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Session 19 - Old Stellar Populations in the Milky Way.
Invited session, Monday, June 09
North Main Hall A,
Hubble Space Telescope is enabling astronomers to expand the scope of a classical astronomical tool, the color--magnitude diagram. Stars well below the the main sequence turnoff are now being studied in detail in globular clusters, even in and near dense cluster cores. This talk will focus on some of the recent HST results concerning main-sequence stars, white dwarfs, and cataclysmic variables in globular clusters.
For the nearest globulars, color--magnitude diagrams extending to V\sim27 reach main sequence and white dwarf stars as faint as M_V\sim13, corresponding to main-sequence masses of close to 0.1 M_ødot, and white-dwarf cooling ages of about 1 Gyr. Measurements of main-sequence stars provide new constraints on models of low-mass, metal-poor stars, while also enabling detailed comparisons to be made of the stellar content of different clusters. Luminosity and mass functions obtained for four metal-poor clusters will be presented and discussed in light of current questions about the halo mass function. A CMD of the cluster NGC 6397 containing an extended white dwarf sequence will also be presented, together with comparisons to model WD sequences.
In globular clusters with sufficiently high stellar densities, close encounters between main-sequence and white-dwarf stars---or between primordial binaries containing them---should produce close binary stars. Cataclysmic variables could be the most common such binary collision product, but exceedingly few are known from ground-based studies. Multiple CVs have now been identified near the centers of two nearby clusters with HST. Recent results concerning the photometric variability and broad-band colors of CVs and other unusual blue stars in one cluster will be presented, and their implications discussed in the context of ongoing efforts to determine the efficacy of stellar encounters in producing close binary systems in globular clusters.
