Binary Yellow Supergiants in the Magellanic Clouds I: Photometric Candidate Identification

Recent works have constrained the binary fraction of evolved populations of massive stars in local galaxies such as red supergiants and Wolf-Rayet stars, but the binary fraction of yellow supergiants (YSGs) in the Hertzsprung Gap remains unconstrained. Binary evolution theory predicts that the Hertzsprung Gap is home to multiple populations of binary systems with varied evolutionary histories. In this paper, we develop a method to distinguish single YSGs from YSG plus O- or B-type main sequence binaries using optical and ultraviolet photometry, and then apply this method to identify candidate YSG binaries in the Magellanic Clouds. After constructing a set of combined stellar atmosphere models, we find that optical photometry is, given typical measurement and reddening uncertainties, sufficient to discern single YSGs from YSG+OB binaries if the OB-star is at least $\sim5$M$_{\odot}$ for T$_{\mathrm{eff,YSG}}\sim$ 4000 K, but requires a $\sim$20M$_{\odot}$ OB star for YSGs up to T$_{\mathrm{eff,YSG}}\sim$ 9000 K. For these hotter YSG temperatures, ultraviolet photometry allows binaries with OB companions as small as $\sim$7M$_{\odot}$ to be identified. We use color-color spaces developed from these models to search for evidence of excess blue or ultraviolet light in a set of $\sim$1000 YSG candidates in the Magellanic Clouds. We identify hundreds of candidate YSG binary systems and report a preliminary fraction of YSGs that show a blue/UV color excess of 20-60\%. Spectroscopic follow-up is now required to confirm the true nature of this population.