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K. Z. Stanek, P. M. Garnavich (Harvard-Smithsonian CfA)
The ideal distance indicator would be a standard candle abundant enough to provide many examples within reach of parallax measurements and sufficiently bright to be seen out to local group galaxies. As developed by Paczynski & Stanek (1998), the red clump method precisely fits this description. These stars are the metal rich equivalent of the better known horizontal branch stars, and their brightness shows only a weak metallicity (Udalski 1998a) and age (Udalski 1998b) dependence. Indeed the absolute magnitude-color diagram of Hipparcos clearly shows how compact the red clump is: Stanek and Garnavich (1998a) determined that the variance in the I-band magnitude is only 0.15 mag.
To date the red clump method was used to measure accurate distances to the Galactic center (Paczynski & Stanek 1998), M31 (Stanek & Garnavich 1998a), LMC (Udalski et al. 1998; Stanek et al.1998) and several clusters within the Galaxy (e.g. 47Tuc: Kaluzny et al. 1998). As with all the distance indicators, the main worry lies in the possible systematics of the method, in this case its dependence on the stellar metallicity and age. Both these dependences were investigated by Udalski (1998a) and Udalski (1998b) using large samples of red clump stars collected by the OGLE collaboration. We are investigating these systematic effects using samples of globular clusters observed in M31 and M33 with the HST. Using the HST archival data and the fact that M31 and M33, including their clusters, span a broad range of ages and metallicities, we will be able to determine the age and metallicity dependence of the red clump brightness directly. The fact that the derived distance moduli for the three fields in M31 used by Stanek & Garnavich (1998a) vary by only 0.035 mag indicates that indeed the population effects on the red clump brightness are small.
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The author(s) of this abstract have provided an email address for comments about the abstract: kstanek@cfa.harvard.edu