwards throughout absorbing cool material. Recent analyses of the line ratios from He-like ions in the X-ray spectra of O-stars highlighted problems with this general paradigm: the measured line ratios of highest ions are consistent with the location of the hottest X-ray emitting plasma very close to the base of the wind, perhaps indicating the presence of a corona, while measurements from lower ions conform with the wind-embedded shock model. Generally, to correctly model the emerging Xray spectra, a detailed knowledge of the cool wind opacities based on stellar atmosphere models is prerequisite. A nearly grey stellar wind opacity for the X-rays is deduced from the analyses of high-resolution X-ray spectra. This indicates that the stellar winds are strongly clumped. Furthermore, the nearly symmetric shape of X-ray emission line profiles can be explained if the wind clumps are radially compressed. In massive binaries the orbital variations of X-ray emission allow to probe the opacity of the stellar wind; results support the picture of strong wind clumping. In high-mass X-ray binaries, the stochastic X-ray variability and the extend of the stellar-wind part photoionized by X-rays provide further strong evidence that stellar winds consist of dense clumps.}, language = {en} } @article{OskinovaFeldmeierHamann2003, author = {Oskinova, Lida and Feldmeier, Achim and Hamann, Wolf-Rainer}, title = {X-ray line profiles from structured stellar winds}, isbn = {1-58381-133-8}, year = {2003}, abstract = {Absorbing material compressed in a number of thin shells is effectively less opaque for X-rays than smoothly distributed gas. The calculated X-ray emission line profiles are red-shifted if the emission arises from the starward side of the shells.}, language = {en} } @article{FeldmeierOskinovaHamann2003, author = {Feldmeier, Achim and Oskinova, Lida and Hamann, Wolf-Rainer}, title = {X-ray line emission from a fragmented stellar wind}, year = {2003}, abstract = {We discuss X-ray line formation in dense O star winds. A random distribution of wind shocks is assumed to emit X-rays that are partially absorbed by cooler wind gas. The cool gas resides in highly compressed fragments oriented perpendicular to the radial flow direction. For fully opaque fragments, we find that the blueshifted part of X-ray line profiles remains flat-topped even after severe wind attenuation, whereas the red part shows a steep decline. These box- type, blueshifted profiles resemble recent Chandra observations of the O3 star zeta Pup. For partially transparent fragments, the emission lines become similar to those from a homogeneous wind.}, language = {en} } @article{OskinovaFeldmeierHamann2004, author = {Oskinova, Lida and Feldmeier, Achim and Hamann, Wolf-Rainer}, title = {X-ray emission lines from inhomogeneous stellar winds}, issn = {0004-6361}, year = {2004}, abstract = {It is commonly adopted that X-rays from O stars are produced deep inside the stellar wind, and transported outwards through the bulk of the expanding matter which attenuates the radiation and affects the shape of emission line profiles. The ability of the X-ray observatories Chandra and XMM-Newton to resolve these lines spectroscopically provided a stringent test for the theory of the X-ray production. It turned out that none of the existing models was able to fit the observations consistently. The possible caveat of these models was the underlying assumption of a smooth stellar wind. Motivated by the evidence that the stellar winds are in fact structured, we present a 2-D numerical model of a stochastic, inhomogeneous wind. Small parcels of hot, X-ray emitting gas are permeated by cool, absorbing wind material which is compressed into thin shell fragments. Wind fragmentation alters the radiative transfer drastically, compared to homogeneous models of the same mass-loss rate. X-rays produced deep inside the wind, which would be totally absorbed in a homogeneous flow, can effectively escape from a fragmented wind. The wind absorption becomes wavelength independent if the individual fragments are optically thick. The X-ray line profiles are flat-topped in the blue part and decline steeply in the red part for the winds with a short acceleration zone. For the winds where the acceleration extends over significant distances, the lines can appear nearly symmetric and only slightly blueshifted, in contrast to the skewed, triangular line profiles typically obtained from homogeneous wind models of high optical depth. We show that profiles from a fragmented wind model can reproduce the observed line profiles from zeta Orionis. The present numerical modeling confirms the results from a previous study, where we derived analytical formulae from a statistical treatment}, language = {en} } @article{OskinovaHamannCassinellietal.2011, author = {Oskinova, Lida and Hamann, Wolf-Rainer and Cassinelli, Joseph P. and Brown, John C. and Todt, Helge Tobias}, title = {X-ray emission from massive stars with magnetic fields}, series = {Astronomische Nachrichten = Astronomical notes}, volume = {332}, journal = {Astronomische Nachrichten = Astronomical notes}, number = {9-10}, publisher = {Wiley-Blackwell}, address = {Malden}, issn = {0004-6337}, doi = {10.1002/asna.201111602}, pages = {988 -- 993}, year = {2011}, abstract = {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.}, language = {en} } @article{HamannGraefenerKoesterke2003, author = {Hamann, Wolf-Rainer and Gr{\"a}fener, G{\"o}tz and Koesterke, Lars}, title = {WR Central Stars}, isbn = {1-583-81148-6}, year = {2003}, abstract = {Wolf-Rayet type central stars have been analyzed with adequate model atmospheres. The obtained stellar parameters and chemical abundances allow for a discussion of their evolutionary origin.