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. (2015) and Barclay et al. (2018). By detecting companions with contrasts of delta m < 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.Comment: Accepted for publication in The Astronomical Journal; 16 pages, 8 figures, 2 table

The Effects of Stellar Companions on the Observed Transiting Exoplanet Radius Distribution

Understanding the distribution and occurrence rate of small planets was a fundamental goal of the Kepler transiting exoplanet mission, and could be improved with K2 and Transiting Exoplanet Survey Satellite (TESS). Deriving accurate exoplanetary radii requires accurate measurements of the host star radii and the planetary transit depths, including accounting for any "third light" in the system due to nearby bound companions or background stars. High-resolution imaging of Kepler and K2 planet candidate hosts to detect very close (within ~0.”5) background or bound stellar companions has been crucial for both confirming the planetary nature of candidates, and the determination of accurate planetary radii and mean densities. Here we present an investigation of the effect of close companions, both detected and undetected, on the observed (raw count) exoplanet radius distribution. We demonstrate that the recently detected "gap" in the observed radius distribution (also seen in the completeness-corrected distribution) is fairly robust to undetected stellar companions, given that all of the systems in the sample have undergone some kind of vetting with high-resolution imaging. However, while the gap in the observed sample is not erased or shifted, it is partially filled in after accounting for possible undetected stellar companions. These findings have implications for the most likely core composition, and thus formation location, of super-Earth and sub-Neptune planets. Furthermore, we show that without high-resolution imaging of planet candidate host stars, the shape of the observed exoplanet radius distribution will be incorrectly inferred, for both Kepler- and TESS-detected systems

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