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

Abstract

We present observations with the Hubble Space Telescope Wide-Field Planetary Camera 2 of three fields centered on super-metal-rich globular clusters in the bulge of M31. Our (I, V-I) color-magnitude diagrams reach as faint as I~26.5 mag 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 become brighter near the center of M31 as concluded by previous investigators. Our observations and artificial star experiments presented in this study strongly support the idea that previous very bright stars were likely the result of spurious detections of blended stars due to crowding in lower resolution images. On the contrary, our observations indicate that, at a mean projected galactocentric distance of 1.1 kpc, the RGB tip is some 1.3 mag fainter than it is at 7 kpc. An analysis of this difference in RGB tip magnitude suggests that the M31 bulge stellar population has a mean metallicity close to that of the Sun.