2017MNRAS.465.4180P

We present a sample of 224 stars that emit Hα (Hα stars) in the Andromeda galaxy (M31). The stars were selected from ∼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 Hα stars. We identified five distinct categories of Hα 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 that ∼12 per cent of B-type stars exhibit Hα 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 Hα stars are Be stars evolving off the MS with fading Hα emission. We separated RHeB from AGB Hα stars. We conclude that the 61 RHeB and AGB stars are likely to be long-period variables. We found that ∼14 per cent of C-rich AGB stars (C stars) emit Hα, which is an upper limit for the ratio of C-rich Miras to C stars. This catalogue of Hα stars will be useful to constrain stellar evolutionary models, calibrate distance indicators for intermediate-age populations, and investigate the properties of M31.