Roy Østensen (Katholieke Universiteit Leuven in Belgium): Hot subdwarf stars: formation and evolution

The hot subdwarf stars are normally identified with extreme horizontal branch stars, a group of core helium-burning stars with extremely thin hydrogen envelopes. Their high temperatures and thin envelopes makes them exceedingly bright in the far-UV part of the spectrum. A long standing problem in galactic astronomy is the far-UV excess observed in most elliptical galaxies. Since ellipticals are not supposed to contain young hot stars, this UV excess is now commonly attributed to the evolved hot subdwarf stars. For this reason investigations of the evolutionary properties of these stars have gained significant interest in the astronomical community. In particular, the ongoing UV survey with the Galaxy Evolution Explorer (GALEX) satellite has this type of investigation as a primary objective. However, the formation of hot subdwarf stars is still somewhat mysterious. The problem is exactly how these stars, unlike normal horizontal branch stars, manage to loose almost its entire hydrogen envelope on the red giant branch. Solving these formation problems is essential for determining how the spectral properties of galaxies evolve with redshift distance.