@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{HamannKoesterkeWesselowski1995,
  author    = {Hamann, Wolf-Rainer and Koesterke, Lars and Wesselowski, U.},
  title     = {Spectral atlas of galactic Wolf-Rayet stars (WN-sequence)},
  year      = {1995},
  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}
}
@article{HamannKoesterke1996,
  author    = {Hamann, Wolf-Rainer and Koesterke, Lars},
  title     = {Spectral analyses with the standard model : Part II: Wolf-Rayet Stars},
  year      = {1996},
  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{KoesterkeHamann1997,
  author    = {Koesterke, Lars and Hamann, Wolf-Rainer},
  title     = {Quantitative spectral analyses of CSPNs of early [WC]-type},
  year      = {1997},
  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     = {Spectrophotometric data of the central star of the large magellanic cloud planetary nebula N66. Quantitative analysis of its WN type spectrum},
  year      = {1997},
  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{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{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{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{KoesterkeDreizlerRauch1998,
  author    = {Koesterke, Lars and Dreizler, S. and Rauch, Thomas},
  title     = {On the mass-loss of PG 1159 stars},
  year      = {1998},
  language  = {en}
}
@article{HamannKoesterke1998,
  author    = {Hamann, Wolf-Rainer and Koesterke, Lars},
  title     = {The nitrogen spectra of Wolf-Rayet stars},
  year      = {1998},
  language  = {en}
}
@article{HamannKoesterke1998,
  author    = {Hamann, Wolf-Rainer and Koesterke, Lars},
  title     = {Spectrum formation in clumped stellar winds : consequences for the analyses of Wolf-Rayet spectra},
  year      = {1998},
  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{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{HamannKoesterke1999,
  author    = {Hamann, Wolf-Rainer and Koesterke, Lars},
  title     = {Spectral analyses of Wolf-Rayet stars : the impact of clumping},
  year      = {1999},
  language  = {en}
}
@article{PenaStastinskaEstebanetal.1999,
  author    = {Pena, M. and Stastinska, G. and Esteban, C. and Koesterke, Lars and Medina, S. and Kingsburgh, R.},
  title     = {Spectroscopy of planetary nebulae with [WR] nuclei},
  year      = {1999},
  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}
}
@article{DeMarcoSchmutzKoesterkeetal.1999,
  author    = {DeMarco, O. and Schmutz, W. and Koesterke, Lars and Hamann, Wolf-Rainer and DeMarco, O. and DeKoter, A.},
  title     = {Why should we compare WR codes?},
  year      = {1999},
  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{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{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{HamannKoesterke2000,
  author    = {Hamann, Wolf-Rainer and Koesterke, Lars},
  title     = {WM stars in the LMC : parameters and atmospheric abundances},
  year      = {2000},
  language  = {en}
}
@article{HamannKoesterke2000,
  author    = {Hamann, Wolf-Rainer and Koesterke, Lars},
  title     = {WN stars in the LMC : parameters and atmospheric abundances},
  year      = {2000},
  abstract  = {The spectra of 18 WN stars in the Large Magellanic Cloud (LMC) are quantitatively analyzed by means of "standard" Wolf-Rayet model atmospheres, using the helium and nitrogen lines as well as the spectral energy distribution. The hydrogen abundance is also determined. Carbon is included for a subset of 4 stars. The studied sample covers all spectral subtypes (WN2 ... WN9) and also includes one WN/WC transition object. The luminosities of the program stars span a wide range ( L/Lsun = 5.0 ... 6.5). Due to the given LMC membership, these results are free from uncertainties inferred from the distance. 50 \% of the studied stars (both, late and early WN subtypes) have rather low luminosity (L/Lsun < 5.5). This puts tough constraints on their evolutionary formation. If coming from single stars, it provides evidence for strong internal mixing processes. The empirical mass-loss rates are scaled down by a factor of about two due to the impact of clumping, compared to previous studies adopting homogeneous winds. There is no obvious strong correlation between the mass-loss rates and other parameters like luminosity, temperature and composition. The stellar parameters for the present LMC sample are not systematically different from those of the Galactic WN stars studied previously with the same techniques, in contrast to the expected metallicity effects.},
  language  = {en}
}
@article{Koesterke2001,
  author    = {Koesterke, Lars},
  title     = {Spectral analyses of WR-type central stars of Planetary Nebulae},
  year      = {2001},
  abstract  = {So far, the evolution of post-AGB stars is not fully understood. In particular the formation of hydrogen- deficient and hydrogen-free Central Stars of Planetary Nebulae (CSPN) is unsettled. New evolution models, which allow for the consistent treatment of the physics of late thermal pulses, promise new insights to the formation of these stars. In this paper we summarize the results of non-LTE analyses of CSPN with wind. By comparing these results with the predictions of the new evolution models, open questions concerning the evolution of the stars might be answered. In addition we discuss the driving mechanism of the winds of Wolf-Rayet CSPN. New models, which account for millions of iron lines, support the assumptions that these winds are driven by radiation.},
  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{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{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{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{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{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}
}