PROPERTIES OF M31. IV. CANDIDATE LUMINOUS BLUE VARIABLES FROM PANDROMEDA

We perform a study on the optical and infrared photometric properties of known luminous blue variables (LBVs) in M31 using a sample of LBV candidates from the Local Group Galaxy Survey by Masset et al. We find that M31 LBV candidates show photometric variability ranging from 0.375 to 1.576 mag in rP1 during a 3 yr time span observed by the Pan-STARRS 1 Andromeda survey (PAndromeda). Their near-infrared colors also follow the distribution of Galactic LBVs as shown by Oksala et al. We use these features as selection criteria to search for unknown LBV candidates in M31. We thus devise a method to search for candidate LBVs using both optical color from the Local Group Galaxy Survey and infrared color from the Two Micron All Sky Survey, as well as photometric variations observed by PAndromeda. We find four sources exhibiting common properties of known LBVs. These sources also exhibit UV emission as seen from Galaxy Evolution Explorer, which is one of the previously adopted methods of searching for LBV candidates. The locations of the LBVs are well aligned with M31 spiral arms as seen in UV light, suggesting that they are evolved stars at a young age given their high-mass nature. We compare these candidates with the latest Geneva evolutionary tracks, which show that our new M31 LBV candidates are massive, evolved stars with ages of 10–100 Myr.