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 s eries using far-UV photometry. Previously, UBV colors were used to derive c luster ages, luminosities, and masses, assuming an extinction interpolated from neighboring stars. Fifteen of 44 clusters imaged in the Hubble Space T elescope WFPC2 with the F170W filter are detected. The far-UV magnitudes pr ovided by this filter are used to rederive young cluster ages by comparing integrated photometry with stellar evolutionary models. Overall, we find ag es consistent with those from our second paper from UBV photometry. However , the addition of the F170W flux provides three major improvements over pre vious work: (1) This band is more sensitive to the temperature range of clu sters younger than similar to 10(8) yr. (2) UBV colors for young clusters w ith contaminated V-band fluxes (from red supergiants or from line emission by surrounding excited gas) provide only an upper age limit. The addition o f 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 fro m the recent models of C. Chiosi. These M/L-V values are used to estimate c luster masses from derived ages and measured luminosities. We find young cl usters 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.