@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{WernerDreizlerRauchetal.1999,
  author    = {Werner, Klaus and Dreizler, S. and Rauch, Thomas and Koesterke, Lars and Heber, Ulrich},
  title     = {Born-again AGB stars : staring point of the H-deficient post-AGB evolutionary sequence?},
  year      = {1999},
  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{KoesterkeWerner1998,
  author    = {Koesterke, Lars and Werner, Klaus},
  title     = {Determination of mass-loss rates of PG 1159 stars from FUV spectroskopy},
  year      = {1998},
  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{WernerDreizlerRauchetal.1999,
  author    = {Werner, Klaus and Dreizler, S. and Rauch, Thomas and Barnstedt, J{\"u}rgen and G{\"o}z, M. and Gringel, W. and Kappelmann, N. and Kr{\"a}mer, G. and Widmann, H. and Koesterke, Lars and Haas, S. and Heber, Ulrich and Appenzeller, Immo},
  title     = {FUV spectroscpy of DO an PG 1159 stars with ORFEUS},
  year      = {1999},
  language  = {en}
}
@article{PenaStastinskaEstebanetal.1998,
  author    = {Pena, M. and Stastinska, G. and Esteban, C. and Koesterke, Lars and Medina, S. and Kingsburgh, R.},
  title     = {Galactic planetary nebulae with Wolf-Rayet nuclei : I. Objects with [WC]-early type stars},
  year      = {1998},
  language  = {en}
}
@article{DeMarcoSchmutzKoesterkeetal.1999,
  author    = {DeMarco, O. and Schmutz, W. and Koesterke, Lars and Hamann, Wolf-Rainer},
  title     = {Gamma 2 Velorum revisited},
  year      = {1999},
  language  = {en}
}
@article{PenaHamannKoesterkeetal.1997,
  author    = {Pena, M. and Hamann, Wolf-Rainer and Koesterke, Lars and Maza, J. and Mendez, R. H. and Peimbert, M. and Ruiz, M. T. and Torres-Peimbert, S.},
  title     = {HST spectrophotometric data of the central star of the planetary nebula LMC-N66},
  year      = {1997},
  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{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{KoesterkeHamannUrrutia2001,
  author    = {Koesterke, Lars and Hamann, Wolf-Rainer and Urrutia, Tanya},
  title     = {Line-Profile Variability in the Wolf-Rayet Stars WR 135 and WR 111},
  year      = {2001},
  abstract  = {We have obtained time-resolved observations of line-profile variations of the two Wolf-Rayet stars WR 135 and WR 111. The spectra, taken during two consecutive nights, cover a broad range from 4470 to 6590 Ang. The profile variability of the C iii emission line at 5696 Ang in WR 135 is shown in detail. The principal difficulties to constrain the velocity law from the frequency drift of discrete spectral features is discussed, emphasizing the crucial dependence on the adopted location of the line-emission region, and the possible necessity to distinguish between the motion of structures and the flow of the matter. - Full access to the observational data is provided via anonymous file transfer.},
  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}
}
@article{DreizlerWernerRauchetal.1997,
  author    = {Dreizler, S. and Werner, Klaus and Rauch, Thomas and Koesterke, Lars and Heber, Ulrich},
  title     = {NLTE Analyses of PG 1159 stars : Contraints for the structure and evolutiuon of Post-AGB stars},
  year      = {1997},
  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{KoesterkeDreizlerRauch1998,
  author    = {Koesterke, Lars and Dreizler, S. and Rauch, Thomas},
  title     = {On the mass-loss of PG 1159 stars},
  year      = {1998},
  language  = {en}
}
@article{KoesterkeHamann1997,
  author    = {Koesterke, Lars and Hamann, Wolf-Rainer},
  title     = {Quantitative spectral analyses of CSPNs of early [WC]-type},
  year      = {1997},
  language  = {en}
}
@article{KoesterkeHamann1995,
  author    = {Koesterke, Lars and Hamann, Wolf-Rainer},
  title     = {Spectral analyses of 25 galactic Wolf-Rayet stars of the carbon sequence},
  year      = {1995},
  language  = {en}
}
@article{KoesterkeHamann1997,
  author    = {Koesterke, Lars and Hamann, Wolf-Rainer},
  title     = {Spectral analyses of central of planetary nebulae of early WC-type / NGC 6751 and Sanduleak 3},
  year      = {1997},
  language  = {en}
}
@article{HamannKoesterkeWesselowski1995,
  author    = {Hamann, Wolf-Rainer and Koesterke, Lars and Wesselowski, U.},
  title     = {Spectral analyses of the galactic Wolf-Rayet stars : hydrogen-helium abundances and improved stellar parameters for the WN class},
  year      = {1995},
  language  = {en}
}