@article{WernerReindlDorschetal.2022,
  author    = {Werner, Klaus and Reindl, Nicole and Dorsch, Matti and Geier, Stephan and Munari, Ulisse and Raddi, Roberto},
  title     = {Non-local thermodynamic equilibrium spectral analysis of five hot, hydrogen-deficient pre-white dwarfs},
  series = {Astronomy and Astrophysics},
  volume    = {658},
  journal   = {Astronomy and Astrophysics},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {0004-6361},
  doi       = {10.1051/0004-6361/202142397},
  pages     = {15},
  year      = {2022},
  abstract  = {Hot, compact, hydrogen-deficient pre-white dwarfs (pre-WDs) with effective temperatures of Teff > 70 000 K and a surface gravity of 5.0 < logg < 7.0 are rather rare objects despite recent and ongoing surveys. It is believed that they are the outcome of either single star evolution (late helium-shell flash or late helium-core flash) or binary star evolution (double WD merger). Their study is interesting because the surface elemental abundances reflect the physics of thermonuclear flashes and merger events. Spectroscopically they are divided in three different classes, namely PG1159, O(He), or He-sdO. We present a spectroscopic analysis of five such stars that turned out to have atmospheric parameters in the range Teff = 70 000-80 000 K and logg = 5.2-6.3. The three investigated He-sdOs have a relatively high hydrogen mass fraction (10\%) that is unexplained by both single (He core flash) and binary evolution (He-WD merger) scenarios. The O(He) star JL 9 is probably a binary helium-WD merger, but its hydrogen content (6\%) is also at odds with merger models. We found that RL 104 is the 'coolest' (Teff = 80 000 K) member of the PG1159 class in a pre-WD stage. Its optical spectrum is remarkable because it exhibits C{\^a}€» IV lines involving Rydberg states with principal quantum numbers up to n = 22. Its rather low mass (0.48-0.02+0.03 M·) is difficult to reconcile with the common evolutionary scenario for PG1159 stars due to it being the outcome of a (very) late He-shell flash. The same mass-problem faces a merger model of a close He-sdO plus CO WD binary that predicts PG1159-like abundances. Perhaps RL 104 originates from a very late He-shell flash in a CO/He WD formed by a merger of two low-mass He-WDs.},
  language  = {en}
}
@article{LoeblingRauchBertolamiMilleretal.2019,
  author    = {L{\"o}bling, Lisa and Rauch, Thomas and Bertolami Miller, Marcelo Miguel and Todt, Helge Tobias and Friederich, F. and Ziegler, M. and Werner, Klaus and Kruk, J. W.},
  title     = {Spectral analysis of the hybrid PG 1159-type central stars of the planetary nebulae Abell 43 and NGC7094},
  series = {Monthly notices of the Royal Astronomical Society},
  volume    = {489},
  journal   = {Monthly notices of the Royal Astronomical Society},
  number    = {1},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  issn      = {0035-8711},
  doi       = {10.1093/mnras/stz1994},
  pages     = {1054 -- 1071},
  year      = {2019},
  abstract  = {Stellar post asymptotic giant branch (post-AGB) evolution can be completely altered by a final thermal pulse (FTP) which may occur when the star is still leaving the AGB (AFTP), at the departure from the AGB at still constant luminosity (late TP, LTP) or after the entry to the white-dwarf cooling sequence (very late TP, VLTP). Then convection mixes the Herich material with the H-rich envelope. According to stellar evolution models the result is a star with a surface composition of H approximate to 20 per cent by mass (AFTP), approximate to 1 per cent (LTP), or (almost) no H (VLTP). Since FTP stars exhibit intershell material at their surface, spectral analyses establish constraints for AGB nucleosynthesis and stellar evolution. We performed a spectral analysis of the so-called hybrid PG 1159-type central stars (CS) of the planetary nebulae Abell 43 and NGC7094 by means of non-local thermodynamical equilibrium models. We confirm the previously determined effective temperatures of T-eff = 115 000 +/- 5 000K and determine surface gravities of log (g /(cm s(-2))) = 5.6 +/- 0.1 for both. From a comparison with AFTP evolutionary tracks, we derive stellar masses of 0.57(-0.04)(+0.07)M(circle dot) and determine the abundances of H, He, and metals up to Xe. Both CS are likely AFTP stars with a surface H mass fraction of 0.25 +/- 0.03 and 0.15 +/- 0.03, respectively, and an Fe deficiency indicating subsolar initial metallicities. The light metals show typical PG 1159-type abundances and the elemental composition is in good agreement with predictions from AFTP evolutionary models. However, the expansion ages do not agree with evolution time-scales expected from the AFTP scenario and alternatives should be explored.},
  language  = {en}
}
@misc{MaierWolfKeiligetal.2018,
  author    = {Maier, Philipp and Wolf, J{\"u}rgen and Keilig, Thomas and Krabbe, Alfred and Duffard, Rene and Ortiz, Jose-Luis and Klinkner, Sabine and Lengowski, Michael and M{\"u}ller, Thomas and Lockowandt, Christian and Krockstedt, Christian and Kappelmann, Norbert and Stelzer, Beate and Werner, Klaus and Geier, Stephan Alfred and Kalkuhl, Christoph and Rauch, Thomas and Schanz, Thomas and Barnstedt, J{\"u}rgen and Conti, Lauro and Hanke, Lars},
  title     = {Towards a European Stratospheric Balloon Observatory},
  series = {Ground-based and Airborne Telescopes VII},
  volume    = {10700},
  journal   = {Ground-based and Airborne Telescopes VII},
  publisher = {SPIE-INT Soc Optical Engineering},
  address   = {Bellingham},
  isbn      = {978-1-5106-1954-8},
  issn      = {0277-786X},
  doi       = {10.1117/12.2319248},
  pages     = {12},
  year      = {2018},
