Circum-planetary discs as bottlenecks for gas accretion onto giant planets

¹ Laboratoire Lagrange, UMR7293, Université de Nice Sophia-Antipolis/CNRS/Observatoire de la Côte d’Azur, BP 4229, 06304 Nice Cedex 4, France
e-mail: aurelien.crida@oca.eu
² Formation Supaéro, Institut Supérieur de l’Aéronautique et de l’Espace, 10 Av. Edouard Belin, BP 94235, 31400 Toulouse Cedex 4, France
³ Laboratoire LERMA/Observatoire de Paris, 61 avenue de l’Observatoire, 75014 Paris, France

Received: 24 January 2012
Accepted: 29 October 2012

Abstract

Context. With hundreds of exoplanets detected, it is necessary to revisit giant planets accretion models to explain their mass distribution. In particular, formation of sub-jovian planets remains unclear, given the short timescale for the runaway accretion of massive atmospheres. However, gas needs to pass through a circum-planetary disc. If the latter has a low viscosity (as expected if planets form in “dead zones”), it might act as a bottleneck for gas accretion.

Aims. We investigate what the minimum accretion rate is for a planet under the limit assumption that the circum-planetary disc is totally inviscid, and the transport of angular momentum occurs solely because of the gravitational perturbations from the star.

Methods. To estimate the accretion rate, we present a steady-state model of an inviscid circum-planetary disc, with vertical gas inflow and external torque from the star. Hydrodynamical simulations of a circum-planetary disc were conducted in 2D, in a planetocentric frame, with the star as an external perturber in order to measure the torque exerted by the star on the disc.

Results. The disc shows a two-armed spiral wave caused by stellar tides, propagating all the way in from the outer edge of the disc towards the planet. The stellar torque is small and corresponds to a doubling time for a Jupiter mass planet of the order of 5 Myr. Given the limit assumptions, this is clearly a lower bound of the real accretion rate.

Conclusions. This result shows that gas accretion onto a giant planet can be regulated by circum-planetary discs. This suggests that the diversity of masses of extra-solar planets may be the result of different viscosities in these discs.