Spectral analysis of four surprisingly similar hot hydrogen-rich subdwarf O stars
Abstract
Context. Post-extreme horizontal branch stars (post-EHB) are helium-shell burning objects evolving away from the EHB and contracting directly towards the white dwarf regime. While the stars forming the EHB have been extensively studied in the past, their hotter and more evolved progeny are not so well characterized.
Aims: We perform a comprehensive spectroscopic analysis of four such bright sdO stars, namely
Methods: We used non-local thermodynamic equilibrium model atmospheres in combination with high quality optical and UV spectra. Photometric data were also used to compute the spectroscopic distances of our stars and to characterize the companion responsible for the infrared excess of
Results: The four bright sdO stars have very similar atmospheric parameters with Teff between 60 000 and 63 000 K and log g (cm s-2) in the range 5.9 to 6.1. This places these objects right on the theoretical post-EHB evolutionary tracks. The UV spectra are dominated by strong iron and nickel lines and suggest abundances that are enriched with respect to those of the Sun by factors of 25 and 60. On the other hand, the lighter elements, C, N, O, Mg, Si, P, and S are depleted. The stars have very similar abundances, although
Conclusions: We performed a spectroscopic analysis of four hot sdO stars that are very similar in terms of atmospheric parameters and chemical compositions. The rotation velocities of our stars are significantly higher than what is observed in their immediate progenitors on the EHB, suggesting that angular momentum may be conserved as the stars evolve away from the EHB.
- Publication:
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Astronomy and Astrophysics
- Pub Date:
- January 2018
- DOI:
- 10.1051/0004-6361/201731496
- arXiv:
- arXiv:1710.06716
- Bibcode:
- 2018A&A...609A..89L
- Keywords:
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- stars: atmospheres;
- stars: abundances;
- subdwarfs;
- stars: fundamental parameters;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- Accepted for publication in Astronomy &