The opacity of grains in protoplanetary atmospheres

TL;DR

This paper models the size distribution and opacity of silicate grains in protoplanetary atmospheres, highlighting how grain growth and breakup influence opacity levels and planetary evolution.

Contribution

It introduces a faster computational method for grain size distribution, including new grain input from planetesimal breakup, and refines opacity estimates in protoplanetary atmospheres.

Findings

Grain opacity is around 10^{-2} cm^2 g^{-1} in most of the radiative zone.

Opacity can increase to about 1 cm^2 g^{-1} near the outer edge of the envelope.

The refined opacity estimates impact models of planetary atmosphere evolution.

Abstract

We have computed the size distribution of silicate grains in the outer radiative region of the envelope of a protoplanet evolving according to the scenario of Pollack et al. (1996). Our computation includes grain growth due to Brownian motion and overtake of smaller grains by larger ones. We also include the input of new grains due to the breakup of planetesimals in the atmosphere. We follow the procedure of Podolak (2003), but have speeded it up significantly. This allows us to test the sensitivity of the code to various parameters. We have also made a more careful estimate of the resulting grain opacity. We find that the grain opacity is of the order of $10^{-2}\ \mathrm{cm^2 g^{-1}}$ throughout most of the outer radiative zone as Hubickyj et al. (2005) assumed for their low opacity case, but near the outer edge of the envelope, the opacity can increase to $\sim{1} \mathrm{cm^2 g^{-1}}$. We discuss the effect of this on the evolution of the models.