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Session 19 - Source Surveys, Galaxy Surveys, Distance Scale II.
Display session, Wednesday, January 07
Exhibit Hall,
We are conducting a survey of post--asymptotic-giant-branch (PAGB) stars throughout the old populations of the Local Group, in order to assess the utility of PAGB stars as standard candles, as probes of the extent and structure of old populations in galaxies, and as tracers of late evolutionary stages of low-mass stars.
We have developed a CCD photometric system combining Gunn u and Johnson-Kron-Cousins BVI that can be used to select PAGB candidates. The low gravities of PAGB stars, as they evolve through types F and A, give them enormous Balmer jumps, making them easily identifiable with uBVI photometry.
A survey of the entire Galactic globular-cluster system is underway to identify PAGB members. These will be used to establish a local luminosity calibration, based on subdwarf parallaxes and other cluster distance indicators.
CCD uBVI photometry will be presented for PAGB candidates in the Magellanic Clouds and the halos of most of the more distant members of the Local Group.
In NGC 205, a dwarf elliptical companion of M31, we have identified a sequence of PAGB stars whose apparent magnitudes place NGC 205 15% more distant than M31 itself. This is in good agreement with other distance indicators, but the measurement took only one hour\/ of 4-meter time.
Other astrophysical results, based on PAGB stars we have found in the halo of M31, include estimates of the PAGB evolutionary timescale (\sim25,000 yr) and of the typical masses of PAGB stars in old populations (about 0.53\,M_ødot).