}, language = {en} } @article{BurgemeisterGvaramadzeStringfellowetal.2013, author = {Burgemeister, S. and Gvaramadze, Visily V. and Stringfellow, G. S. and Kniazev, Alexei Y. and Todt, Helge Tobias and Hamann, Wolf-Rainer}, title = {WR 120bb and WR 120bc: a pair of WN9h stars with possibly interacting circumstellar shells}, series = {Monthly notices of the Royal Astronomical Society}, volume = {429}, journal = {Monthly notices of the Royal Astronomical Society}, number = {4}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/sts588}, pages = {3305 -- 3315}, year = {2013}, abstract = {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.}, language = {en} } @article{HamannHeberJeffrey1996, author = {Hamann, Wolf-Rainer and Heber, Ulrich and Jeffrey, C. S.}, title = {Wolf-Rayet stars of high and low mass}, year = {1996}, language = {en} } @article{ShenarHainichTodtetal.2016, author = {Shenar, Tomer and Hainich, Rainer and Todt, Helge Tobias and Sander, Andreas Alexander Christoph and Hamann, Wolf-Rainer and Moffat, Anthony F. J. and Eldridge, J. J. and Pablo, H. and Oskinova, Lida and Richardson, N. D.}, title = {Wolf-Rayet stars in the Small Magellanic Cloud II. Analysis of the binaries}, series = {American mineralogist : an international journal of earth and planetary materials}, volume = {591}, journal = {American mineralogist : an international journal of earth and planetary materials}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201527916}, pages = {25}, year = {2016}, abstract = {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.}, language = {en} } @article{HainichPasemannTodtetal.2015, author = {Hainich, Rainer and Pasemann, Diana and Todt, Helge Tobias and Shenar, Tomer and Sander, Andreas Alexander Christoph and Hamann, Wolf-Rainer}, title = {Wolf-Rayet stars in the Small Magellanic Cloud I. Analysis of the single WN stars}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {581}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201526241}, pages = {30}, year = {2015}, abstract = {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.}, language = {en} } @article{HamannGraefenerKoesterke2003, author = {Hamann, Wolf-Rainer and Gr{\"a}fener, G{\"o}tz and Koesterke, Lars}, title = {Wolf-Rayet star parameters from spectral analyses}, isbn = {1-58381-133-8}, year = {2003}, abstract = {The Potsdam Non-LTE code for expanding atmospheres, which accounts for clumping and iron-line blanketing, has been used to establish a grid of model atmospheres for WC stars. A parameter degeneracy is discovered for early-type WC models which do not depend on the "stellar temperature". 15 galactic WC4-7 stars are analyzed, showing a very uniform carbon abundance (He:C=55:40) with only few exceptions.}, language = {en} } @article{TodtHamann2015, author = {Todt, Helge Tobias and Hamann, Wolf-Rainer}, title = {Wolf-Rayet central stars of planetary nebulae}, series = {Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.-5. June 2015}, journal = {Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.-5. June 2015}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-88147}, pages = {253 -- 258}, year = {2015}, abstract = {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.}, language = {en} } @article{HamannKoesterke2000, author = {Hamann, Wolf-Rainer and Koesterke, Lars}, title = {WN stars in the LMC : parameters and atmospheric abundances}, year = {2000}, abstract = {The spectra of 18 WN stars in the Large Magellanic Cloud (LMC) are quantitatively analyzed by means of "standard" Wolf-Rayet model atmospheres, using the helium and nitrogen lines as well as the spectral energy distribution. The hydrogen abundance is also determined. Carbon is included for a subset of 4 stars. The studied sample covers all spectral subtypes (WN2 ... WN9) and also includes one WN/WC transition object. The luminosities of the program stars span a wide range ( L/Lsun = 5.0 ... 6.5). Due to the given LMC membership, these results are free from uncertainties inferred from the distance. 50 \% of the studied stars (both, late and early WN subtypes) have rather low luminosity (L/Lsun < 5.5). This puts tough constraints on their evolutionary formation. If coming from single stars, it provides evidence for strong internal mixing processes. The empirical mass-loss rates are scaled down by a factor of about two due to the impact of clumping, compared to previous studies adopting homogeneous winds. There is no obvious strong correlation between the mass-loss rates and other parameters like luminosity, temperature and composition. The stellar parameters for the present LMC sample are not systematically different from those of the Galactic WN stars studied previously with the same techniques, in contrast to the expected metallicity effects.}, language = {en} } @article{HamannKoesterke2000, author = {Hamann, Wolf-Rainer and Koesterke, Lars}, title = {WM stars in the LMC : parameters and atmospheric abundances}, year = {2000}, language = {en} } @article{Hamann2015, author = {Hamann, Wolf-Rainer}, title = {Wind models and spectral analyses}, series = {Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.-5. June 2015}, journal = {Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.