Yellow Post-Asymptotic-Giant-Branch Stars as Standard Candles. I.
  Calibration of the Luminosity Function in Galactic Globular Clusters
  We use results of a survey for low-surface-gravity stars in Galactic (and
LMC) globular clusters to show that "yellow" post-asymptotic-giant-branch
(yPAGB) stars are likely to be excellent extragalactic standard candles,
capable of producing distances to early-type galaxies that are accurate to a
few percent. We show that the mean bolometric magnitude of the 10 known yPAGB
stars in globular clusters is <Mbol> = -3.38 +/- 0.03, a value that is ~0.2 mag
brighter than that predicted from the latest post-horizontal-branch
evolutionary tracks. More importantly, we show that the observed dispersion in
the distribution is only 0.10 mag, i.e., smaller than the scatter for
individual Cepheids. We describe the physics that can produce such a small
dispersion, and show that, if one restricts surveys to the color range 0 <
(B-V)0 < 0.5, then samples of non-variable yPAGB stars can be identified quite
easily with a minimum of contamination. The bright absolute V magnitudes of
these stars (<Mv> = -3.37) make them, by far, the visually brightest objects in
old stellar populations and ideal Population II standard candles for
measurements out to ~10 Mpc with current instrumentation. A Hubble Space
Telescope survey in the halos of galaxies in the M81 and Sculptor groups could
therefore serve as an effective cross-check on both the Cepheid and TRGB
distance scales.
