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 .