-5. June 2015}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-87748}, pages = {91 -- 96}, year = {2015}, abstract = {The emission-line dominated spectra of Wolf-Rayet stars are formed in expanding layers of their atmosphere, i.e. in their strong stellar wind. Adequate modeling of such spectra has to face a couple of difficulties. Because of the supersonic motion, the radiative transfer is preferably formulated in the co-moving frame. The strong deviations from local thermodynamical equilibrium (LTE) require to solve the equations of statistical equilibrium for the population numbers, accounting for many hundred atomic energy levels and thousands of line transitions. Moreover, millions of lines from iron-group elements must be taken into account for their blanketing effect. Model atmospheres of the described kind can reproduce the observed WR spectra satisfyingly, and have been widely applied for corresponding spectral analyses.}, language = {en} } @article{DeMarcoSchmutzKoesterkeetal.1999, author = {DeMarco, O. and Schmutz, W. and Koesterke, Lars and Hamann, Wolf-Rainer and DeMarco, O. and DeKoter, A.}, title = {Why should we compare WR codes?}, year = {1999}, language = {en} } @article{SteffenHubrigTodtetal.2014, author = {Steffen, M. and Hubrig, Swetlana and Todt, Helge Tobias and Schoeller, M. and Hamann, Wolf-Rainer and Sandin, Christer and Sch{\"o}nberner, Detlef}, title = {Weak magnetic fields in central stars of planetary nebulae?}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {570}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201423842}, pages = {15}, year = {2014}, abstract = {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.}, language = {en} } @misc{TodtPenaZuehlkeetal.2011, author = {Todt, Helge Tobias and Pe{\~n}a, Miriam and Z{\"u}hlke, Julia and Oskinova, Lida and Hamann, Wolf-Rainer and Gr{\"a}fener, G{\"o}tz}, title = {Weak emission line central stars of planetary nebulae}, series = {Planetary Nebulae: an Eye to the Future}, journal = {Planetary Nebulae: an Eye to the Future}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-413775}, pages = {2}, year = {2011}, abstract = {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.}, language = {en} } @misc{BarniskeOskinovaHamann2016, author = {Barniske, Andreas and Oskinova, Lida and Hamann, Wolf-Rainer}, title = {Two extremely luminous WN stars in the Galactic center with circumstellar emission from dust and gas (vol 486, pg 971, 2008)}, series = {Physical chemistry, chemical physics : a journal of European Chemical Societies}, volume = {587}, journal = {Physical chemistry, chemical physics : a journal of European Chemical Societies}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/200809568e}, pages = {1}, year = {2016}, language = {en} } @article{SurlanHamannKubatetal.2012, author = {Surlan, B. and Hamann, Wolf-Rainer and Kubat, Jirij and Oskinova, Lida and Feldmeier, Achim}, title = {Three-dimensional radiative transfer in clumped hot star winds I influence of clumping on the resonance line formation}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {541}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201118590}, pages = {11}, year = {2012}, abstract = {Context. The true mass-loss rates from massive stars are important for many branches of astrophysics. For the correct modeling of the resonance lines, which are among the key diagnostics of stellar mass-loss, the stellar wind clumping has been found to be very important. To incorporate clumping into a radiative transfer calculation, three-dimensional (3D) models are required. Various properties of the clumps may have a strong impact on the resonance line formation and, therefore, on the determination of empirical mass-loss rates. Aims. We incorporate the 3D nature of the stellar wind clumping into radiative transfer calculations and investigate how different model parameters influence the resonance line formation. Methods. We develop a full 3D Monte Carlo radiative transfer code for inhomogeneous expanding stellar winds. The number density of clumps follows the mass conservation. For the first time, we use realistic 3D models that describe the dense as well as the tenuous wind components to model the formation of resonance lines in a clumped stellar wind. At the same time, we account for non-monotonic velocity fields. Results. The 3D density and velocity wind inhomogeneities show that there is a very strong impact on the resonance line formation. The different parameters describing the clumping and the velocity field results in different line strengths and profiles. We present a set of representative models for various sets of model parameters and investigate how the resonance lines are affected. Our 3D models show that the line opacity is lower for a larger clump separation and shallower velocity gradients within the clumps. Conclusions. Our model demonstrates that to obtain empirically correct mass-loss rates from the UV resonance lines, the wind clumping and its 3D nature must be taken into account.