Institut für Astronomie und Astrophysik Abteilung Astronomie Sand 1, D-72076 Tübingen, Germany

Dissertation Nicole Reindl

Spectroscopic Analysis of Hot (Pre-) White Dwarfs

Institut für Astronomie und Astrophysik

Summary.
In this work, different kinds of hot (pre-) white dwarfs (WD) were analyzed by means of static and expanding non-LTE model atmospheres to obtain a better understanding of the late, hot stages of stellar evolution.
In the first paper, Reindl et al. 2014a, we derived for the first time the temporal evolution of the atmospheric parameters of the unusually quick evolving, hydrogen-rich central star of planetary nebula (CSPN) SAO 244567. We confirm that SAO 244567 must be a low-mass star (M < 0.55 M_sun). The slow evolution of the respective canonical stellar evolutionary models is, however, in strong contradiction to the observed fast evolution and the young planetary nebula. We speculate that the star could be a late He-shell flash object. Alternatively, it could be the outcome of a close-binary evolution. Then SAO 244567 would be a low-mass (0.35 M_sun) helium pre-WD after the common-envelope phase, during which the planetary nebula was ejected.
The paper Reindl et al. 2014b aimed to obtain clues about the nature of the exotic O(He) stars. The new optical spectra allowed a more precise determination of the atmospheric parameters of the two CSPNe K 1-27 and LoTr 4. Furthermore, upper limits for the mass-loss rates of K 1-27, LoTr 4, HS2209+8229, and HS1522+6615 were derived. Finally, the atmospheric parameters of the O(He) stars were compared to those of other helium-dominated stars and stellar evolution calculations in order to explain a helium-dominated stellar evolution sequence.
In Reindl et al, 2014, we identified 24 DO WDs in the tenth data release of the Sloan Digital Sky Survey and analyzed them for the first time by means of non-LTE model atmospheres. Two of our objects are the coolest DO WDs ever discovered that still show a considerable amount of carbon in the atmosphere. This is in strong contradiction with diffusion calculations and we suggested that a weak mass-loss is present in DO WDs. The mass distribution of DO WDs beyond the wind limit strongly deviates from the mass distribution of the objects before the wind limit. We address this phenomenon to different evolutionary input channels. We argue that the DO WD channel may be fed by about 13\% by post-extreme-horizontal branch stars and that PG 1159 stars and O(He) stars may contribute in a similar extent to the non-DA WD channel.
The final paper DeMarco, 2014, contributes to the search for close binary CSPNe. We examined the Kepler lightcurves of five CSPNe and searched for photometric variabilities. My contribution to this paper was the determination of the atmospheric parameters of three CSPNe. By that, we could identify Pa 5 as the tenth known O(He) star. Although the photometric variability of only one CSPN can certainly be ascribed to a close companion, we conclude that there are a lot more short periodic close binaries than the currently estimated fraction of 15%.
 

Online-Publikation: http://hdl.handle.net/10900/62963

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