Age Constraints for an M31 Globular Cluster from Main Sequence   Photometry
  We present a color-magnitude diagram (CMD) of the globular cluster SKHB-312 in the Andromeda galaxy (M31), obtained with the Advanced Camera for Surveys on the Hubble Space Telescope. The cluster was included in deep observations taken to measure the star formation history of the M31 halo. Overcoming a very crowded field, our photometry of SKHB-312 reaches V ~ 30.5 mag, more than 1 mag below the main sequence turnoff. These are the first observations to allow a direct age estimate from the turnoff in an old M31 cluster. We analyze its CMD and luminosity function using a finely-spaced grid of isochrones that have been calibrated using observations of Galactic clusters taken with the same camera and filters. The luminosity difference between the subgiant and horizontal branches is ~0.2 mag smaller in SKHB-312 than in the Galactic clusters 47 Tuc and NGC 5927, implying SKHB-312 is 2-3 Gyr younger. A quantitative comparison to isochrones yields an age of 10 +2.5/-1 Gyr. 
Thomas M. Brown, Henry C. Ferguson, Ed Smith (STScI), Randy A. Kimble,   Allen V. Sweigart (GSFC), Alvio Renzini (ESO), R. M...