}, language = {en} } @article{IgnaceGayleyHamannetal.2013, author = {Ignace, Rico and Gayley, Kenneth G. and Hamann, Wolf-Rainer and Huenemoerder, David P. and Oskinova, Lida and Pollock, Andy M. T. and McFall, Michael}, title = {THE XMM-NEWTON/EPIC X-RAY LIGHT CURVE ANALYSIS OF WR 6}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {775}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {1}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.1088/0004-637X/775/1/29}, pages = {12}, year = {2013}, abstract = {We obtained four pointings of over 100 ks each of the well-studied Wolf-Rayet star WR 6 with the XMM-Newton satellite. With a first paper emphasizing the results of spectral analysis, this follow-up highlights the X-ray variability clearly detected in all four pointings. However, phased light curves fail to confirm obvious cyclic behavior on the well-established 3.766 day period widely found at longer wavelengths. The data are of such quality that we were able to conduct a search for event clustering in the arrival times of X-ray photons. However, we fail to detect any such clustering. One possibility is that X-rays are generated in a stationary shock structure. In this context we favor a corotating interaction region (CIR) and present a phenomenological model for X-rays from a CIR structure. We show that a CIR has the potential to account simultaneously for the X-ray variability and constraints provided by the spectral analysis. Ultimately, the viability of the CIR model will require both intermittent long-term X-ray monitoring of WR 6 and better physical models of CIR X-ray production at large radii in stellar winds.}, language = {en} } @article{GraefenerKoesterkeHamann2003, author = {Gr{\"a}fener, G{\"o}tz and Koesterke, Lars and Hamann, Wolf-Rainer}, title = {The WR population in CTS 1026}, isbn = {1-58381-133-8}, year = {2003}, abstract = {The blue compact H II galaxy CTS 1026 shows very strong WR emission features around 4686 AA and 5800 AA. We present high S/N optical spectra of the nucleus of this object. Byanalysis of the WR profile shapes, we determine the dominant spectral types and the WN/WC ratio in the starforming region. The ratio WR/O is determined via standard nebular diagnostics.}, language = {en} } @article{SteinkeOskinovaHamannetal.2015, author = {Steinke, M. and Oskinova, Lida and Hamann, Wolf-Rainer and Sander, A.}, title = {The Wolf-Rayet stars WR102c and 102ka and their isolation}, series = {Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.-5. June 2015}, journal = {Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.-5. June 2015}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-88503}, pages = {365}, year = {2015}, abstract = {While the majority of very massive stars is clearly found in clusters, there are also very massive objects not associated with any cluster, suggesting they may have been born in isolation. In order to gain more insights, we studied the regions around two WR stars in the Galactic Center region. To understand the nature of the potential cluster around massive stars, photometry alone is not sufficient. We therefore used the ESO VLT/SINFONI integral field spectrograph to obtain photometry and spectra for the whole region around our two candidate stars. In total, more than 60 stars have been found and assigned a spectral type.}, language = {en} } @article{HainichRuehlingTodtetal.2014, author = {Hainich, Rainer and Ruehling, Ute and Todt, Helge Tobias and Oskinova, Lida and Liermann, A. and Graefener, G. and Foellmi, C. and Schnurr, O. and Hamann, Wolf-Rainer}, title = {The Wolf-Rayet stars in the Large Magellanic Cloud - A comprehensive analysis of the WN class}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {565}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201322696}, pages = {62}, year = {2014}, abstract = {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.}, language = {en} } @article{SanderTodtHainichetal.2014, author = {Sander, Andreas Alexander Christoph and Todt, Helge Tobias and Hainich, Rainer and Hamann, Wolf-Rainer}, title = {The Wolf-Rayet stars in M31 I. Analysis of the late-type WN stars}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {563}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201323240}, pages = {22}, year = {2014}, abstract = {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.}, language = {en} } @article{ShenarSablowskiHainichetal.2019, author = {Shenar, Tomer and Sablowski, D. P. and Hainich, Rainer and Todt, Helge Tobias and Moffat, Anthony F. J. and Oskinova, Lida and Ramachandran, Varsha and Sana, Hugues and Sander, Andreas Alexander Christoph and Schnurr, O. and St-Louis, N. and Vanbeveren, D. and Gotberg, Y. and Hamann, Wolf-Rainer}, title = {The Wolf-Rayet binaries of the nitrogen sequence in the Large Magellanic Cloud Spectroscopy, orbital analysis, formation, and evolution}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {627}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201935684}, pages = {68}, year = {2019}, abstract = {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.}, language = {en} } @article{HainichRuehlingPasemannetal.2015, author = {Hainich, Rainer and R{\"u}hling, U. and Pasemann, D. and Hamann, Wolf-Rainer}, title = {The WN population in the Magellanic Clouds}, series = {Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.-5. June 2015}, journal = {Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.-5. June 2015}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-87806}, pages = {117 -- 120}, year = {2015}, abstract = {A detailed and comprehensive study of the Wolf-Rayet stars of the nitrogen sequence (WN stars) in the Small Magellanic Cloud (SMC) and the Large Magellanic Cloud (LMC) is presented. We derived the fundamental stellar and wind parameters for more than 100 massive stars, encompassing almost the whole WN population in the Magellanic Clouds (MCs). The observations are fitted with synthetic spectra, using the PotsdamWolf-Rayet model atmosphere code (PoWR). For this purpose, large grids of line-blanket models for different metallicities have been calculated, covering a wide range of stellar temperatures, mass-loss rates, and hydrogen abundances. Our comprehensive sample facilitates statistical studies of the WN properties in the MCs without selection bias. To investigate the impact of the low LMC metallicity and the even lower SMC metallicity, we compare our new results to previous analyses of the Galactic WN population and the late type WN stars from M31. Based on these studies we derived an empirical relation between the WN mass-loss rates and the metallicity. Current stellar evolution tracks, even when accounting for rotationally induced mixing, partly fail to reproduce the observed ranges of luminosities and initial masses.