The Impact of Stellar Multiplicity on Planetary Systems. I. The Ruinous Influence of Close Binary Companions

Abstract

The dynamical influence of binary companions is expected to profoundly influence planetary systems. However, the difficulty of identifying planets in binary systems has left the magnitude of this effect uncertain; despite numerous theoretical hurdles to their formation and survival, at least some binary systems clearly host planets. We present high-resolution imaging of 382 Kepler Objects of Interest (KOIs) obtained using adaptive-optics imaging and nonredundant aperture-mask interferometry (NRM) on the Keck-II telescope. Among the full sample of 506 candidate binary companions to KOIs, we super-resolve some binary systems to projected separations of <5 AU, showing that planets might form in these dynamically active environments. However, the full distribution of projected separations for our planet-host sample more broadly reveals a deep paucity of binary companions at solar-system scales. For a field binary population, we should have found 58 binary companions with projected separation ρ < 50 AU and mass ratio q > 0.4; we instead only found 23 companions (a 4.6σ deficit), many of which must be wider pairs that are only close in projection. When the binary population is parametrized with a semimajor axis cutoff a cut and a suppression factor inside that cutoff S bin , we find with correlated uncertainties that inside a cut = 47 +59 −23 AU, the planet occurrence rate in binary systems is only S bin = 0.34 +0.14 −0.15 times that of wider binaries or single stars. Our results demonstrate that a fifth of all solar-type stars in the Milky Way are disallowed from hosting planetary systems due to the influence of a binary companion.

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

“…We note that choosing the multiplicity statistics from Raghavan et al (2010) may not be accurate for this sample, as previous work on the Kepler field planet host stars has shown that stellar multiplicity may be affected by the presence of planets (Wang et al 2014a(Wang et al , 2014bKraus et al 2016). However, the specifics of this multiplicity effect are not yet well understood, so we default to using the known multiplicity statistics of the solar neighborhood.…”

“…Comparable multiplicity to the solar neighborhood was shown to be a reasonable assumption by Horch et al (2014) based on high-resolution imaging, which is most sensitive to stellar companions at fairly large physical separations. However, there are indications that multiplicity may be suppressed at smaller separations (e.g., Wang et al 2014aWang et al , 2014bKraus et al 2016), and other effects such as Malmquist bias may also play a role in determining what fraction of Kepler planet hosts have unknown stellar companions.…”

“…The full implications of stellar multiplicity on planet formation and evolution are still being explored. Previous studies (e.g., Wang et al 2014b;Kraus et al 2016) indicate that stellar multiplicity may be suppressed in planethosting Kepler systems, compared to field stars in the solar neighborhood (Raghavan et al 2010). This may indicate that planet formation is rarer in close binary systems than in singlestar systems.…”

Eccentricity in planetary systems and the role of binarity

“…Our results thus confirm the paucity of planet-host binaries with less than ∼50-100au separation found in previous studies. Wang et al (2014b,a) and Kraus et al (2016) have reported on such a paucity for a sample of Kepler Objects of Interest, while Eggenberger et al (2007) has also previously reported a less frequent occurrence of binaries with mean semimajor axes between 35 and 100 au around planet-host stars. Finally, Bonavita & Desidera (2007) also found a lower frequency of planets around binaries with separation smaller than 50-100 au with respect to single stars and members of wide binaries.…”

“…Finally, Bonavita & Desidera (2007) also found a lower frequency of planets around binaries with separation smaller than 50-100 au with respect to single stars and members of wide binaries. This first study included a comparison sample of stars for which no radial-velocity companions were detected, while Wang et al (2014b) and Kraus et al (2016) based their analysis on a comparison with the binary distribution over the field stars. In our analysis, the comparison is made over the population of known exoplanetary systems with a distinction between dynamically active and inactive systems based on the measured eccentricity of the outer companion.…”