Hunting for red supergiant binaries: UVIT photometry of the SMC

TL;DR

This study uses UVIT photometry to identify and analyze binary systems involving red supergiants in the Small Magellanic Cloud, revealing a significant binary fraction and a flat mass-ratio distribution.

Contribution

First direct measurement of the mass-ratio distribution of RSG binaries in the SMC using UV photometry, providing new insights into their binary properties.

Findings

Detected 88 RSGs with far-UV emission indicating binarity.

Determined an intrinsic binary fraction of approximately 19%.

Found a flat mass-ratio distribution up to 15 solar masses.

Abstract

We present UVIT/Astrosat UV photometry of the RSG population of the Small Cloud galaxy (SMC). As RSGs are extremely faint in the far-UV, these observations directly probe potential companion stars. From a sample of 861 SMC RSGs, we find 88 have detections at far-UV wavelengths: a clear signature of binarity. Stellar parameters are determined for both components, which allows us to study - for the first time - the mass-ratio (q) distribution of RSG binary systems. We find a flat mass-ratio distribution best describes the observations up to M{RSG}~15 Msun. We account for our main observing bias (i.e. the limiting magnitude of the UVIT survey) to determine the intrinsic RSG binary fraction of 18.8+/-1.5 %, for mass-ratios in the range 0.3 < q < 1.0 and orbital periods approximately in the range 3 < log P[ days] < 8.