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D.R. Alves, H.E. Bond (STScI)
Population~II stars evolving off the asymptotic giant branch (AGB) and passing through spectral types F and A are excellent candidates for a new extragalactic standard candle. These ``yellow'' post-AGB (PAGB) stars are the visually brightest members of old populations. They should have a narrow luminosity function (LF), because essentially a single main-sequence turnoff mass is feeding the PAGB region of the Hertzprung-Russell diagram. We have recently published new photometry of the southern globular cluster NGC\,5986, which contains two yellow PAGB stars. Based on these data, and published photometry of the star ROA\,24 in \omega~Cen, we have suggested that the V-band LF of yellow PAGB stars is sharply peaked at MV(PAGB) = -3.28 ±0.07.
Yellow PAGB stars are easily recognized by their enormous Balmer jumps. In order to develop these stars as standard candles, we have devised a hybrid photometric system combining Gunn's u band with the Johnson-Kron-Cousins BVI\, bands. The resulting high-throughput uBVI\, photometric system is optimally sensitive to surface gravity for A-F-type stars. We describe the standardization of this system. Repeated uBVI\, observations using the CTIO and KPNO 0.9-m telescopes have been obtained of Landolt standard stars in fields around the celestial equator on nearly 50 photometric nights between 1996 and 1999. We also present uBVI\, photometry of yellow dwarfs, giants, and supergiants, including known examples of Population~II yellow PAGB stars in the field and in globular clusters.
Supported by NASA grant NAG5-6821 under the ``UV, Visible, and Gravitational Astrophysics Research and Analysis'' program.