Clues about the Star Formation History of the M31 Disk from WFPC2 Photometry

Over the past several years, the Hubble Space Telescope (HST) has acquired many broad-band images of various regions in the M31 disk. I have obtained 27 such fields from the HST data archive in order to produce color-magnitude diagrams (CMDs) of the stellar populations contained within these areas of the disk. I have attempted to reproduce these CMDs using theoretical stellar evolution models in conjunction with statistical tools for determining the star formation history which best fits the observations. The wide range of extinction values within any given field makes the data difficult to accurately reproduce; nevertheless, I have managed to find star formation histories which roughly reproduce the data. These statistically determined star formation histories reveal that, like the disk of the Galaxy, the disk of M31 contains very few old metal-poor stars. The histories also suggest that the star formation rate of the disk as a whole has been low over the past ~1 Gyr