Far-ultraviolet Variables in M31: Concentration in Spiral Arms and Association with Young Stars

Abstract

The Andromeda galaxy (M31) is an object of ongoing study with the Ultraviolet Imaging Telescope (UVIT) on AstroSat. Field 2, which is 6.4 kpc in diameter at the distance of M31, includes a substantial part of the NE spiral arms of the galaxy. We have obtained a second observation of Field 2 with the far-ultraviolet (FUV) F148W (148 nm) filter, separated from the first observation by 1465 days. Both observations are analyzed to detect sources that are variable at a >3 sigma confidence level. For sources with less than similar to 2 '' separation, we apply multi-Gaussian fits, to obtain reliable magnitudes in the presence of source crowding. The variable sources are found to be concentrated in the spiral arms, with fraction of variable to nonvariable sources similar to 2 times higher than for interarm regions, indicating an association of FUV variables with young stellar populations. UVIT FUV-NUV color-magnitude diagrams confirm the association of FUV variables with young massive/hot stars. Using existing catalogs, we obtain counterparts for 64 of the 82 most variable sources (>5 sigma). The counterparts include 13 star clusters, three ionized hydrogen (H ii) regions, three novae, one S Doradus star, eight eclipsing binaries, 20 foreground sources, two regular variables, and 14 unspecified variables.