Grain Sedimentation in a Giant Gaseous Protoplanet

TL;DR

This study models grain sedimentation in giant gaseous protoplanets formed by disk instability, showing that small grains can form refractory cores over hundreds of thousands of years, with implications for planetary composition.

Contribution

It provides the first detailed calculation of grain sedimentation in protoplanets from disk instability, demonstrating core formation potential and composition based on grain evaporation and convection effects.

Findings

Small grains sediment to form cores within 4×10^5 years.

Core masses range from 1 to 10 Earth masses with additional heavy elements.

Refractory materials dominate the core, while envelope volatiles are mostly water vapor and organics.

Abstract

We present a calculation of the sedimentation of grains in a giant gaseous protoplanet such as that resulting from a disk instability of the type envisioned by Boss (1998). Boss (1998) has suggested that such protoplanets would form cores through the settling of small grains. We have tested this suggestion by following the sedimentation of small silicate grains as the protoplanet contracts and evolves. We find that during the course of the initial contraction of the protoplanet, which lasts some $4\times 10^5$ years, even very small (> 1 micron) silicate grains can sediment to create a core both for convective and non-convective envelopes, although the sedimentation time is substantially longer if the envelope is convective, and grains are allowed to be carried back up into the envelope by convection. Grains composed of organic material will mostly be evaporated before they get to the core region, while water ice grains will be completely evaporated. These results suggest that if giant planets are formed via the gravitational instability mechanism, a small heavy element core can be formed due to sedimentation of grains, but it will be composed almost entirely of refractory material. Including planetesimal capture, we find core masses between 1 and 10 M$_{\oplus}$, and a total high-Z enhancement of ~40 M$_{\oplus}$. The refractories in the envelope will be mostly water vapor and organic residuals.