ALMA observations require slower Core Accretion runaway growth

S Nayakshin,J Szulágyi,G Dipierro

doi:10.1093/mnrasl/slz087

Abstract

ABSTRACT Due to recent high-resolution ALMA observations, there is an accumulating evidence for presence of giant planets with masses from ${\sim } 0.01 \, {\rm {M}}_{\rm {J}}$ to a few $\, {\rm {M}}_{\rm {J}}$ with separations up to 100 au in the annular structures observed in young protoplanetary discs. We point out that these observations set unique ‘live’ constraints on the process of gas accretion on to sub-Jovian planets that were not previously available. Accordingly, we use a population synthesis approach in a new way: we build time-resolved models and compare the properties of the synthetic planets with the ALMA data at the same age. Applying the widely used gas accretion formulae leads to a deficit of sub-Jovian planets and an overabundance of a few Jupiter mass planets compared to observations. We find that gas accretion rate on to planets needs to be suppressed by about an order of magnitude to match the observed planet mass function. This slower gas giant growth predicts that the planet mass should correlate positively with the age of the protoplanetary disc, albeit with a large scatter. This effect is not clearly present in the ALMA data but may be confirmed in the near future with more observations.

Highlights

In the Core Accretion paradigm, a solid core grows by accretion of solids (Safronov 1972; Pollack et al 1996)
Applying the widely used gas accretion formulae leads to a deficit of sub-Jovian planets and an overabundance of a few Jupiter mass planets compared to observations
We find that gas accretion rate on to planets needs to be suppressed by about an order of magnitude to match the observed planet mass function

Summary

INTRODUCTION

In the Core Accretion paradigm, a solid core grows by accretion of solids (Safronov 1972; Pollack et al 1996). A planet is destined to become a massive gas giant if it enters the runaway accretion growth phase while the disc is still present This runaway gas accretion scenario produces a valley in the planet mass function from Mp ∼ 0.1 to Mp ∼ 1 MJ or more (Ida & Lin 2004a; Mordasini et al 2009). The trend is continuous into the brown dwarf regime (e.g. Troup et al 2016), and suggests that at least a fraction of the most massive planets forms ‘as stars’ – by disc fragmentation This suggests that gas accretion onto 1 MJ mass planets is inefficient as otherwise the positive host metallicity correlation of low-mass ‘seed’ giants would be passed on to higher mass planets, and even strengthened (Mordasini et al 2012a). At large separations the runaway accretion should start at lower core masses (Piso, Youdin & Murray-Clay 2015) and terminate at larger masses (∼[3–10] MJ, because disc gap opening is harder), producing a very wide valley from ∼0.1 to ∼3 MJ, and an excess of Mp ∼ 10 MJ planets

THE DATA

POPULATION SYNTHESIS MODELLING

COMPARISON OF RESULTS TO O B S E RVAT I O N S

DISCUSSION AND CONCLUSIONS