Metal loading of giant gas planets

Sergei Nayakshin

doi:10.1093/mnras/stu2074

Abstract

One of many challenges in forming giant gas planets via Gravitational disc Instability model (GI) is an inefficient radiative cooling of the pre-collapse fragments. Since fragment contraction times are as long at $10^5 -10^7$ years, the fragments may be tidally destroyed sooner than they contract into gas giant planets. Here we explore the role of "pebble accretion" onto the pre-collapse giant planets and find an unexpected result. Despite larger dust opacity at higher metallicities, addition of metals actually accelerates -- rather than slows down -- collapse of high opacity, relatively low mass giant gas planets ($M_p$ below a few Jupiter masses). A simple analytical theory that explains this result exactly in idealised simplified cases is presented. The theory shows that planets with the central temperature in the range between $\sim$ 1000 to 2000K are especially sensitive to pebble accretion: addition of just $\sim 5$ to 10 % of metals by weight is sufficient to cause their collapse. These results show that dust grain physics and dynamics is essential for an accurate modelling of self-gravitating disc fragments and their near environments in the outer massive and cold protoplanetary discs.

Highlights

AND BACKGROUNDGravitational disc Instability (GI) theory for giant planet formation (e.g., Kuiper 1951; Cameron et al 1982; Boss 1997, 1998) posits that gravitational instability of the disc leads to formation of self-gravitating gas fragments that later contract into present day planets
In this paper we have shown that accretion of pebbles from the surrounding protoplanetary disc is a surprisingly efficient way for pre-collapse H2 dominated planets to contract and eventually collapse in high opacity regime
Metal loading makes it much more likely that these planets are able to contract and collapse despite a rapid inward migration. This may help to resolve the challenges to forming Gravitational disc Instability model (GI) planets at short separations by migration of fragments born at separations ∼ 100 AU emphasised by Zhu et al (2012) and Vazan & Helled (2012)

Summary

Introduction

Gravitational disc Instability (GI) theory for giant planet formation (e.g., Kuiper 1951; Cameron et al 1982; Boss 1997, 1998) posits that gravitational instability of the disc leads to formation of self-gravitating gas fragments that later contract into present day planets This view has been strongly challenged in the last decade since it was shown that protoplanetary discs do not cool rapidly enough to fragment onto gas clumps inside several tens to a hundred AU (Gammie 2001; Mayer et al 2004; Rice et al 2005; Rafikov 2005; Durisen et al 2007; Stamatellos & Whitworth 2008; Meru & Bate 2011). This scheme, named ”Tidal Downsizing” by Nayakshin (2010a), may potentially explain any mass planets at arbitrary separation from the host star within a single framework (Forgan & Rice 2013)

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