No Increase of the Red Giant Branch Tip Luminosity toward the Center of M31

We present observations with the Hubble Space Telescope Wide Field PlanetaryCamera 2 of three fields centered on super-metal-rich globular clusters in thebulge of M31. Our (I,V-I) color-magnitude diagrams reach as faint as I ~ 26.5mag and clearly reveal the magnitude of the first ascent red giant branch (RGB)tip. We find that the apparent I magnitude of the RGB tip does not becomebrighter near the center of M31 as concluded by previous investigators. Ourobservations and artificial star experiments presented in this study stronglysupport the idea that previous very bright stars were likely the result ofspurious detections of blended stars due to crowding in lower resolutionimages. On the contrary, our observations indicate that, at a mean projectedgalactocentric distance of 1.1 kpc, the RGB tip is some 1.3 magnitudes fainterthan it is at 7 kpc. An analysis of this difference in RGB tip magnitudesuggests that the M31 bulge stellar population has a mean metallicity close tothat of the Sun.Comment: Accepted for publication in ApJ, June 20, 1999 issu