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 - 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 - 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 - 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 - 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 - 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 - 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 - 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 - 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 - 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 - 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 - 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 - 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 - 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 - 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 - 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 - 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 - 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 - 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 -