Observational Constraints on the Red Supergiant Binary Fraction and Merger Rate

Massive stars in binary systems are the progenitors to many exciting phenomena, such as TZOs and the gravitational wave events we can now detect with LIGO. The binary fraction of massive main-sequence OB stars is thought to be as high as 70% or greater. As these stars burn through their hydrogen, many of them will experience strong Roche-Lobe overflow while on the main sequence and merge. In the systems that remain, the more massive star will evolve off the main sequence and expand into a red supergiant (RSG). In binaries with close separations, the RSG will consume its companion and a merger will occur. In longer period systems, the companion will remain. Eventually, the RSG will end its life as a supernova, and the resulting system will contain an evolved star with a compact object, the progenitor to a TZO. As part of my PhD research, I characterized the RSG binary fraction as a function of metallicity in the Local Group galaxies M31, M33 and the Large Magellanic Cloud and am currently investigating the RSG merger fraction in our own Galaxy. In this talk, I’ll discuss how these results help us better understand the numbers and behaviors of TZO progenitors.