Evidence of fast pebble growth near condensation fronts in the HL Tau protoplanetary disk

TL;DR

This study uses ALMA observations of HL Tau to provide evidence that rapid pebble growth occurs near condensation fronts of volatiles like water and ammonia, influencing planetesimal formation.

Contribution

It presents observational evidence linking pebble growth to condensation fronts in a protoplanetary disk, supported by spectral index analysis and dust population modeling.

Findings

The innermost emission dip at 13 AU coincides with the water ice condensation front.

Dips at 32 and 63 AU align with ammonia and hydrate condensation fronts.

Spectral index variations suggest the presence of decimeter-sized pebbles inside the dips.

Abstract

Water and simple organic molecular ices dominate the mass of solid materials available for planetesimal and planet formation beyond the water snow line. Here we analyze ALMA long baseline 2.9, 1.3 and 0.87 mm continuum images of the young star HL Tau, and suggest that the emission dips observed are due to rapid pebble growth around the condensation fronts of abundant volatile species. Specifically, we show that the prominent innermost dip at 13 AU is spatially resolved in the 0.87 mm image, and its center radius is coincident with the expected mid-plane condensation front of water ice. In addition, two other prominent dips, at distances of 32 and 63 AU, cover the mid-plane condensation fronts of pure ammonia or ammonia hydrates and clathrate hydrates (especially with CO and N$_2$) formed from amorphous water ice. The spectral index map of HL Tau between 1.3 and 0.87 mm shows that the flux ratios inside the dips are statistically larger than those of nearby regions in the disk. This variation can be explained by a model with two dust populations, where most of solid mass resides in a component that has grown into decimeter size scales inside the dips. Such growth is in accord with recent numerical simulations of volatile condensation, dust coagulation and settling.