HUBBLE-SPACE-TELESCOPE OBSERVATIONS OF GLOBULAR-CLUSTERS IN M31 .1. COLOR-MAGNITUDE DIAGRAMS, HORIZONTAL-BRANCH METALLICITY DEPENDENCE, ANDTHE DISTANCE TO M31

This paper presents Hubble Space Telescope observations of four globul ar clusters in M31 with [Fe/H] ranging from -0.6 to -2.0. The Wide-Fie ld and Planetary Camera-2 (WFPC-2) imaged these clusters through the F 555W (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 st andard giant branches in V and I. In addition, two methods give simila r reddening estimates for each cluster. The color-magnitude diagrams o f the four clusters extend about 1 mag fainter than the horizontal bra nches 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. Su rface brightness fluctuation measurements on one cluster in the I band produced a distance modulus to M31 of 24.56+/-0.12, in statistical ag reement with the Cepheid distance modulus of 24.43 (0.77 Mpc) of Freed man & Madore [ApJ, 365, 186 (1990)]. The best estimate of the mean bri ghtnesses of RR Lyrae stars, which includes only three clusters, yield s [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 depen dence measured here is slightly lower than slopes of similar to 0.15 ( common in the literature), but the values agree within the errors. Slo pes of similar to 0.30 or higher appear less likely, Adopting a metall icity dependence of 0.15 for the three clusters yields a mean zero poi nt of 0.97+/-0.12 mag, implying that the Cepheid distance scale and th e RR Lyrae scale of Carney et al, [ApJ, 386, 663 (1992)] are in reason ably good agreement. However, current uncertainties about the WFPC-2 p hotometric behavior make final conclusions about the photometric zero point less certain. Additional M31 and other Local Group globular clus ter observations are needed to calibrate the RR Lyrae stars definitive ly relative to Cepheids. (C) 1996 American Astronomical Society.