}, language = {en} } @article{EvansSmarttLeeetal.2005, author = {Evans, C. J. and Smartt, S. J. and Lee, J. K. and Lennon, D. J. and Kaufer, A. and Dufton, P. L. and Trundle, C. and Herrero, A. and Simon D{\´i}az, Sergio and de Koter, A. and Hamann, Wolf-Rainer and Hendry, M. A. and Hunter, I. and Irwin, M. J. and Korn, A. J. and Kudritzki, R. P. and Langer, Norbert and Mokiem, M. R. and Najarro, F. and Pauldrach, A. W. A. and Przybilla, Norbert and Puls, J. and Ryans, R. S. I. and Urbaneja, M. A. and Venn, K. A. and Villamariz, M. R.}, title = {The VLT-FLAMES survey of massive stars : Observations in the Galactic clusters NGC3293, NGC4755 and NGC6611}, year = {2005}, abstract = {We introduce a new survey of massive stars in the Galaxy and the Magellanic Clouds using the Fibre Large Array Multi- Element Spectrograph ( FLAMES) instrument at the Very Large Telescope ( VLT). Here we present observations of 269 Galactic stars with the FLAMES- Giraffe Spectrograph ( R similar or equal to 25 000), in fields centered on the open clusters NGC3293, NGC4755 and NGC6611. These data are supplemented by a further 50 targets observed with the Fibre- Fed Extended Range Optical Spectrograph ( FEROS, R = 48 000). Following a description of our scientific motivations and target selection criteria, the data reduction methods are described; of critical importance the FLAMES reduction pipeline is found to yield spectra that are in excellent agreement with less automated methods. Spectral classifications and radial velocity measurements are presented for each star, with particular attention paid to morphological peculiarities and evidence of binarity. These observations represent a significant increase in the known spectral content of NGC3293 and NGC4755, and will serve as standards against which our subsequent FLAMES observations in the Magellanic Clouds will be compared}, language = {en} } @article{ShenarRichardsonSablowskietal.2017, author = {Shenar, Tomer and Richardson, N. D. and Sablowski, Daniel P. and Hainich, Rainer and Sana, H. and Moffat, A. F. J. and Todt, Helge Tobias and Hamann, Wolf-Rainer and Oskinova, Lida and Sander, Andreas Alexander Christoph and Tramper, Frank and Langer, Norbert and Bonanos, Alceste Z. and de Mink, Selma E. and Gr{\"a}fener, G. and Crowther, Paul and Vink, J. S. and Almeida, Leonardo A. and de Koter, A. and Barb{\´a}, Rodolfo and Herrero, A. and Ulaczyk, Krzysztof}, title = {The tarantula massive binary monitoring}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {598}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201629621}, pages = {16}, year = {2017}, abstract = {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.}, language = {en} } @article{HamannGraefener2003, author = {Hamann, Wolf-Rainer and Gr{\"a}fener, G{\"o}tz}, title = {The surface composition of hydrogen-deficient Post-AGB stars}, year = {2003}, abstract = {Most Central Stars of Planetary Nebulae exhibit a spectrum of a hydrogen-rich hot star with little or no stellar wind. About 20 \% of the CSPN, however, show entirely different spectra dominated by bright and broad emission lines of carbon, oxygen and helium, resembling the so-called Wolf-Rayet (WR) spectral class originally established for massive, Pop. I stars. These spectra indicate a hydrogen-deficient surface composition and, at the same time, strong mass-loss. As the WR spectra are formed entirely in a dense stellar wind, their spectral analysis requires adequate modelling. Corresponding Non-LTE model atmospheres have been developed in the last decade and became more and more sophisticated. They have been applied yet for analyzing almost all available WR-type CSPN spectra, establishing the stellar parameters. The obtained surface abundances are not understandable in terms of "classical" evolutionary calculations, but agree in principle with the advanced models for AGB evolution which account consistently for diffusive mixing and nuclear burning. The underabundance of iron, which we established in a recent study of a WC-type central star (LMC-SMP 61), gives indirect evidence that neutron-capture synthesis has converted Fe into s-process elements.}, language = {en} } @article{HamannGraefener2003, author = {Hamann, Wolf-Rainer and Gr{\"a}fener, G{\"o}tz}, title = {The surface composition of hydrogen-deficient Post-AGB stars}, year = {2003}, language = {en} } @article{BozzoOskinovaLobeletal.2017, author = {Bozzo, Enrico and Oskinova, Lida and Lobel, A. and Hamann, Wolf-Rainer}, title = {The super-orbital modulation of supergiant high-mass X-ray binaries}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {606}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, organization = {LIGO Sci Collaboration \& Virgo;Fermi GBM;INTERGRAL;IceCube Collaboration;AstroSat Cadmium Zinc Telluride;IPN Collaboration;Insight-Hxmt Collaboration;ANTARES Collaboration;Swift Collaboration;AGILE Team;The 1M2H Team;Dark Energy Camera GW-EM;DLT40 Collaboration;GRAWITA GRAvitational Wave;Fermi Large Area Telescope;ATCA Australia