ar X iv : a st ro - p h / 97 02 00 8 v 1 1 F eb 1 99 7 Post - AGB Stars as Standard Extragalactic Candles

Stars evolving off the asymptotic giant branch and passing through spectral types F and A are excellent candidates for a new extragalactic standard candle. These post-AGB (PAGB) stars are the visually brightest members of Population II systems. They should have a narrow luminosity function, bounded from above by the shorter transition times of more massive and more luminous remnants, and from below by the core mass corresponding to the lowest-mass stars that are leaving the main sequence. Moreover, PAGB A-F supergiants are easily recognized because of their enormous Balmer jumps, and should lie both in ellipticals and the halos of spirals. I describe a photometric system that combines the Gunn u filter (lying below the Balmer jump) with the standard Johnson-Kron-Cousins BVI bandpasses, and report a successful search for PAGB stars in the halo of M31 using this uBVI system. The zero-point calibration will come from PAGB A and F stars in Galactic globular clusters. Four are presently known, and have a mean MV = −3.4 with a scatter of only 0.2 mag. Two are in the same cluster, NGC 5986, and their V magnitudes differ by only 0.09 mag, strongly suggesting a narrow luminosity function. Adopting this MV and calculating the M31 distance from its halo PAGB stars, we exactly reproduce the accepted value. Future plans include a uBVI survey of all Milky Way globular clusters for PAGB stars in order to strengthen the zero-point calibration, and a survey of Local Group galaxies to check the calibration. Ultimately we believe we can reach the Virgo Cluster through a distance ladder with only three rungs: subdwarf parallaxes, Milky Way globular clusters, and then directly to Virgo (with HST).