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 . The mass-loss rates scale very tightly with L 0.8 . The two WO stars in our sample turn out to be outstandingly hot (≈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 . 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 .

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 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 ) = 5.3 ... 5.8. Conclusions. While the few extremely luminous stars (log (L/L ) > 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 ) = 5.3 ... 5.8, these stars originate from initial masses between 20 and 40 M . 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.

Context. Massive Wolf-Rayet (WR) stars are evolved massive stars (M i 20 M ) 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 Potsdam Wolf-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 massluminosity 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 AB 6 is found to be very luminous (log L ≈ 6.3 [L ]) given its orbital mass (≈10 M ), 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 ( 60 M ) 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.

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, M 31, and the LMC, we derive stellar parameters and massloss 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 . The WN stars in the SMC exhibit on average lower mass-loss rates and weaker winds than their counterparts in the Milky Way, M 31, 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.

We present a comprehensive study of X-ray emission and wind properties of massive magnetic early B-type stars. Dedicated XMM-Newton observations were obtained for three stars xi1 CMa, V2052 Oph, and zeta Cas. We report the first detection of X-ray emission from V2052 Oph and zeta Cas. The observations show that the X-ray spectra of our program stars are quite soft. We compile the complete sample of early B-type stars with detected magnetic fields to date and existing X-ray measurements, in order to study whether the X-ray emission can be used as a general proxy for stellar magnetism. We find that hard and strong X-ray emission does not necessarily correlate with the presence of a magnetic field. We analyze the UV spectra of five non-supergiant B stars with magnetic fields by means of non-LTE iron-blanketed model atmospheres. The latter are calculated with the PoWR code, which treats the photosphere as well as the the wind, and also accounts for X-rays. Our models accurately fit the stellar photospheric spectra in the optical and the UV. The parameters of X-ray emission, temperature and flux are included in the model in accordance with observations. We confirm the earlier findings that the filling factors of X-ray emitting material are very high. Our analysis reveals that the magnetic early type B stars studied here have weak winds. The mass-loss rates are significantly lower than predicted by hydrodynamically consistent models. We find that, although the X-rays strongly affect the ionization structure of the wind, this effect is not sufficient in reducing the total radiative acceleration. When the X-rays are accounted for at the intensity and temperatures observed, there is still sufficient radiative acceleration to drive stronger mass-loss than we empirically infer from the UV spectral lines. (abridged)Comment: 20 pages, accepted by MNRA