@article{AguileraDenaLangerAntoniadisetal.2022,
  author    = {Aguilera-Dena, David R. and Langer, Norbert and Antoniadis, John and Pauli, Daniel and Dessart, Luc and Vigna-G{\´o}mez, Alejandro and Gr{\"a}fener, G{\"o}tz and Yoon, Sung-Chul},
  title     = {Stripped-envelope stars in different metallicity environments: I. Evolutionary phases, classification, and populations},
  series = {Astronomy and astrophysics},
  volume    = {661},
  journal   = {Astronomy and astrophysics},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {0004-6361},
  doi       = {10.1051/0004-6361/202142895},
  pages     = {20},
  year      = {2022},
  abstract  = {Massive stars that become stripped of their hydrogen envelope through binary interaction or winds can be observed either as Wolf-Rayet stars, if they have optically thick winds, or as transparent-wind stripped-envelope stars. We approximate their evolution through evolutionary models of single helium stars, and compute detailed model grids in the initial mass range 1.5-70 M. for metallicities between 0.01 and 0.04, from core helium ignition until core collapse. Throughout their lifetimes some stellar models expose the ash of helium burning. We propose that models that have nitrogen-rich envelopes are candidate WN stars, while models with a carbon-rich surface are candidate WC stars during core helium burning, and WO stars afterwards. We measure the metallicity dependence of the total lifetimes of our models and the duration of their evolutionary phases. We propose an analytic estimate of the wind's optical depth to distinguish models of Wolf-Rayet stars from transparent-wind stripped-envelope stars, and find that the luminosity ranges at which WN-, WC-, and WO-type stars can exist is a strong function of metallicity. We find that all carbon-rich models produced in our grids have optically thick winds and match the luminosity distribution of observed populations. We construct population models and predict the numbers of transparent-wind stripped-envelope stars and Wolf-Rayet stars, and derive their number ratios at different metallicities. We find that as metallicity increases, the number of transparent-wind stripped-envelope stars decreases and the number of Wolf-Rayet stars increases. At high metallicities WC- and WO-type stars become more common. We apply our population models to nearby galaxies, and find that populations are more sensitive to the transition luminosity between Wolf-Rayet stars and transparent-wind helium stars than to the metallicity-dependent mass loss rates.},
  language  = {en}
}
@misc{TodtPenaZuehlkeetal.2011,
  author    = {Todt, Helge Tobias and Pe{\~n}a, Miriam and Z{\"u}hlke, Julia and Oskinova, Lidia M. 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}
}
@article{TodtPenaHamannetal.2010,
  author    = {Todt, Helge Tobias and Pena, Maria A. and Hamann, Wolf-Rainer and Gr{\"a}fener, G{\"o}tz},
  title     = {The central star of the planetary nebula PB8 : a Wolf-Rayet-type wind of an unusual WN/WC chemical composition},
  issn      = {0004-6361},
  doi       = {10.1051/0004-6361/200912183},
  year      = {2010},
  abstract  = {A considerable fraction of the central stars of planetary nebulae (CSPNe) are hydrogen-deficient. As a rule, these CSPNe exhibit a chemical composition of helium, carbon, and oxygen with the majority showing Wolf-Rayet-like emission line spectra. These stars are classified as CSPNe of a spectral type [WC]. We perform a spectral analysis of CSPN PB 8 with the Potsdam Wolf-Rayet (PoWR) models for expanding atmospheres. The source PB8 displays wind-broadened emission lines from strong mass loss. Most strikingly, we find that its surface composition is hydrogen-deficient, but not carbon-rich. With mass fractions of 55\% helium, 40\% hydrogen, 1.3\% carbon, 2\% nitrogen, and 1.3\% oxygen, it differs greatly from the 30-50\% of carbon which are typically seen in [WC]-type central stars. The atmospheric mixture in PB8 has an analogy in the WN/WC transition type among the massive Wolf-Rayet stars. Therefore we suggest to introduce a new spectral type [WN/WC] for CSPNe, with PB8 as its first member. The central star of PB8 has a relatively low temperature of T-* = 52 kK, as expected for central stars in their early evolutionary stages. Its surrounding nebula is less than 3000 years old, i.e. relatively young. Existing calculations for the post-AGB evolution can produce hydrogen-deficient stars of the [WC] type, but do not predict the composition found in PB8. We discuss various scenarios that might explain the origin of this unique object.},
  language  = {en}
}
@article{GraefenerHamannHillieretal.1998,
  author    = {Gr{\"a}fener, G{\"o}tz and Hamann, Wolf-Rainer and Hillier, D. J. and Koesterke, Lars},
  title     = {Spectral analyses of WC stars in the LMC},
  year      = {1998},
  language  = {en}
}
@article{KoesterkeHamannGraefener1999,
  author    = {Koesterke, Lars and Hamann, Wolf-Rainer and Gr{\"a}fener, G{\"o}tz},
  title     = {Inhomogeneities in Wolf-Rayet atmospheres},
  year      = {1999},
  language  = {en}
}
@article{GraefenerHamannKoesterke1999,
  author    = {Gr{\"a}fener, G{\"o}tz and Hamann, Wolf-Rainer and Koesterke, Lars},
  title     = {Spectral analyses of WC stars in the LMC},
  year      = {1999},
