Fast formation of large ice pebbles after FU Orionis outbursts

TL;DR

This paper investigates how rapid cooling after FU Orionis outbursts influences water ice particle formation in protoplanetary discs, potentially facilitating planetesimal formation near the ice line.

Contribution

It introduces a model linking cooling timescales to ice nucleation and growth, showing how outbursts can promote formation of large icy pebbles.

Findings

Fast cooling leads to high nucleation rates and small ice particles.

Slow cooling allows for larger ice-formed pebbles of centimeters to decimeters.

Large pebbles may trigger planetesimal formation via streaming instability.

Abstract

During their formation, nascent planetary systems are subject to FU Orionis outbursts that heat a substantial part of the disc. This causes water ice in the affected part of the disc to sublimate as the ice line moves outwards to several to tens of astronomical units. In this paper, we investigate how the subsequent cooling of the disc impacts the particle sizes. We calculate the resulting particle sizes in a disc model with cooling times between 100 and 1,000 years, corresponding to typical FU Orionis outbursts. As the disc cools and the ice line retreats inwards, water vapour forms icy mantles on existing silicate particles. This process is called heterogeneous nucleation. The nucleation rate per surface area of silicate substrate strongly depends on the degree of super-saturation of the water vapour in the gas. Fast cooling results in high super-saturation levels, high nucleation rates, and limited condensation growth because the main ice budget is spent in the nucleation. Slow cooling, on the other hand, leads to rare ice nucleation and efficient growth of ice-nucleated particles by subsequent condensation. We demonstrate that close to the quiescent ice line, pebbles with a size of about centimetres to decimetres form by this process. The largest of these are expected to undergo cracking collisions. However, their Stokes numbers still reach values that are high enough to potentially trigger planetesimal formation by the streaming instability if the background turbulence is weak. Stellar outbursts may thus promote planetesimal formation around the water ice line in protoplanetary discs.