Red supergiant stars in binary systems. I. Identification and characterization in the small magellanic cloud from the UVIT ultraviolet imaging survey

We aim to identify and characterize binary systems containing red supergiant (RSG) stars in the Small Magellanic Cloud (SMC) using a newly available ultraviolet (UV) point source catalogue obtained using the Ultraviolet Imaging Telescope (UVIT) on board AstroSat. We select a sample of 560 SMC RSGs based on photometric and spectroscopic observations at optical wavelengths and cross-match this with the far-UV point source catalogue using the UVIT F172M filter, finding 88 matches down to M-F172M = 20.3 ABmag, which we interpret as hot companions to the RSGs. Stellar parameters (luminosities, effective temperatures, and masses) for both components in all 88 binary systems are determined and we find mass distributions in the ranges 6.1 < M/M-circle dot < 22.3 for RSGs and 3.7 < M/M-circle dot < 15.6 for their companions. The most massive RSG binary system in the SMC has a combined mass of 32 +/- 4 M-circle dot, with a mass ratio (q) of 0.92. By simulating observing biases, we find an intrinsic multiplicity fraction of 18.8 +/- 1.5 per cent for mass ratios in the range 0.3 < q < 1.0 and orbital periods approximately in the range 3 < log P[days] < 8. By comparing our results with those of a similar mass on the main sequence, we determine the fraction of single stars to be similar to 20 per cent and argue that the orbital period distribution declines rapidly beyond log P similar to 3.5. We study the mass-ratio distribution of RSG binary systems and find that a uniform distribution best describes the data below 14 M-circle dot. Above 15 M-circle dot, we find a lack of high mass ratio systems.