  language  = {en}
}
@article{HamannKoesterkeGraefener1999,
  author    = {Hamann, Wolf-Rainer and Koesterke, Lars and Gr{\"a}fener, G{\"o}tz},
  title     = {Modelling and quantitative analyses of WR spectra : recent progress and results},
  year      = {1999},
  language  = {en}
}
@phdthesis{Graefener1999,
  author    = {Gr{\"a}fener, G{\"o}tz},
  title     = {Spektralanalysen von WC-Sternen in der LMC},
  pages     = {93 S. : graph. Darst.},
  year      = {1999},
  language  = {de}
}
@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{HamannKoesterkeGraefener2000,
  author    = {Hamann, Wolf-Rainer and Koesterke, Lars and Gr{\"a}fener, G{\"o}tz},
  title     = {Non-LTE models of WR winds},
  year      = {2000},
  language  = {en}
}
@article{HamannKoesterkeGraefener2002,
  author    = {Hamann, Wolf-Rainer and Koesterke, Lars and Gr{\"a}fener, G{\"o}tz},
  title     = {Spectral analyses of Wolf-Rayet winds},
  year      = {2002},
  abstract  = {The analysis of Wolf-Rayet spectra requires adequate model atmospheres which treat the non-LTE radiation transfer in a spherically expanding medium. Present state-of-the-art calculations account for complex model atoms with, typically, a few hundred energy levels and a few thousand spectral lines of He and CNO elements. In the most recent version of our model code, blanketing by millions of lines from iron-group elements is also included. These models have been widely applied for the spectral analysis of WN stars in the Galaxy and LMC. WN spectra can be well reproduced in most cases. WC stars have not yet been analyzed comprehensively, because the agreement with observations becomes satisfactory only when line-blanketed models are applied. The introduction of inhomogeneities (clumping), although treated in a rough approximation, has significantly improved the fit between synthetic and observed spectra with respect to the electron-scattering wings of strong lines. The mass-loss rates obtained from spectral analyses become smaller by a factor 2-3 if clumping is accounted for. A pre-specified velocity law is adopted for our models, but the radiation pressure can be evaluated from our detailed calculation and can be compared a posteriori with the required wind acceleration. Surprisingly we find that the line-blanketed models are not far from being hydrodynamically consistent, thus indicating that radiation pressure is probably the main driving force for the mass-loss from WR stars.},
  language  = {en}
}
@article{KoesterkeHamannGraefener2002,
  author    = {Koesterke, Lars and Hamann, Wolf-Rainer and Gr{\"a}fener, G{\"o}tz},
  title     = {Expanding atmospheres in non-LTE : Radiation transfer using short characteristics},
  year      = {2002},
  abstract  = {We present our technique for solving the equations of radiation transfer in spherically expanding atmospheres. To ensure an efficient treatment of the Thomson scattering, the mean intensity J is derived by solving the moment equations in turn with the angle-dependent transfer equation. The latter provide the Eddington factors. Two different methods for the solution of the angle dependent equation are compared. Thereby the integration along short characteristics turned out to be superior in our context over the classical differencing scheme. The method is the basis of a non-LTE code suitable for the atmospheres of hot stars with high mass-loss.},
  language  = {en}
}
@article{GraefenerKoesterkeHamann2002,
  author    = {Gr{\"a}fener, G{\"o}tz and Koesterke, Lars and Hamann, Wolf-Rainer},
  title     = {Line-blanketed model atmospheres for WR star},
  year      = {2002},
  abstract  = {We describe the treatment of iron group line-blanketing in non-LTE model atmospheres for WR stars. As an example, a blanketed model for the early-type WC star WR 111 is compared to its un-blanketed counterpart. Blanketing affects the ionization structure and the emergent flux distribution of our models. The radiation pressure, as computed within our models, falls short by only a factor of two to provide the mechanical power of the WR wind.},
  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{GraefenerHamann2003,
  author    = {Gr{\"a}fener, G{\"o}tz and Hamann, Wolf-Rainer},
  title     = {Spectral analysis of the LMC [WC] star SMP 61},
  isbn      = {1-583-81148-6},
  year      = {2003},
  abstract  = {HST UV and optical spectra of the early-type [WC] star SMP 61 in the LMC are analyzed by means of line blanketed non-LTE models for expanding atmospheres. The known distance to the LMC allows a reliable determination of the stellar parameters. The low iron surface abundance of the object possibly indicates a preceding evolution through a very late thermal pulse (VLTP).},
  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}
}
@phdthesis{HamannPenaGraefeneretal.2003,
  author    = {Hamann, Wolf-Rainer and Pena, M. and Gr{\"a}fener, G{\"o}tz and Ruiz, M. T.},
  title     = {The central star of the planetary nebula N66 in the Large Magellanic Cloud : a detailed analysis of its dramatic evolution 1983 - 2000},
  issn      = {0004-6361},
  year      = {2003},
  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{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{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}
}