Extreme Horizontal Branch Stars

TL;DR

This review discusses the properties, origins, and evolutionary links of extreme Horizontal Branch stars, highlighting their binary nature, formation channels, and significance in asteroseismology and galactic dynamics.

Contribution

It provides a comprehensive overview of the current understanding of hot subluminous stars, including recent discoveries and models of their formation and evolution.

Findings

Many field sdBs are in close binaries with white dwarf or low-mass companions.

Some unseen companions may be neutron stars or black holes exceeding Chandrasekhar mass.

A hyper-velocity sdO star suggests ejection by the Galactic center black hole.

Abstract

A review is presented on the properties, origin and evolutionary links of hot subluminous stars which are generally believed to be extreme Horizontal Branch stars or closely related objects. Amongst the field stars a large fraction of sdBs are found to reside in close binaries. The companions are predominantly white dwarfs, or low mass main sequence stars. Systems with sufficiently massive WD companions may qualify as SN Ia progenitors. Recently evidence has been found that the masses of some unseen companions might exceed the Chandrasekhar mass, hence they must be neutron stars or black holes. Even a planet has recently been detected orbiting the pulsating sdB star V391 Peg. Quite to the opposite, in globular clusters, only very few sdB binaries are found indicating that the dominant sdB formation processes is different in a dense environment. Binary population synthesis models identify three formation channels, (i) stable Roche lobe overflow, (ii) one or two common envelope ejection phases and (iii) the merger of two helium white dwarfs. The latter channel may explain the properties of the He-enriched sdO stars because their binary fraction is lower than that of the sdBs by a factor of ten or more. Pulsating subluminous B (sdB) stars play an important role for asteroseismology as this technique has already led to mass determinations for a handful of stars. A unique 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. (abridged)