Giant planet formation: episodic impacts versus gradual core growth

Center for Space and Habitability and Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
e-mail: broeg@space.unibe.ch

Received: 6 April 2011
Accepted: 17 November 2011

Abstract

Aims. We describe the growth of gas giant planets in the core accretion scenario. The core growth is not modeled as a gradual accretion of planetesimals but as episodic impacts of large mass ratios, i.e. we study impacts of 0.02–1 M_⊕ onto cores of 1–15 M_⊕. Such impacts could deliver the majority of solid matter in the giant impact regime. We focus on the thermal response of the envelope to the energy delivery. Previous studies have shown that sudden shut off of core accretion can dramatically speed up gas accretion. We therefore expect that giant impacts followed by periods of very low core accretion will result in a net increase in gas accretion rate. This study aims at modelling such a sequence of events and to understand the reaction of the envelope to giant impacts in more detail.

Methods. To model this scenario, we spread the impact energy deposition over a time that is long compared to the sound crossing time, but very short compared to the Kelvin-Helmholtz time. The simulations are done in spherical symmetry and assume quasi-hydrostatic equilibrium.

Results. Results confirm what could be inferred from previous studies: gas can be accreted faster onto the core for the same net core growth speed while at the same time rapid gas accretion can occur for smaller cores – significantly smaller than the usual critical core mass. Furthermore our simulations show, that significant mass fractions of the envelope can be ejected by such an impact.

Conclusions. Large impacts are an efficient process to remove the accretion energy by envelope ejection. In the time between impacts, very fast gas accretion can take place. This process could significantly shorten the formation time of gas giant planets. As an important side-effect, the episodic ejection of the envelope will reset the envelope composition to nebula conditions.