Session 8 - Distance Indicators.
Display session, Monday, January 15
North Banquet Hall, Convention Center

## [8.07] Post-AGB A and F Supergiants as Standard Candles

L. K. Fullton, H. E. Bond, A. Saha, K. G. Schaefer (STScI)

Low-mass stars leaving the asymptotic giant branch (AGB) and passing through spectral types F and A should, theoretically, have a very narrow luminosity function. The upper limit is set by the much shorter lifetimes of the more luminous post-AGB (PAGB) stars, and the lower limit corresponds to the turnoff mass of the oldest stars in the parent population. A handful of PAGB A-F supergiants are known in Milky Way globular clusters, and gratifyingly show a very small scatter around absolute magnitude M_V = -3.4. Moreover, PAGB A-F stars are readily recognized because of their enormous Balmer jumps, lie in regions of spirals that are relatively free of internal absorption, should also exist in ellipticals, and do not require a long time series of observations for their detection.

In order to calibrate PAGB stars as standard candles, we are searching for them with Gunn u plus Johnson-Kron-Cousins BVI CCD photometry in old populations of Local Group galaxies, and we report preliminary results here. In the halo of M31, we have used the KPNO 4-m telescope to find PAGB stars in the numbers expected from theoretical evolutionary lifetimes, with a scatter in absolute magnitude of only \sigma=0.3 mag. We have also used the Curtis Schmidt and 1.5-m telescope at CTIO to search for PAGB stars in the two Magellanic Clouds, and in NGC 6822 and IC 1613, in order to calibrate any metallicity effects.

Assuming that the predicted sharp luminosity function is confirmed within the Local Group, we next plan to apply the method to the Sculptor and M81 Groups with ground-based telescopes. The ultimate aim will be to use HST\/ and its Advanced Camera to determine the distance to the Virgo Cluster with this Population II'' candle, which will be directly calibrated within the Milky Way and entirely independent of the Population I Cepheid distance scale.

Supported by NASA Grant NAGW-4361.