M31 globular clusters in the Hubble Space Telescope archive. I. Cluster detection and completeness

Globular clusters at the distance of M31 have apparent angular sizes of a f ew arcseconds. While many M31 globular clusters have been detected and stud ied from ground-based images, the high spatial resolution of the Hubble Spa ce Telescope (HST) allows much more robust detection and characterization o f star cluster properties. We present the results of a search of 157 HST Wi de Field Planetary Camera 2 images of M31. We found 82 previously cataloged globular cluster candidates, as well as 32 new globular cluster candidates and 20 open cluster candidates. We present images of the new candidates an d photometry for all clusters. We assess existing cluster catalogs' complet eness and use the results to estimate the total number of globular clusters in M31 as 460 +/- 70. The specific frequency is S-N = 1.2 +/- 0.2 and the mass specific frequency T = 2.4 +/- 0.4; these values are at the upper end of the range seen for spiral galaxies.