Subluminous O Stars - Origin and Evolutionary Links

TL;DR

This paper reviews the origin and evolution of hot subluminous O stars, discussing their binary nature, spectral properties, and possible formation scenarios, highlighting the white dwarf merger as a favored model.

Contribution

It synthesizes recent spectral analyses and evaluates different evolutionary models, emphasizing the role of binary interactions and mergers in sdO star development.

Findings

Helium-deficient sdO stars are likely successors to sdB stars.

Helium-enriched sdO stars can be explained by merger or hot flasher scenarios.

White dwarf mergers best explain the scarcity of close binaries among helium-enriched sdO stars.

Abstract

Hot subluminous stars can be roughly divided into B- and O-types. Unlike the latter many sdBs are found in close binaries, indicating that binary evolution plays a vital role. Recent NLTE spectral analyses revealed that an evolutionary link between sdB stars and sdO stars is plausible only for the helium-deficient sdO stars, i.e. they are the likely successors to sdB stars. The atmospheric properties of helium-enriched sdO stars can be explained by the late hot flasher as well as by the white-dwarf merger scenarios, although both models do not match the observed properties of helium-enriched sdO stars in detail. The white dwarf merger scenario is favoured because it naturally explains the scarcity of close binaries amongst helium-enriched sdO stars. A hyper-velocity sdO star moving so fast that it is unbound to the Galaxy has probably been ejected by the super-massive black hole in the Galactic centre.