@article{KoesterkeDreizlerRauch1998,
  author    = {Koesterke, Lars and Dreizler, S. and Rauch, Thomas},
  title     = {On the mass-loss of PG 1159 stars},
  year      = {1998},
  language  = {en}
}
@article{StastinskaGraefenerPenaetal.2004,
  author    = {Stastinska, G. and Gr{\"a}fener, G{\"o}tz and Pena, M. and Hamann, Wolf-Rainer and Koesterke, Lars and Szczerba, Ryszard},
  title     = {Comprehensive modelling of the planetary nebula LMC-SMP 61 and its [WC]-type central star},
  issn      = {0004-6361},
  year      = {2004},
  abstract  = {We present a comprehensive study of the Magellanic Cloud planetary nebula SMP 61 and of its nucleus, a Wolf- Rayet type star classified [WC 5-6]. The observational material consists of HST STIS spectroscopy and imaging, together with optical and UV spectroscopic data collected from the literature and infrared fluxes measured by IRAS. We have performed a detailed spectral analysis of the central star, using the Potsdam code for expanding atmospheres in non-LTE. For the central star we determine the following parameters: L-star = 10(3.96) L-., R-star = 0.42 R-., T-star = 87.5 kK, (M) over dot = 10(-6.12) M-. yr(-1), v(infinity) = 1400 km s(-1), and a clumping factor of D = 4. The elemental abundances by mass are X-He = 0.45, X-C = 0.52, X-N < 5 x 10(-5), X-O = 0.03, and X-Fe < 1 x 10(-4). The fluxes from the model stellar atmosphere were used to compute photoionization models of the nebula. All the available observations, within their error bars, were used to constrain these models. We find that the ionizing fluxes predicted by the stellar model are consistent with the fluxes needed by the photoionization model to reproduce the nebular emission, within the error margins. However, there are indications that the stellar model overestimates the number and hardness of Lyman continuum photons. The photoionization models imply a clumped density structure of the nebular material. The observed C III] lambda1909/C II lambda4267 line ratio implies the existence of carbon-rich clumps in the nebula. Such clumps are likely produced by stellar wind ejecta, possibly mixed with the nebular material. We discuss our results with regard to the stellar and nebular post-AGB evolution. The observed Fe-deficiency for the central star indicates that the material which is now visible on the stellar surface has been exposed to s-process nucleosynthesis during previous thermal pulses. The absence of nitrogen allows us to set an upper limit to the remaining H-envelope mass after a possible AGB final thermal pulse. Finally, we infer from the total amount of carbon detected in the nebula that the strong [WC] mass- loss may have been active only for a limited period during the post-AGB evolution},
  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{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{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{KoesterkeHamann2002,
  author    = {Koesterke, Lars and Hamann, Wolf-Rainer},
  title     = {[WC]-type CSPN : clumping and wind-driving},
  year      = {2002},
  abstract  = {Many Central Stars of Planetary Nebulae are very similar to massive Wolf-Rayet stars of the carbon sequence with respect to their spectra, chemical composition and wind properties. Therefore their study opens an additional way towards the understanding of the Wolf-Rayet phenomenon. While the study of Line Profile Variation will be difficult, espescially for the very compact early types, the comparision with other hydrogen-deficient Central Stars illuminates the driving mechanism of their winds. We speculate that at least two ingredients are needed. The ionization of their atmpospheres has to be stratified to enable multi-scattering processes and the amount of carbon and oxygen has to be high (more than a few percent by mass).},
  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{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{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{KoesterkeWerner1998,
  author    = {Koesterke, Lars and Werner, Klaus},
  title     = {Determination of mass-loss rates of PG 1159 stars from FUV spectroskopy},
  year      = {1998},
  language  = {en}
}