S. Moehler

The UV spectrum of the Galactic bulge

The elliptical galaxies show in their spectra at ultraviolet wavelengths an unexpected flux . SdB (alias called Extreme Horizontal Branch) stars are believed to be responsible of this feature. A spectroscopic analysis showed that sdB stars are present also in the bulge of our Galaxy. By means of spectroscopic and photometric data, the Galactic bulge integrated spectrum from the UV to the optical was constructed, showing that the Galactic bulge has only a very small (or not at all) UV excess. A parallel study of Horizontal Branch stars in two Globular clusters hints to more than one star formation episode in these systems.

Hot stars in globular clusters

Globular clusters are ideal laboratories to study the evolution of low-mass stars. In this review, I shall concentrate on two types of hot stars observed in globular clusters: horizontal branch stars and UV bright stars. The third type, the white dwarfs, are covered by Bono in this volume. While the morphology of the horizontal branch correlates strongly with metallicity, it has been known for a long time that one parameter is not sufficient to describe the diversity of observed horizontal branch morphologies. A veritable zoo of candidates for this elusive “2nd parameter” has been suggested over the past decades, and the most prominent ones will be briefly discussed here. Adding to the complications, diffusion is active in the atmospheres of hot horizontal branch stars, which makes their analysis much more diffcult. The latest twist along the horizontal branch was added by the recent discovery of an extension to hotter temperatures and fainter magnitudes, the so-called “blue hook”. The evolutionary origin of these stars is still under debate. I shall also give a brief overview of our current knowledge about hot UV bright stars and use them to illustrate the adverse effects of selection bias.

Subdwarf B–Stars and Binarity

Spectroscopic and photometric data give evidence for a large fraction of binary stars among hot subdwarfs. Since our work aims at determining atmospheric parameters and distances of sdB and sdOB stars, we have to take special care of this aspect. We try to deconvolve the photometric data and derive colours for the subdwarf component of the binary. The impact of binarity in hot subdwarfs to their evolutionary history is briefly discussed.