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Session 47 - Old Stellar Populations Beyond The Milky Way - II.
Oral session, Wednesday, June 11
North Main Hall A,
The visually brightest members of Population II are post-AGB (PAGB) stars passing through spectral types F and A (where the bolometric correction is smallest). These objects are 4 mag brighter than RR Lyrae stars.
Moreover, PAGB stars are easy to recognize because of their extremely large Balmer jumps. We have begun programs aimed at identifying PAGB stars in the halos of Local Group galaxies, and (as calibrators) in Milky Way globular clusters. We use a photometric system that combines Gunn u (lying below the Balmer jump) with the standard BVI.
Four PAGB stars in Galactic globulars have a mean M_V=-3.4. We have identified PAGB stars in several fields in the halo of M31, from uBVI frames obtained with the KPNO 4-m. Their number counts imply PAGB lifetimes of \sim25,000 yr, and their mean apparent magnitudes near V=21 reproduce the accepted M31 distance if we use the globular-cluster calibration. PAGB evolutionary tracks imply a mean mass slightly below 0.55M_ødot, in good agreement with masses of white dwarfs in globular clusters.
These PAGB stars should find application as primary standard candles, detectable as far as the Virgo Cluster with HST; and as tracers of the extent and dynamics of galaxy halos.
