Dust processing in protoplanetary envelopes as the origin of hot minerals in comets

TL;DR

This study proposes that crystalline silicates found in comets could originate from thermal processing inside protoplanetary envelopes around young protoplanets, which then transport these minerals into the comet formation regions.

Contribution

It introduces a new model where silicate crystallization occurs in protoplanetary envelopes, explaining the presence of crystalline minerals in comets without requiring transport from the hot inner disk.

Findings

Envelopes can reach temperatures >800 K, vaporizing silicates.

Envelopes become convective, enabling dust ejection into the disk.

Crystalline silicates can form and be transported back to the disk.

Abstract

Crystalline silicates are found in a large number of comets. These pose a long-standing conundrum for solar system formation models as they can only be created in the inner hot disk at temperatures higher than 800 K, and there is no obvious mechanism to transport them out into the comets formation region. Here we propose that these particles could have formed inside the hydrostatic envelopes surrounding young protoplanets still embedded in the protoplanetary disk. Using a simplified 1D model we investigate the thermal structure of these envelopes, and find that for core masses ranging from 0.08 to 1.5 M_Earth, located anywhere between 1 and 30 AU, the temperature and pressure at the base of the envelopes are high enough to quickly vaporize silicate particles of various sizes. Moreover, if the grain abundance is at least solar, these envelopes become fully convective, allowing for dust ejection across the Bondi radius back into the disk. Amorphous silicates are hence thermally processed into crystalline particles in these envelopes, and then transported back to disk through convective diffusion to be finally incorporated into the cometary building blocks.