  Be stars are a class of rapidly rotating B stars with circumstellar disks
that cause Balmer and other line emission. There are three possible reasons for
the rapid rotation of Be stars: they may have been born as rapid rotators, spun
up by binary mass transfer, or spun up during the main-sequence (MS) evolution
of B stars. To test the various formation scenarios, we have conducted a
photometric survey of 55 open clusters in the southern sky. Of these, five
clusters are probably not physically associated groups and our results for two
other clusters are not reliable, but we identify 52 definite Be stars and an
additional 129 Be candidates in the remaining clusters. We use our results to
examine the age and evolutionary dependence of the Be phenomenon. We find an
overall increase in the fraction of Be stars with age until 100 Myr, and Be
stars are most common among the brightest, most massive B-type stars above the
zero-age MS (ZAMS). We show that a spin-up phase at the terminal-age MS (TAMS)
cannot produce the observed distribution of Be stars, but up to 73% of the Be
stars detected may have been spun-up by binary mass transfer. Most of the
remaining Be stars were likely rapid rotators at birth.
  Previous studies have suggested that low metallicity and high cluster density
may also favor Be star formation. Our results indicate a possible increase in
the fraction of Be stars with increasing cluster distance from the Galactic
center (in environments of decreasing metallicity). However, the trend is not
significant and could be ruled out due to the intrinsic scatter in our data. We
also find no relationship between the fraction of Be stars and cluster density.