Telescope;ASKAP Australian SKA Pathfinder;Las Cumbres Observatory Grp;OzGrav DWF Deeper Wider Faster;VINROUGE Collaboration;MASTER Collaboration;J-GEM;GROWTH JAGWAR CALTECH;Pan-STARRS;MAXI Team;TZAC Consortium;KU Collaboration;Nordic Optical Telescope;ePESSTO;GROUND;Texas Tech Univ;Salt Grp;Toros Transient Robotic Observat;BOOTES Collaboration;MWA Murchison Widefield Array;CALET Collaboration;IKI-GW Follow-up Collaboration;H E S S Collaboration;LOFAR Collaboration;LWA Long Wavelength Array;HAWC Collaboration;Pierre Auger Collaboration;ALMA Collaboration;Euro VLBI Team;Pi Sky Collaboration;Chandra Team McGill Univ;DFN Desert Fireball Network;ATLAS;High Time Resolution Universe;RIMAS RATIR;SKA South Africa MeerKAT}, issn = {1432-0746}, doi = {10.1051/0004-6361/201731930}, pages = {4}, year = {2017}, abstract = {The long-term X-ray light curves of classical supergiant X-ray binaries and supergiant fast X-ray transients show relatively similar super-orbital modulations, which are still lacking a sound interpretation. We propose that these modulations are related to the presence of corotating interaction regions (CIRs) known to thread the winds of OB supergiants. To test this hypothesis, we couple the outcomes of three-dimensional (3D) hydrodynamic models for the formation of CIRs in stellar winds with a simplified recipe for the accretion onto a neutron star. The results show that the synthetic X-ray light curves are indeed modulated by the presence of the CIRs. The exact period and amplitude of these modulations depend on a number of parameters governing the hydrodynamic wind models and on the binary orbital configuration. To compare our model predictions with the observations, we apply the 3D wind structure previously shown to well explain the appearance of discrete absorption components in the UV time series of a prototypical B0.5I-type supergiant. Using the orbital parameters of IGRJ 16493-4348, which has the same B0.5I donor spectral type, the period and modulations in the simulated X-ray light curve are similar to the observed ones, thus providing support to our scenario. We propose that the presence of CIRs in donor star winds should be considered in future theoretical and simulation efforts of wind-fed X-ray binaries.}, language = {en} } @article{ShenarHainichTodtetal.2018, author = {Shenar, Tomer and Hainich, Rainer and Todt, Helge Tobias and Moffat, Anthony F. J. and Sander, Andreas Alexander Christoph and Oskinova, Lida and Ramachandran, Varsha and Munoz, M. and Pablo, H. and Sana, Hugues and Hamann, Wolf-Rainer}, title = {The shortest-period Wolf-Rayet binary in the small magellanic cloud}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {616}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201833006}, pages = {15}, year = {2018}, abstract = {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.}, language = {en} } @article{ReindlRauchParthasarathyetal.2014, author = {Reindl, Nicole and Rauch, Thomas and Parthasarathy, M. and Werner, K. and Kruk, J. W. and Hamann, Wolf-Rainer and Sander, Andreas Alexander Christoph and Todt, Helge Tobias}, title = {The rapid evolution of the exciting star of the Stingray nebula}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {565}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201323189}, pages = {14}, year = {2014}, abstract = {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.}, language = {en} } @misc{LiermannHamannOskinova2014, author = {Liermann, Angelika and Hamann, Wolf-Rainer and Oskinova, Lida}, title = {The quintuplet cluster III. Hertzsprung-Russell diagram and cluster age (vol 540, pg A14, 2012)}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {563}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201117534e}, pages = {2}, year = {2014}, language = {en} } @article{LiermannHamannOskinova2012, author = {Liermann, A. and Hamann, Wolf-Rainer and Oskinova, Lida}, title = {The Quintuplet cluster III. Hertzsprung-Russell diagram and cluster age}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {540}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201117534}, pages = {9}, year = {2012}, abstract = {The Quintuplet, one of three massive stellar clusters in the Galactic center (GC), is located about 30 pc in projection from Sagittarius A*. We aim at the construction of the Hertzsprung-Russell diagram (HRD) of the cluster to study its evolution and to constrain its star-formation history. For this purpose we use the most complete spectral catalog of the Quintuplet stars. Based on the K-band spectra we determine stellar temperatures and luminosities for all stars in the catalog under the assumption of a uniform reddening towards the cluster. We find two groups in the resulting HRD: early-type OB stars and late-type KM stars, well separated from each other. By comparison with Geneva stellar evolution models we derive initial masses exceeding 8 M-circle dot for the OB stars. In the HRD these stars are located along an isochrone corresponding to an age of about 4 Myr. This confirms previous considerations, where a similar age estimate was based on the presence of evolved Wolf-Rayet stars in the cluster. We derive number ratios for the various spectral subtype groups (e.g. N-WR/N-O, N-WC/N-WN) and compare them with predictions of population synthesis models. We find that an instantaneous burst of star formation at about 3.3 to 3.6 Myr ago is the most likely scenario to form the Quintuplet cluster. Furthermore, we apply a mass-luminosity relation to construct the initial mass function (IMF) of the cluster. We find indications