  abstract  = {This paper presents the concept of a community-accessible stratospheric balloon-based observatory that is currently under preparation by a consortium of European research institutes and industry. We present the technical motivation, science case, instrumentation, and a two-stage image stabilization approach of the 0.5-m UV/visible platform. In addition, we briefly describe the novel mid-sized stabilized balloon gondola under design to carry telescopes in the 0.5 to 0.6 m range as well as the currently considered flight option for this platform. Secondly, we outline the scientific and technical motivation for a large balloon-based FIR telescope and the ESBO DS approach towards such an infrastructure.},
  language  = {en}
}
@article{RichterdeBoerWerneretal.2015,
  author    = {Richter, Philipp and de Boer, Klaas S. and Werner, Klaus and Rauch, Thomas},
  title     = {High-velocity gas toward the LMC resides in the Milky Way halo},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {584},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {1432-0746},
  doi       = {10.1051/0004-6361/201527451},
  pages     = {4},
  year      = {2015},
  abstract  = {Aims. To explore the origin of high-velocity gas in the direction of the Large Magellanic Cloud, (LMC) we analyze absorption lines in the ultraviolet spectrum of a Galactic halo star that is located in front of the LMC at d = 9.2(-7.2)(+4.1) kpc distance. Methods. We study the velocity-component structure of low and intermediate metal ions (CII, SiII, SiIII) in the spectrum of RXJ0439.8-6809, as obtained with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST), and measure equivalent widths and column densities for these ions. We supplement our COS data with a Far-Ultraviolet Spectroscopic Explorer (FUSE) spectrum of the nearby LMC star Sk-69 59 and with Hi 21 cm data from the Leiden-Argentina-Bonn (LAB) survey. Results. Metal absorption toward RXJ0439.8-6809 is unambiguously detected in three different velocity components near v(LSR) = 0, + 60, and + 150 km s(-1). The presence of absorption proves that all three gas components are situated in front of the star, thus located in the disk and inner halo of the Milky Way. For the high-velocity cloud (HVC) at v(LSR) = + 150 km s(-1), we derive an oxygen abundance of [O/H] = -0.63 (similar to 0.2 solar) from the neighboring Sk-69 59 sight line, in accordance with previous abundance measurements for this HVC. From the observed kinematics we infer that the HVC hardly participates in the Galactic rotation. Conclusions. Our study shows that the HVC toward the LMC represents a Milky Way halo cloud that traces low column density gas with relatively low metallicity. We rule out scenarios in which the HVC represents material close to the LMC that stems from a LMC outflow.},
  language  = {en}
}
@article{ReindlRauchWerneretal.2014,
  author    = {Reindl, Nicole and Rauch, Thomas and Werner, Klaus and Kruk, J. W. and Todt, Helge Tobias},
  title     = {On helium-dominated stellar evolution: the mysterious role of the O(He)-type stars},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {566},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {0004-6361},
  doi       = {10.1051/0004-6361/201423498},
  pages     = {23},
  year      = {2014},
  abstract  = {Context. About a quarter of all post-asymptotic giant branch (AGB) stars are hydrogen-deficient. Stellar evolutionary models explain the carbon-dominated H-deficient stars by a (very) late thermal pulse scenario where the hydrogen-rich envelope is mixed with the helium-rich intershell layer. Depending on the particular time at which the final flash occurs, the entire hydrogen envelope may be burned. In contrast, helium-dominated post-AGB stars and their evolution are not yet understood. Aims. A small group of very hot, helium-dominated stars is formed by O(He)-type stars. A precise analysis of their photospheric abundances will establish constraints to their evolution. Methods. We performed a detailed spectral analysis of ultraviolet and optical spectra of four O(He) stars by means of state-of-the-art non-LTE model-atmosphere techniques. Results. We determined effective temperatures, surface gravities, and the abundances of H, He, C, N, O, F, Ne, Si, P, S, Ar, and Fe. By deriving upper limits for the mass-loss rates of the O(He) stars, we found that they do not exhibit enhanced mass-loss. The comparison with evolutionary models shows that the status of the O(He) stars remains uncertain. Their abundances match predictions of a double helium white dwarf (WD) merger scenario, suggesting that they might be the progeny of the compact and of the luminous helium-rich sdO-type stars. The existence of planetary nebulae that do not show helium enrichment around every other O(He) star precludes a merger origin for these stars. These stars must have formed in a different way, for instance via enhanced mass-loss during their post-AGB evolution or a merger within a common-envelope (CE) of a CO-WD and a red giant or AGB star. Conclusions. A helium-dominated stellar evolutionary sequence exists that may be fed by different types of mergers or CE scenarios. It appears likely that all these pass through the O(He) phase just before they become WDs.},
  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{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{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{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{WernerDreizlerPakulletal.1996,
  author    = {Werner, Klaus and Dreizler, S. and Pakull, M. W. and Cowley, A. P. and Schmidtke, P. C. and Hutchings, J. B. and Crampton, D.},
  title     = {Non-LTE model atmosphere analysis of the supersoft X-ray source RX J0122.9-7521},
  year      = {1996},
  language  = {en}
}