Assessing the Effect of Stellar Companions from High-resolution Imaging of Kepler Objects of Interest

Abstract

We report on 176 close (<2″) stellar companions detected with high-resolution imaging near 170 hosts of Kepler Objects of Interest (KOIs). These Kepler targets were prioritized for imaging follow-up based on the presence of small planets, so most of the KOIs in these systems (176 out of 204) have nominal radii <6 Å R . Each KOI in our sample was observed in at least two filters with adaptive optics, speckle imaging, lucky imaging, or the Hubble Space Telescope. Multi-filter photometry provides color information on the companions, allowing us to constrain their stellar properties and assess the probability that the companions are physically bound. We find that 60%-80% of companions within 1″ are bound, and the bound fraction is >90% for companions within 0 5; the bound fraction decreases with increasing angular separation. This picture is consistent with simulations of the binary and background stellar populations in the Kepler field. We also reassess the planet radii in these systems, converting the observed differential magnitudes to a contamination in the Kepler bandpass and calculating the planet radius correction factor, X R =R p (true)/R p (single). Under the assumption that planets in bound binaries are equally likely to orbit the primary or secondary, we find a mean radius correction factor for planets in stellar multiples of X R = 1.65. If stellar multiplicity in the Kepler field is similar to the solar neighborhood, then nearly half of all Kepler planets may have radii underestimated by an average of 65%, unless vetted using high-resolution imaging or spectroscopy.

Average Albedos of Close-in Super-Earths and Super-Neptunes from Statistical Analysis of Long-cadence Kepler Secondary Eclipse Data

“…The albedo, proportional to (a/R p ) 2 from Equation 1, will be underestimated, because the effect on R p will dominate. We simulated this effect for our shortened 1 to 2 R ⊕ sample, based on the results from Hirsch et al (2017). The authors found that one third of planet-hosting stars have companions.…”

“…The authors found that one third of planet-hosting stars have companions. We used a Monte-Carlo procedure of assigning companions randomly to one third of our sample, drawing the magnitude difference also randomly from the list of Hirsch et al (2017). We calculated the effect on each inferred planetary radius and semi-major axis, and thus on the average albedo of the group.…”

Validation of Small Kepler Transiting Planet Candidates in or near the Habitable Zone

“…For the r »  1. 94 companion to KOI-5236, the situation is less clear, as Hirsch et al (2017) found that wider companions out to 2″ are about equally likely to be bound as unbound. However, the BLENDER constraints for both the HTP and the BP scenarios indicate that this star is too faint to be the source of the transit for any reasonable size planet.…”

“…Studies by Horch et al (2014) and Hirsch et al (2017) have concluded that the vast majority of sub-arcsecond companions to KOIs are bound to their primary stars. All but one of the companions to our targets are within 1″, the exception being the r »  1.…”

Robo-AO Kepler Survey. IV. The Effect of Nearby Stars on 3857 Planetary Candidate Systems

“…These stars are often much fainter than the primary star and the assumption that each star is equally likely to host the planet results in a large number of gas giant planets, which are inherently rare compared to terrestrial planets (Howard et al 2012). Simulations from galactic stellar models suggest that the majority of nearby stars to KOIs at separations larger than 1″ are likely unbound (Horch et al 2014), a conclusion borne out by observations (Atkinson et al 2017;Hirsch et al 2017). If we limit our survey to just those likely bound nearby stars within 1″, we find radius correction factors of 1.18, 1.88, and 1.54 for the scenarios where all planets orbit the primary star, all planets orbit a bound secondary star, and all planets are equally likely to orbit either star, respectively.…”