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Paper: The Origin of Subdwarf O Stars
Volume: 392, Hot Subdwarf Stars and Related Objects
Page: 139
Authors: Napiwotzki, R.
Abstract: An overview is given on the evolutionary status of the different types of subdwarf O stars (sdO). The hydrogen-rich sdOs are well explained in the context of stars evolving away from the asymptotic giant branch (AGB) or extreme horizontal branch (EHB). The origin of helium-rich sdO stars (He-sdO) is still unclear. Significant progress has been made recently in placing them in the Hertzsprung–Russell diagram. However, the question of their progenitors is still unsolved. It is unclear which types of stars evolve into He-sdOs and whether they can be formed by all galactic populations. Here I will present results of a radial velocity (RV) survey of compact He-sdOs. It turns out that the close binary frequency among them is very low, which is a strong argument against a direct evolutionary link with the subdwarf B stars (sdB). Absolute RVs are used to perform a statistical analysis of the membership of observed He-sdOs in the different galactic populations (thin disk, thick disk, halo). The observational evidence available so far is best explained with merging of two He-core white dwarfs being the dominant formation mechanism for compact He-sdOs.
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