@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{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{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{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{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{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{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{ToalaGuerreroTodtetal.2015,
  author    = {Toala, Jes{\´u}s Alberto and Guerrero, Mart{\´i}n A. and Todt, Helge Tobias and Hamann, Wolf-Rainer and Chu, Y.-H. and Gruendl, R. A. and Sch{\"o}nberner, Detlef and Oskinova, Lida and Marquez-Lugo, R. A. and Fang, X. and Ramos-Larios, Gerardo},
  title     = {The born-again Planetary nebula A78: an X-RAY twin of A30},
  series = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  volume    = {799},
  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/799/1/67},
  pages     = {10},
  year      = {2015},
  abstract  = {We present the XMM-Newton discovery of X-ray emission from the planetary nebula (PN) A78, the second born-again PN detected in X-rays apart from A30. These two PNe share similar spectral and morphological characteristics: they harbor diffuse soft X-ray emission associated with the interaction between the H-poor ejecta and the current fast stellar wind and a point-like source at the position of the central star (CSPN). We present the spectral analysis of the CSPN, using for the first time an NLTE code for expanding atmospheres that takes line blanketing into account for the UV and optical spectra. The wind abundances are used for the X-ray spectral analysis of the CSPN and the diffuse emission. The X-ray emission from the CSPN in A78 can be modeled by a single C VI emission line, while the X-ray emission from its diffuse component is better described by an optically thin plasma emission model with a temperature of kT = 0.088 keV (T approximate to 1.0 x 10(6) K). We estimate X-ray luminosities in the 0.2-2.0 keV energy band of L-X,L-CSPN =(1.2 +/- 0.3) x 10(31) erg s(-1) and L-X,L-DIFF =(9.2 +/- 2.3) x 10(30) erg s(-1) for the CSPN and diffuse components, respectively.},
  language  = {en}
}
@article{HuenemoerderGayleyHamannetal.2015,
  author    = {Huenemoerder, David P. and Gayley, K. G. and Hamann, Wolf-Rainer and Ignace, R. and Nichols, J. S. and Oskinova, Lida and Pollock, A. M. T. and Schulz, Norbert S. and Shenar, Tomer},
  title     = {Probing Wolf-Rayet winds: Chandra/HETG X-ray spectra of WR 6},
  series = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  volume    = {815},
  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/815/1/29},
  pages     = {16},
  year      = {2015},
  abstract  = {With a deep Chandra/HETGS exposure of WR 6, we have resolved emission lines whose profiles show that the X-rays originate from a uniformly expanding spherical wind of high X-ray-continuum optical depth. The presence of strong helium-like forbidden lines places the source of X-ray emission at tens to hundreds of stellar radii from the photosphere. Variability was present in X-rays and simultaneous optical photometry, but neither were correlated with the known period of the system or with each other. An enhanced abundance of sodium revealed nuclear-processed material, a quantity related to the evolutionary state of the star. The characterization of the extent and nature of the hot plasma in WR 6 will help to pave the way to a more fundamental theoretical understanding of the winds and evolution of massive stars.},
  language  = {en}
}
@article{TodtSanderHainichetal.2015,
  author    = {Todt, Helge Tobias and Sander, Angelika and Hainich, Rainer and Hamann, Wolf-Rainer and Quade, Markus and Shenar, Tomer},
  title     = {Potsdam Wolf-Rayet model atmosphere grids for WN stars},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {579},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {0004-6361},
  doi       = {10.1051/0004-6361/201526253},
  pages     = {6},
  year      = {2015},
  abstract  = {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.},
  language  = {en}
}