for a slightly top-heavy IMF. The late-type stars in the LHO catalog are red giant branch (RGB) stars or red supergiants (RSGs) according to their spectral signatures. Under the assumption that they are located at about the distance of the Galactic center we can derive their luminosities. The comparison with stellar evolution models reveals that the initial masses of these stars are lower than 15 M-circle dot implying that they needed about 15 Myr (RSG) or even more than 30 Myr (RGB) to evolve into their present stage. It might be suspected that these late-type stars do not physically belong to the Quintuplet cluster. Indeed, most of them disqualify as cluster members because their radial velocities differ too much from the cluster average. Nevertheless, five of the brightest RGB/RSG stars from the LHO catalog share the mean radial velocity of the Quintuplet, and thus remain highly suspect for being gravitationally bound members. If so, this would challenge the cluster formation and evolution scenario.}, language = {en} } @misc{DolezalovaKubatovaKubatetal.2019, author = {Dolezalova, Barbora and Kubatova, Brankica and Kubat, Jiri and Hamann, Wolf-Rainer}, title = {The Quasi-WR Star HD 45166 Revisited}, series = {Radiative signatures from the cosmos}, volume = {519}, journal = {Radiative signatures from the cosmos}, publisher = {Astronomical soc pacific}, address = {San Fransisco}, isbn = {978-1-58381-925-8}, issn = {1050-3390}, pages = {197 -- 200}, year = {2019}, abstract = {We studied the wind of the quasi Wolf-Rayet (qWR) star HD 45166. As a first step we modeled the observed UV spectra of this star by means of the state-of-the-art Potsdam Wolf-Rayet (PoWR) atmosphere code. We inferred the wind parameters and compared them with previous findings.}, language = {en} } @article{HamannKoesterke1998, author = {Hamann, Wolf-Rainer and Koesterke, Lars}, title = {The nitrogen spectra of Wolf-Rayet stars}, year = {1998}, language = {en} } @article{PenaHamannRuiz2003, author = {Pena, M. and Hamann, Wolf-Rainer and Ruiz, M. T.}, title = {The LMC planetary nebula N66 revisited. Nebular kinematics and stellar models}, isbn = {1-583-81148-6}, year = {2003}, language = {en} } @article{ShenarHamannTodt2015, author = {Shenar, Tomer and Hamann, Wolf-Rainer and Todt, Helge Tobias}, title = {The impact of rotation on the line profiles of Wolf-Rayet stars}, series = {Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.-5. June 2015}, journal = {Wolf-Rayet Stars : Proceedings of an International Workshop held in Potsdam, Germany, 1.-5. June 2015}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-88008}, pages = {193 -- 196}, year = {2015}, abstract = {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).}, language = {en} } @article{ShenarHamannTodt2014, author = {Shenar, Tomer and Hamann, Wolf-Rainer and Todt, Helge Tobias}, title = {The impact of rotation on the line profiles of Wolf-Rayet stars}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {562}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201322496}, pages = {13}, year = {2014}, abstract = {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).}, language = {en} } @article{GraefenerHamannKoesterke2000, author = {Gr{\"a}fener, G{\"o}tz and Hamann, Wolf-Rainer and Koesterke, Lars}, title = {The impact of iron group elements on the ionizatin structure of WC star atmospheres : WR111}, year = {2000}, language = {en} } @article{HamannGraefenerLiermannetal.2019, author = {Hamann, Wolf-Rainer and Gr{\"a}fener, G. and Liermann, A. and Hainich, Rainer and Sander, Andreas Alexander Christoph and Shenar, Tomer and Ramachandran, Varsha and Todt, Helge Tobias and Oskinova, Lida}, title = {The Galactic WN stars revisited}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {625}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201834850}, pages = {11}, year = {2019}, abstract = {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.}, language = {en} } @article{HamannGraefenerLiermann2006, author = {Hamann, Wolf-Rainer and Graefener, G. and Liermann, A.}, title = {The galactic WN stars - Spectral analyses with line-blanketed model atmospheres versus stellar evolution models with and without rotation}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {457}, journal = {Astronomy and astrophysics : an international weekly journal}, number = {3}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361:20065052}, pages = {1015 -- 1031}, year = {2006}, abstract = {Context. Very massive stars pass through the Wolf-Rayet (WR) stage before they finally explode. Details of their evolution have not yet been safely established, and their physics are not well understood. Their spectral analysis requires adequate model atmospheres, which have been developed step by step during the past decades and account in their recent version for line blanketing by the millions of lines from iron and iron-group elements. However, only very few WN stars have been re-analyzed by means of line-blanketed models yet. Aims. The quantitative spectral analysis of a large sample of Galactic WN stars with the most advanced generation of model atmospheres should provide an empirical basis for various studies about the origin, evolution, and physics of the Wolf-Rayet stars and their powerful winds. Methods. We analyze a large sample of Galactic WN stars by means of the Potsdam Wolf-Rayet (PoWR) model atmospheres, which account for iron line blanketing and clumping. The results are compared with a synthetic population, generated from the Geneva tracks for massive star evolution. Results. We obtain a homogeneous set of stellar and atmospheric parameters for the GalacticWN stars, partly revising earlier results. Conclusions. Comparing the results of our spectral analyses of the Galactic WN stars with the predictions of the Geneva evolutionary calculations, we conclude that there is rough qualitative agreement. However, the quantitative discrepancies are still severe, and there is no preference for the tracks that account for the effects of rotation. It seems that the evolution of massive stars is still not satisfactorily understood.