Detecting Unresolved Binaries in TESS Data with Speckle Imaging

The Transiting Exoplanet Survey Satellite (TESS) is conducting a two-year wide-field survey searching for transiting exoplanets around nearby bright stars that will be ideal for follow-up characterization. To facilitate studies of planet compositions and atmospheric properties, accurate and precise planetary radii need to be derived from the transit light curves. Since 40%–50% of exoplanet host stars are in multiple star systems, however, the observed transit depth may be diluted by the flux of a companion star, causing the radius of the planet to be underestimated. High angular resolution imaging can detect companion stars that are not resolved in the TESS Input Catalog, or by seeing-limited photometry, to validate exoplanet candidates and derive accurate planetary radii. We examine the population of stellar companions that will be detectable around TESS planet candidate host stars, and those that will remain undetected, by applying the detection limits of speckle imaging to the simulated host star populations of Sullivan et al. and Barclay et al. By detecting companions with contrasts of Δm ≲ 7–9 and separations of ∼0.″02–1.″2, speckle imaging can detect companion stars as faint as early M stars around A–F stars and stars as faint as mid-M around G–M stars, as well as up to 99% of the expected binary star distribution for systems located within a few hundred parsecs.