TY  - JOUR
A1  - Werner, Klaus
A1  - Barstow, Martin A.
A1  - Holberg, J. B.
A1  - Koester, D.
A1  - Nousek, J. A.
T1  - Extreme ultraviolet spectroscopy of white dwarfs
Y1  - 1994
ER  - 
TY  - JOUR
A1  - Werner, Klaus
T1  - Spectra of GW Vir pulsators
Y1  - 1995
ER  - 
TY  - JOUR
A1  - Werner, Klaus
A1  - Dreizler, S.
A1  - Wolff, B.
T1  - Spectral analysis of the hot DO white dwarf PG 1034+001
Y1  - 1995
ER  - 
TY  - JOUR
A1  - Werner, Klaus
A1  - Dreizler, S.
A1  - Heber, Ulrich
A1  - Rauch, Thomas
T1  - Triple-alpha burning products on the surface of peculiar post-AGB stars
Y1  - 1995
ER  - 
TY  - JOUR
A1  - Werner, Klaus
A1  - Rauch, Thomas
A1  - Dreizler, S.
A1  - Heber, Ulrich
T1  - Confining the edges of the GW Vir Instability Strip
Y1  - 1995
ER  - 
TY  - JOUR
A1  - Werner, Klaus
A1  - Dreizler, S.
A1  - Heber, Ulrich
A1  - Rauch, Thomas
T1  - Confining the edges of the GW Vir instability strip
Y1  - 1995
ER  - 
TY  - JOUR
A1  - Werner, Klaus
A1  - Dreizler, S.
A1  - Heber, Ulrich
A1  - Rauch, Thomas
A1  - Wisotzki, Lutz
A1  - Hagen, H.-J.
T1  - Discovery of two hot DO white dwarfs exhibiting ultrahigh-excitation absorption lines
Y1  - 1995
ER  - 
TY  - JOUR
A1  - Werner, Klaus
A1  - Dreizler, S.
A1  - Pakull, M. W.
A1  - Cowley, A. P.
A1  - Schmidtke, P. C.
A1  - Hutchings, J. B.
A1  - Crampton, D.
T1  - Non-LTE model atmosphere analysis of the supersoft X-ray source RX J0122.9-7521
Y1  - 1996
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
A1  - Dreizler, S.
A1  - Heber, Ulrich
A1  - Rauch, Thomas
A1  - Fleming, T. A.
A1  - Sion, E. M.
A1  - Vauclair, G.
T1  - High resolution spectroscopy of two hot (pre-) white dwarfs with the Hubble space telescope : KPD 0005+5106 and RXJ 2117+3412
Y1  - 1996
ER  - 
TY  - JOUR
A1  - Werner, Klaus
A1  - Dreizler, S.
A1  - Heber, Ulrich
A1  - Rauch, Thomas
T1  - PG 1159 stars
Y1  - 1996
ER  - 
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  - Werner, Klaus
A1  - Dreizler, S.
A1  - Rauch, Thomas
A1  - Koesterke, Lars
A1  - Heber, Ulrich
T1  - Born-again AGB stars : staring point of the H-deficient post-AGB evolutionary sequence?
Y1  - 1999
ER  - 
TY  - JOUR
A1  - Werner, Klaus
A1  - Dreizler, S.
A1  - Rauch, Thomas
A1  - Barnstedt, Jürgen
A1  - Göz, M.
A1  - Gringel, W.
A1  - Kappelmann, N.
A1  - Krämer, G.
A1  - Widmann, H.
A1  - Koesterke, Lars
A1  - Haas, S.
A1  - Heber, Ulrich
A1  - Appenzeller, Immo
T1  - FUV spectroscpy of DO an PG 1159 stars with ORFEUS
Y1  - 1999
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  - 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  -