Star clusters in M33: updated UBVRI photometry, ages, metallicities, and
  masses
  The photometric characterization of M33 star clusters is far from complete.
In this paper, we present homogeneous $UBVRI$ photometry of 708 star clusters
and cluster candidates in M33 based on archival images from the Local Group
Galaxies Survey, which covers 0.8 deg$^2$ along the galaxy's major axis. Our
photometry includes 387, 563, 616, 580, and 478 objects in the $UBVRI$ bands,
respectively, of which 276, 405, 430, 457, and 363 do not have previously
published $UBVRI$ photometry. Our photometry is consistent with previous
measurements (where available) in all filters. We adopted Sloan Digital Sky
Survey $ugriz$ photometry for complementary purposes, as well as Two Micron
All-Sky Survey near-infrared $JHK$ photometry where available. We fitted the
spectral-energy distributions of 671 star clusters and candidates to derive
their ages, metallicities, and masses based on the updated {\sc parsec} simple
stellar populations synthesis models. The results of our $\chi^2$ minimization
routines show that only 205 of the 671 clusters ($31\%$) are older than 2 Gyr,
which represents a much smaller fraction of the cluster population than that in
M31 ($56\%$), suggesting that M33 is dominated by young star clusters ($<1$
Gyr). We investigate the mass distributions of the star clusters---both open
and globular clusters---in M33, M31, the Milky Way, and the Large Magellanic
Cloud. Their mean values are $\log(M_{\rm cl}/M_{\odot})=4.25$, 5.43, 2.72, and
4.18, respectively. The fraction of open to globular clusters is highest in the
Milky Way and lowest in M31. Our comparisons of the cluster ages, masses, and
metallicities show that our results are basically in agreement with previous
studies (where objects in common are available); differences can be traced back
to differences in the models adopted, the fitting methods used, and stochastic
sampling effects.
