Star Clusters in M33. III. The Youngest Population

Abstract

We investigate the properties of young ({\lt}=10^8 yr) M33 star clustersfrom the samples presented in the first two papers of this series usingfar-UV photometry. Previously, UBV colors were used to derive clusterages, luminosities, and masses, assuming an extinction interpolated fromneighboring stars. Fifteen of 44 clusters imaged in the Hubble SpaceTelescope WFPC2 with the F170W filter are detected. The far-UVmagnitudes provided by this filter are used to rederive young clusterages by comparing integrated photometry with stellar evolutionarymodels. Overall, we find ages consistent with those from our secondpaper from UBV photometry. However, the addition of the F170W fluxprovides three major improvements over previous work: (1) This band ismore sensitive to the temperature range of clusters younger than ~10^8yr. (2) UBV colors for young clusters with contaminated V-band fluxes(from red supergiants or from line emission by surrounding excited gas)provide only an upper age limit. The addition of the far-UV flux allowsus to extract precise ages. (3) Far-UV flux helps to constrain clusterextinction. New theoretical M/L_V ratios are presented for cluster agesbetween 4x10^6 and 10^10 yr and metallicities of (Z=0.02, Y=0.28),(Z=0.008, Y=0.25), (Z=0.004, Y=0.24), (Z=0.001, Y=0.23), and (Z=0.0004,Y=0.23), extracted from the recent models of C. Chiosi. These M/L_Vvalues are used to estimate cluster masses from derived ages andmeasured luminosities. We find young clusters to have masses in therange 6x10^2-2x10^4 M_solar. These values are smaller than the mostmassive old clusters in M33 (which have masses up to a few times 10^5M_solar) in the sample presented in our second paper.