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Session 12 - Distance Scale.
Display session, Monday, January 13
Low-mass stars leaving the asymptotic giant branch (AGB) and passing through spectral types F and A should make excellent Population II standard candles. Theoretically, they should 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 progenitors. Moreover, PAGB A-F stars are readily recognized because of their enormous Balmer jumps. They should be found in the halos of spiral galaxies where internal absorption is minimized and also in elliptical galaxies.
As part of an ongoing project to calibrate PAGB stars as standard candles, we are searching for them in old populations of Local Group galaxies. We employ a hybrid CCD photometric system that combines the Gunn u filter with Johnson-Kron-Cousins BVI. We have used Kurucz/Hubeny model atmospheres and stellar emergent spectra along with filter transmission functions and instrument sensitivity curves to generate synthetic photometric indices that can be parameterized in terms of stellar \logg and T_eff.
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 observed several other Local Group galaxies, including the Magellanic Clouds, NGC 185, NGC 205, NGC 6822, and IC 1613.
A handful of A-F PAGB supergiants are members of Galactic globular clusters, and as predicted, these show a very small scatter around absolute magnitude M_V = -3.4. We have begun a systematic uBVI survey of the entire Galactic globular cluster system to look for additional PAGB member stars for use in a fundamental calibration that can be tied directly to distances from subdwarf parallaxes.
Supported by NASA Grant NAGW-4361.
Program listing for Monday