}, language = {en} } @article{SanderHamannTodt2012, author = {Sander, A. and Hamann, Wolf-Rainer and Todt, Helge Tobias}, title = {The Galactic WC stars Stellar parameters from spectral analyses indicate a new evolutionary sequence}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {540}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201117830}, pages = {79}, year = {2012}, abstract = {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.}, language = {en} } @article{SanderHamannTodtetal.2019, author = {Sander, Andreas Alexander Christoph and Hamann, Wolf-Rainer and Todt, Helge Tobias and Hainich, Rainer and Shenar, Tomer and Ramachandran, Varsha and Oskinova, Lida}, title = {The Galactic WC and WO stars}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {621}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201833712}, pages = {19}, year = {2019}, abstract = {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.}, language = {en} } @article{GrunerHainichSanderetal.2018, author = {Gruner, David and Hainich, Rainer and Sander, Andreas Alexander Christoph and Shenar, Tomer and Todt, Helge Tobias and Oskinova, Lida and Ramachandran, Varsha and Ayres, T. and Hamann, Wolf-Rainer}, title = {The extreme O-type spectroscopic binary HD 93129A A quantitative, multiwavelength analysis}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {621}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201833178}, pages = {16}, year = {2018}, abstract = {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.}, language = {en} } @article{PenaPeimbertHamannetal.2004, author = {Pena, M and Peimbert, A. and Hamann, Wolf-Rainer and Ruiz, M. T. and Peimbert, M.}, title = {The extraordinary planetary nebula N66 in the LMC}, isbn = {3-12-283174-0}, year = {2004}, abstract = {Morphology of the planetary nebula LMC-N66 (ionized by a [WN] star) indicates that the nebula is a multipolar object with a very narrow waist. It shows several jets, knots and filaments in opposite directions from the central star. A couple of twisted long filaments could be interpreted as due to point-symmetric type ejection. If such is the case, the progenitor would be a binary precessing system. High resolution spectroscopy shows that most of the material is approaching or receding from the star. However the line profiles are very complex, showing several components at different velocities. Our high resolution spectroscopic data show that the different structures (knots, filaments, ...) present different radial velocities spreading from 240 to more than 400 km/s. The system velocity is 300 km/s. There are high velocity knots located to the north of the central star, moving at more than 100 km/s relative to the system velocity.}, language = {en} } @article{MartinezNunezSanderGimenezGarciaetal.2015, author = {Martinez-Nunez, Silvia and Sander, Angelika and Gimenez-Garcia, Angel and Gonzalez-Galan, Ana and Torrejon, Jose Miguel and Gonzalez-Fernandez, Carlos and Hamann, Wolf-Rainer}, title = {The donor star of the X-ray pulsar X1908+075}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {578}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201424823}, pages = {9}, year = {2015}, abstract = {High-mass X-ray binaries consist of a massive donor star and a compact object. While several of those systems have been well studied in X-rays, little is known for most of the donor stars as they are often heavily obscured in the optical and ultraviolet regime. There is an opportunity to observe them at infrared wavelengths, however. The goal of this study is to obtain the stellar and wind parameters of the donor star in the X1908+075 high-mass X-ray binary system with a stellar atmosphere model to check whether previous studies from X-ray observations and spectral morphology lead to a sufficient description of the donor star. We obtained H-and K-band spectra of X1908+075 and analysed them with the Potsdam Wolf-Rayet (PoWR) model atmosphere code. For the first time, we calculated a stellar atmosphere model for the donor star, whose main parameters are: M-spec = 15 +/- 6 M-circle dot, T-* = 23(-3)(+6) kK, log g(eff) = 3.0 +/- 0.2 and log L/L-circle dot = 4.81 +/- 0.25. The obtained parameters point towards an early B-type (B0-B3) star, probably in a supergiant phase. Moreover we determined a more accurate distance to the system of 4.85 +/- 0.50 kpc than the previously reported value.}, language = {en} } @article{HajdukTodtHamannetal.2020, author = {Hajduk, Marcin and Todt, Helge Tobias and Hamann, Wolf-Rainer and Borek, Karolina and van Hoof, Peter A. M. and Zijlstra, Albert A.}, title = {The cooling-down central star of the planetary nebula SwSt 1}, series = {Monthly notices of the Royal Astronomical Society}, volume = {498}, journal = {Monthly notices of the Royal Astronomical Society}, number = {1}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/staa2274}, pages = {1205 -- 1220}, year = {2020}, abstract = {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.}, language = {en} }