Star clusters in M33. III. The youngest population

We investigate the properties of young (less than or equal to 10(8) yr) M33 star clusters from the samples presented in the first two papers of this series using far-UV photometry. Previously, UBV colors were used to derive cluster ages, luminosities, and masses, assuming an extinction interpolated from neighboring stars. Fifteen of 44 clusters imaged in the Hubble Space Telescope WFPC2 with the F170W filter are detected. The far-UV magnitudes provided by this filter are used to rederive young cluster ages by comparing integrated photometry with stellar evolutionary models. Overall, we find ages consistent with those from our second paper from UBV photometry. However, the addition of the F170W flux provides three major improvements over previous work: (1) This band is more sensitive to the temperature range of clusters younger than similar to 10(8) yr. (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 allows us to extract precise ages. (3) Far-UV flux helps to constrain cluster extinction. New theoretical M/L-V ratios are presented for cluster ages between 4 x 10(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-V values are used to estimate cluster masses from derived ages and measured luminosities. We find young clusters to have masses in the range 6 x 10(2)-2 x 10(4) M.. These values are smaller than the most massive old clusters in M33 (which have masses up to a few times 10(5) M.) in the sample presented in our second paper.