Hot subluminous stars

Abstract

Hot subluminous stars of spectral type B and O are core helium-burning stars at the blue end of the horizontal branch or have evolved even beyond that stage. Strikingly, the distribution in the Hertzsprung-Russell diagram of He-rich vs. He-poor hot subdwarf stars of the globular clusters omega Cen and NGC~2808 differ from that of their field counterparts. The metal-abundance patterns of hot subdwarfs are typically characterized by strong deficiencies of some lighter elements as well as large enrichments of heavy elements. A large fraction of sdB stars are found in close binaries with white dwarf or very low-mass main sequence companions, which must have gone through a common-envelope phase of evolution.They provide a clean-cut laboratory to study this important but yet purely understood phase of stellar evolution. Substellar companions to sdB stars have also been found. For HW~Vir systems the companion mass distribution extends from the stellar into the brown dwarf regime. A giant planet to the pulsator V391 Peg was the first discovery of a planet that survived the red giant evolution of its host star. Several types of pulsating star have been discovered among hot subdwarf stars, the most common are the gravity-mode sdB pulsators (V1093 Her) and their hotter siblings, the p-mode pulsating V361 Hya stars. Another class of multi-periodic pulsating hot subdwarfs has been found in the globular cluster omega Cen that is unmatched by any field star. The masses of hot subdwarf stars are the key to understand the stars' evolution. A few pulsating sdB stars in eclipsing binaries have been found that allow mass determination. The results are in good agreement with predictions from binary population synthesis. New classes of binaries, hosting extremely low mass (ELM) white dwarfs (M<0.3 Msun), have recently been discovered, filling a gap in the mosaic of binary stellar evolution. (abbreviated)