Hubble Space Telescope WFPC2 Photometry of M33: Properties of the Halo Star Clusters and Surrounding Fields*

Abstract

We present CCD photometry for 10 fields centered on halo star clusters in the nearby spiral galaxy M33. The data are based on Hubble Space Telescope Wide-Field Planetary Camera 2 observations in the F555W (~V) and F814W (~I) filters. The resultant color-magnitude diagrams (CMDs) extend ~1.5 mag below the horizontal branch (HB). From these diagrams, we draw the following conclusions: Eight of the 10 M33 halo clusters possess HBs that are located entirely redward of the RR Lyrae instability strip. For nine of these clusters, we are able to use the slope of the red giant branch to measure metal abundances; we find their metal abundances to be in the range -1.64 ≥ [Fe/H] ≥ -0.65 with a mean of [Fe/H] = -1.27 ± 0.11. Utilizing this information, we are able to place these objects in the [Fe/H]-HB type diagram from which we conclude that the halo clusters in M33 suffer from the second-parameter effect; this suggests that another parameter, in addition to metal abundance, is affecting the morphology of their HBs. We present several arguments in favor of the hypothesis that this second parameter is cluster age. If this is indeed the case, while two of these clusters are likely to be as old as Galactic globular clusters at their metallicity, the remaining seven clusters have a mean age of ~7 Gyr. Using the two clusters in which the magnitude of the RR Lyrae variables can be inferred and adopting the RR Lyrae luminosity-metallicity relation favored by Chaboyer, we calculate a distance modulus of (m - M)0 = 24.84 ± 0.16 for M33. Our CMDs of the fields onto which the clusters are projected reveal the M33 disk population to be quite complicated. Our main conclusion regarding the disk field stars is that the youngest population is concentrated toward the center of M33.