FUV Variable Sources in M31

TL;DR

This study used UVIT on AstroSat to identify and analyze FUV variable sources in M31, revealing a high fraction of young, hot stars and related objects, with a catalog of these variables provided.

Contribution

First detailed UV variability survey of M31's inner spiral arms using high-resolution UVIT data, identifying and cataloging FUV variable sources and their counterparts.

Findings

Higher fraction of FUV variables in spiral arms indicates association with hot young stars.

42 out of 86 FUV variables have identified counterparts, including star clusters and HII regions.

Color-magnitude diagrams confirm most variables are hot, young stellar objects.

Abstract

The Ultraviolet Imaging Telescope (UVIT) onboard the AstroSat observatory has imaged the Andromeda Galaxy (M31) from 2017 to 2019 in FUV and NUV with the high spatial resolution of ~1". The survey covered the large sky area of M31 with a set of observations (Fields) each 28 arcminutes in diameter. Field 1 was observed in two epochs with the F148W filter, separated by 1133 days (~3.10 years). The 6.4 kpc diameter Field 1 (at the distance of M31) includes a substantial part of the inner spiral arms of the galaxy. We identify UVIT sources in both epochs of Field 1 and obtain catalogs of sources that are variable in FUV at > 3 sigma and > 5 sigma confidence level. The fraction of FUV variable sources is higher for brighter sources, and the fraction is higher in the two main spiral arms compared to other areas. This is evidence that a significant fraction of the FUV variables are associated with hot young stars. Source counterparts are found for 42 of the 86 > 5 sigma FUV variables using existing catalogs. The counterparts include 10 star clusters, 6 HII regions, 5 regular or semiregular variables, 6 other variables and 6 nova or nova candidates. The UVIT FUV-NUV and FUV-FUV color-magnitude diagrams confirm the association of most of the FUV variables with hot young stars. A catalog of UVIT photometry for the variable sources is presented.