Gas Giant Formation with Small Cores Triggered by Envelope Pollution by Icy Planetesimals

TL;DR

This study shows that pollution of protoplanetary envelopes by icy planetesimals lowers the critical core mass needed for gas giant formation and accelerates gas accretion, highlighting the importance of considering envelope pollution in core-accretion models.

Contribution

It introduces the effect of icy planetesimal pollution on envelope composition, demonstrating its role in reducing core mass requirements for gas giant formation.

Findings

Envelope pollution lowers critical core mass.

Polluted envelopes accelerate gas accretion.

Small cores can form gas giants with pollution.

Abstract

We have investigated how envelope pollution by icy planetesimals affects the critical core mass for gas giant formation and the gas accretion time-scales. In the core-accretion model, runaway gas accretion is triggered after a core reaches a critical core mass. All the previous studies on the core-accretion model assumed that the envelope has the solar composition uniformly. In fact, the envelope is likely polluted by evaporated materials of icy planetesimals because icy planetesimals going through the envelope experience mass loss via strong ablation and most of their masses are deposited in the deep envelope. In this paper, we have demonstrated that envelope pollution in general lowers the critical core masses and hastens gas accretion on to the protoplanet because of the increase in the molecular weight and reduction of adiabatic temperature gradient. Widely- and highly-polluted envelopes allow smaller cores to form massive envelopes before disc dissipation. Our results suggest that envelope pollution in the course of planetary accretion has the potential to trigger gas giant formation with small cores. We propose that it is necessary to take into account envelope pollution by icy planetesimals when we discuss gas giant formation based on the core-accretion model.