A high binary fraction for the most massive close-in giant planets and brown dwarf desert members

Abstract

Stellar multiplicity is believed to influence planetary formation and evolution, although the precise nature and extent of this role remain ambiguous. We present a study aimed at testing the role of stellar multiplicity in the formation and/or evolution of the most massive, closein planetary and substellar companions. Using past and new direct imaging observations, as well as the Gaia DR2 catalogue, we searched for wide binary companions to 38 stars hosting massive giant planets or brown dwarfs (M > 7 M Jup ) on orbits shorter than ∼1 AU. We report the discovery of a new component in the WASP-14 system, and present an independent confirmation of a comoving companion to WASP-18. From a robust Bayesian statistical analysis, we derived a binary fraction of 79.0 +13.2 −14.7 % between 20−10,000 AU for our sample, twice as high as for field stars with a 3-σ significance. This binary frequency was found to be larger than for lower-mass planets on similar orbits, and we observed a marginally higher binary rate for inner companions with periods shorter than 10 days. These results demonstrate that stellar companions greatly influence the formation and/or evolution of these systems, suggesting that the role played by binary companions becomes more important for higher-mass planets, and that this trend may be enhanced for systems with tighter orbits. Our analysis also revealed a peak in binary separation at 250 AU, highlighting a shortfall of close binaries among our sample. This indicates that the mechanisms affecting planet and brown dwarf formation or evolution in binaries must operate from wide separations, although we found that the Kozai-Lidov mechanism is unlikely to be the dominant underlying process. We conclude that binarity plays a crucial role in the existence of very massive short-period giant planets and brown dwarf desert inhabitants, which are almost exclusively observed in multiple systems.

Planets in Binaries: Formation and Dynamical Evolution

“…On the other hand, there is a clear tendency towards a very high fraction of hot-Jupiter in moderate-to-wide binaries. For the 50 ≤ ρ ≤ 2000 au separation range, reference [17] indeed find a multiplicity fraction that is 2.9 times higher for hot-Jupiter hosts than for field stars, a result which was confirmed by [19] for close-in companions up to the brown dwarf regime.…”

A new method for target selection in direct imaging programmes with COPAINS

“…We also queried the position of the star in the Gaia DR2 catalogue and searched for comoving sources within a angular radius corresponding to a projected separation of 10 4 AU given the parallax of the star. We define comoving objects in Gaia as sources with a fractional difference of < 20% in parallax and at least one of the proper motion coordinates, following the approach from Fontanive et al (2019) to search for wide binary companions in Gaia DR2. This ensures that binary components showing significant disparities in their kinematics due to their gravitational influence on one another (i.e.…”

“…The VLT/NACO (L filter) and VLT/SPHERE (Y JH + K1K2) 5-σ detection limits were taken from the median performances achieved in Rameau et al (2013) and Maire et al (2017), respectively. The Gaia limit corresponds to the Gaia DR2 99% binary completeness limit described in Fontanive et al (2019). The sensitivity limits in terms of contrast for the future E-ELT/METIS instrument were obtained through end-to-end simulations of a 1-hour observation in the L band for a star of magnitude L = 6, using the classical vortex coronagraph and assuming an ADI sequence with 40 deg parallac-tic angle rotation (B. Carlomagno et al, in prep.).…”

NGTS-7Ab: an ultrashort-period brown dwarf transiting a tidally locked and active M dwarf

“…Bolmont et al 2012). From a sample of 38 high mass (> 7 M J ) exoplanets and brown dwarfs, Fontanive et al (2019) found that companions with orbital periods less than 10 days have circularisation timescales consistent with the Kozai-Lidov mechanism. We have estimated the timescale of the Kozai-Lidov mechanism (τ K L ) for the NGTS-7 system using the formalism from Kiseleva et al (1998) and find τ K L <55 Myr for initial orbits beyond 0.1 AU.…”