TY - JOUR A1 - Dreizler, S. A1 - Werner, Klaus A1 - Rauch, Thomas A1 - Koesterke, Lars A1 - Heber, Ulrich T1 - NLTE Analyses of PG 1159 stars : Contraints for the structure and evolutiuon of Post-AGB stars Y1 - 1997 ER - TY - JOUR A1 - Koesterke, Lars A1 - Werner, Klaus T1 - Determination of mass-loss rates of PG 1159 stars from FUV spectroskopy Y1 - 1998 ER - TY - JOUR A1 - Löbling, Lisa A1 - Rauch, Thomas A1 - Bertolami Miller, Marcelo Miguel A1 - Todt, Helge Tobias A1 - Friederich, F. A1 - Ziegler, M. A1 - Werner, Klaus A1 - Kruk, J. W. T1 - Spectral analysis of the hybrid PG 1159-type central stars of the planetary nebulae Abell 43 and NGC7094 JF - Monthly notices of the Royal Astronomical Society N2 - 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. KW - stars: abundances KW - stars: AGB and post-AGB KW - stars: atmospheres KW - stars: evolution KW - stars: individual: WD1751+106 KW - stars: individual: WD2134+125 Y1 - 2019 U6 - https://doi.org/10.1093/mnras/stz1994 SN - 0035-8711 SN - 1365-2966 VL - 489 IS - 1 SP - 1054 EP - 1071 PB - Oxford Univ. Press CY - Oxford ER - TY - GEN A1 - Maier, Philipp A1 - Wolf, Jürgen A1 - Keilig, Thomas A1 - Krabbe, Alfred A1 - Duffard, Rene A1 - Ortiz, Jose-Luis A1 - Klinkner, Sabine A1 - Lengowski, Michael A1 - Müller, Thomas A1 - Lockowandt, Christian A1 - Krockstedt, Christian A1 - Kappelmann, Norbert A1 - Stelzer, Beate A1 - Werner, Klaus A1 - Geier, Stephan A1 - Kalkuhl, Christoph A1 - Rauch, Thomas A1 - Schanz, Thomas A1 - Barnstedt, Jürgen A1 - Conti, Lauro A1 - Hanke, Lars T1 - Towards a European Stratospheric Balloon Observatory BT - the ESBO design study T2 - Ground-based and Airborne Telescopes VII N2 - 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. KW - astronomy KW - balloon telescopes KW - UV KW - far infrared KW - detectors KW - observatory Y1 - 2018 SN - 978-1-5106-1954-8 U6 - https://doi.org/10.1117/12.2319248 SN - 0277-786X SN - 1996-756X VL - 10700 PB - SPIE-INT Soc Optical Engineering CY - Bellingham ER - TY - JOUR A1 - Reindl, Nicole A1 - Rauch, Thomas A1 - Werner, Klaus A1 - Kruk, J. W. A1 - Todt, Helge Tobias T1 - On helium-dominated stellar evolution: the mysterious role of the O(He)-type stars JF - Astronomy and astrophysics : an international weekly journal N2 - 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. KW - stars: AGB and post-AGB KW - stars: evolution KW - stars: fundamental parameters KW - stars: abundances Y1 - 2014 U6 - https://doi.org/10.1051/0004-6361/201423498 SN - 0004-6361 SN - 1432-0746 VL - 566 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Richter, Philipp A1 - de Boer, Klaas S. A1 - Werner, Klaus A1 - Rauch, Thomas T1 - High-velocity gas toward the LMC resides in the Milky Way halo JF - Astronomy and astrophysics : an international weekly journal N2 - 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. KW - Galaxy: halo KW - Galaxy: evolution KW - ISM: abundances KW - ISM: structure Y1 - 2015 U6 - https://doi.org/10.1051/0004-6361/201527451 SN - 1432-0746 VL - 584 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Werner, Klaus T1 - On the Balmer line problem Y1 - 1996 ER - TY - JOUR A1 - Werner, Klaus T1 - Search for trace amounts of hydrogen in hot DO white dwarfs Y1 - 1996 ER - TY - JOUR A1 - Werner, Klaus T1 - X-ray emission from hot hydrogen-deficient white dwarfs Y1 - 1996 ER - TY - JOUR A1 - Werner, Klaus T1 - Spectra of GW Vir pulsators Y1 - 1995 ER -