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On Helium-Dominated Stellar Evolution: The Mysterious Role of the O(He)-Type StarsContext. 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.
Document ID
20150001302
Acquisition Source
Goddard Space Flight Center
Document Type
Reprint (Version printed in journal)
Authors
Reindl, N.
(Eberhard Karls Universitat Tuebingen Tuebingen, Germany)
Rauch, T.
(Eberhard Karls Universitat Tuebingen Tuebingen, Germany)
Werner, K.
(Eberhard Karls Universitat Tuebingen Tuebingen, Germany)
Kruk, J. W.
(NASA Goddard Space Flight Center Greenbelt, MD United States)
Todt, H.
(Potsdam Univ. Potsdam, Germany)
Date Acquired
February 3, 2015
Publication Date
June 25, 2014
Publication Information
Publication: Astronomy & Astrophysics
Publisher: Astronomy & Astrophysics
Volume: 566
Issue: A&A
Subject Category
Astronomy
Astrophysics
Report/Patent Number
GSFC-E-DAA-TN17940
Funding Number(s)
CONTRACT_GRANT: NAS5-26555
CONTRACT_GRANT: NNX09AF08G
CONTRACT_GRANT: NAS5-26666
Distribution Limits
Public
Copyright
Public Use Permitted.
Keywords
evolution
helium-dominated
stellar
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