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We aim to identify and characterise binary systems containing red supergiant(RSG) stars in the Small Magellanic Cloud (SMC) using a newly availableultraviolet (UV) point source catalogue obtained using the Ultraviolet ImagingTelescope (UVIT) on board AstroSat. We select a sample of 560 SMC RSGs based onphotometric and spectroscopic observations at optical wavelengths andcross-match this with the far-UV point source catalogue using the UVIT F172Mfilter, finding 88 matches down to m$_{F172M}$=20.3 ABmag, which we interpretas hot companions to the RSGs. Stellar parameters (luminosities, effectivetemperatures and masses) for both components in all 88 binary systems aredetermined and we find mass distributions in the ranges 6.1 to 22.3 Solarmasses for RSGs and 3.7 to 15.6 Solar masses for their companions. The mostmassive RSG binary system in the SMC has a combined mass of 32 $\pm$4M$_\odot$, with a mass ratio (q) of 0.92. By simulating observing biases, wefind an intrinsic multipliciy fraction of 18.8 $\pm$ 1.5% for mass ratios inthe range 0.3 < q < 1.0 and orbital periods approximately in the range 3 < logP [days] < 8. By comparing our results with those of a similar mass on themain-sequence, we determine the fraction of single stars to be ~20% and arguethat the orbital period distribution declines rapidly beyond log P ~ 3.5. Westudy the mass-ratio distribution of RSG binary systems and find that a uniformdistribution best describes the data below 14 M$_\odot$. Above 15 M$_\odot$, wefind a lack of high mass-ratio systems.

monthly notices of the royal astronomical society

Red supergiant stars in binary systems. I. Identification and  characterisation in the Small Magellanic Cloud from the UVIT ultraviolet  imaging survey