Planetesimal and gas dynamics in binaries

TL;DR

This study investigates how gas disc eccentricity and binary star gravitational effects influence planetesimal collision velocities, revealing increased challenges for planet formation in close binary systems.

Contribution

It provides analytical formulas to estimate planetesimal encounter velocities considering gas disc eccentricity, advancing understanding of planet formation in binary systems.

Findings

Encounter velocities depend strongly on gas disc eccentricity.

Full gas dynamics increase encounter velocities compared to static discs.

Gas dynamical evolution hampers planetesimal accretion in binaries.

Abstract

Observations of extrasolar planets reveal that planets can be found in close binary systems, where the semi-major axis of the binary orbit is less than 20 AU. The existence of these planets challenges planet formation theory, because the strong gravitational perturbations due to the companion increase encounter velocities between planetesimals and make it difficult for them to grow through accreting collisions. We study planetesimal encounter velocities in binary systems, where the planetesimals are embedded in a circumprimary gas disc that is allowed to evolve under influence of the gravitational perturbations of the companion star. We find that the encounter velocities between planetesimals of different size strongly depend on the gas disc eccentricity. In all cases studied, inclusion of the full gas dynamics increases the encounter velocity compared to the case of a static, circular gas disc. Full numerical parameter exploration is still impossible, but we derive analytical formulae to estimate encounter velocities between bodies of different sizes given the gas disc eccentricity. The gas dynamical evolution of a protoplanetary disc in a binary system tends to make planetesimal accretion even more difficult than in a static, axisymmetric gas disc.