Hubble space telescope observations of globular clusters in M31 .1. Color-magnitude diagrams, horizontal branch metallicity dependence, and the distance to M31

This paper presents Hubble Space Telescope observations of four globular clusters in M31 with [Fe/H] ranging from -0.6 to -2.0. The Wide-Field and Planetary Camera-2 (WFPC-2) imaged these clusters through the F555W (V) and F814W (I) filters with total exposure times of 2000 s in each color. The ground-based spectroscopic metallicities are generally confirmed by the shapes of the red giant branches when compared to standard giant branches in V and I. In addition, two methods give similar reddening estimates for each cluster. The color-magnitude diagrams of the four clusters extend about 1 mag fainter than the horizontal branches of the clusters, allowing easy identification of the horizontal branches. Horizontal branch morphologies change from blue to red with increasing metallicity as expected from Galactic globular clusters. Surface brightness fluctuation measurements on one cluster in the I band produced a distance modulus to M31 of 24.56+/-0.12, in statistical agreement with the Cepheid distance modulus of 24.43 (0.77 Mpc) of Freedman & Madore [ApJ, 365, 186 (1990)]. The best estimate of the mean brightnesses of RR Lyrae stars, which includes only three clusters, yields [M(V)(RR)]=(0.08+/-0.13) [Fe/H]+(0.88+/-0.21), where the zero point assumes a distance modulus to M31 of 24.43. The mild metallicity dependence measured here is slightly lower than slopes of similar to 0.15 (common in the literature), but the values agree within the errors. Slopes of similar to 0.30 or higher appear less likely, Adopting a metallicity dependence of 0.15 for the three clusters yields a mean zero point of 0.97+/-0.12 mag, implying that the Cepheid distance scale and the RR Lyrae scale of Carney et al, [ApJ, 386, 663 (1992)] are in reasonably good agreement. However, current uncertainties about the WFPC-2 photometric behavior make final conclusions about the photometric zero point less certain. Additional M31 and other Local Group globular cluster observations are needed to calibrate the RR Lyrae stars definitively relative to Cepheids. (C) 1996 American Astronomical Society.