ASTROSAT/UVIT Survey of M31, First Results: UV-bright Stars in the Bulge

A survey of M31 is being carried out with the ASTROSAT Observatory in ultraviolet (UV), soft X-rays, and hard X-rays using the UltraViolet Imaging Telescope (UVIT), soft X-ray telescope, LAXPC, and CZT instruments. Here we analyze UVIT observations of the first field covering the central (7‧) part of the bulge of M31. The UV bands observed cover the range 120-280 nm with four filters with a spatial resolution of 1″. They are sensitive to the UV emission from hot stars (T _eff > 10000 K), so can be used to detect the hottest stars in the bulge of M31. We cross-match the UVIT sources with sources from Panchromatic Hubble Andromeda Treasury to obtain multi-band spectral energy distributions (SEDs) for 26 sources. Fits of the SEDs with stellar models show that these are likely main-sequence stars with masses in the range 5-20 M _⊙. This provides evidence for a low, but significant (∼10^-5 M _⊙ yr^-1), rate of star formation in the bulge of M31, which has occurred in the very recent past (∼10-100 million years).