Extreme Horizontal Branch Stars
Ulrich Heber

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…
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Taxonomy
TopicsStellar, planetary, and galactic studies · Astrophysics and Star Formation Studies · Gamma-ray bursts and supernovae
