Emission-line stars in M31 from the SPLASH and PHAT surveys
  We present a sample of 224 stars that emit H$\alpha$ (H$\alpha$ stars) in the
Andromeda galaxy (M31). The stars were selected from $\sim$ 5000 spectra,
collected as part of the Spectroscopic and Photometric Landscape of Andromeda's
Stellar Halo survey using Keck II/DEIMOS. We used six-filter Hubble Space
Telescope photometry from the Panchromatic Hubble Andromeda Treasury survey to
classify and investigate the properties of the H$\alpha$ stars. We identified
five distinct categories of H$\alpha$ star: B-type main sequence (MS) stars,
`transitioning'-MS (T-MS) stars, red core He burning (RHeB) stars, non-C-rich
asymptotic giant branch (AGB) stars, and C-rich AGB stars. We found $\sim$ 12
per cent of B-type stars exhibit H$\alpha$ emission (Be stars). The frequency
of Be to all B stars is known to vary with the metallicity of their
environment. Comparing this proportion of Be stars with other environments
around the Local Group, the result could indicate that M31 is more metal rich
than the Milky Way. We predict that the 17 T-MS H$\alpha$ stars are Be stars
evolving off the MS with fading H$\alpha$ emission. We separated RHeB from AGB
H$\alpha$ stars. We conclude that the 61 RHeB and AGB stars are likely to be
Long Period Variables. We found that $\sim$ 14 per cent of C-rich AGB stars (C
stars) emit H$\alpha$, which is an upper limit for the ratio of C-rich Miras to
C stars. This catalogue of H$\alpha$ stars will be useful to constrain stellar
evolutionary models, calibrate distance indicators for intermediate age
populations, and investigate the properties of M31.
