The Hubble Space Telescope Advanced Camera for Surveys Emission Line
  Survey of Andromeda. I: Classical Be Stars
  We present results from a 2-epoch HST H$\alpha$ emission line survey of the
Andromeda Galaxy that overlaps the footprint of the Panchromatic Hubble
Andromeda Treasury (PHAT) survey. We find 552 (542) classical Be stars and 8429
(8556) normal B-type stars in epoch # 1 (epoch # 2), yielding an overall
fractional Be content of 6.15% $\pm$0.26% (5.96% $\pm$0.25%). The fractional Be
content decreased with spectral sub-type from $\sim$23.6% $\pm$2.0%
($\sim$23.9% $\pm$2.0%) for B0-type stars to $\sim$3.1% $\pm$0.34% ($\sim$3.4%
$\pm$0.35%) for B8-type stars in epoch # 1 (epoch # 2). We observe a clear
population of cluster Be stars at early fractional main sequence lifetimes,
indicating that a subset of Be stars emerge onto the ZAMS as rapid rotators. Be
stars are 2.8x rarer in M31 for the earliest sub-types compared to the SMC,
confirming that the fractional Be content decreases in significantly more metal
rich environments (like the Milky Way and M31). However, M31 does not follow a
clear trend of Be fraction decreasing with metallicity compared to the Milky
Way, which may reflect that the Be phenomenon is enhanced with evolutionary
age. The rate of disk-loss or disk-regeneration episodes we observe, 22% $\pm$
2% yr$^{-1}$, is similar to that observed for seven other Galactic clusters
reported in the literature, assuming these latter transient fractions scale by
a linear rate. The similar number of disk-loss events (57) as disk-renewal
events (43) was unexpected since disk dissipation time-scales can be $\sim$2x
the typical time-scales for disk build-up phases.
