The Binary Fraction of Red Supergiants

Abstract

The binary fraction of massive main sequence OB stars is thought to be as high as 70%. However, until recently, only around a dozen binary red supergiants (RSGs) had been identified, despite the fact that these stars are the evolved descendants of most OB stars. As dictated by stellar evolution, binary RSGs will most commonly have B-type companions and such systems will have unique photometric signatures due to the shape of their spectral energy distributions. Using these signatures, we spectroscopically confirmed 250 new RSG+B binaries in the Local Group galaxies M31, M33, and the Magellanic Clouds. Near-IR photometric surveys of these galaxies additionally lead to the identification of a complete sample of RSGs down to a limiting luminosity. We estimated the binary fraction using a k-nearest neighbors algorithm that classifies RSGs as single or binary based on photometry with our spectroscopic sample as a training set. Taking into account observational biases, we calculated a RSG binary fraction as a function of metallicity in M31, M33, and the Large Magellanic Cloud and found that binary fraction appears to decrease with decreasing metallicity. This dependence most likely stems not from changes in the physical properties of RSGs due to metallicity, but changes in the parent distribution of OB binaries. Understanding this dependence is vital to supernovae population studies, predicting gravitational wave events, and deciphering how these massive stars enrich our Universe with heavy elements.