Chemistry of a protoplanetary disk with grain settling and Lyman alpha radiation

TL;DR

This study models the chemical evolution of protoplanetary disks, highlighting the effects of grain settling and Lyman alpha radiation on molecular composition and UV radiation penetration.

Contribution

It introduces a comprehensive model that includes Lyman alpha radiation and dust grain settling, providing new insights into disk chemistry and molecular distributions.

Findings

Grain settling reduces freeze-out regions and increases molecular layers.

Lyman alpha radiation significantly decreases the column densities of HCN, NH3, and CH4.

Some species like CO2 and SO are modestly enhanced by Lyman alpha radiation.

Abstract

We present results from a model of the chemical evolution of protoplanetary disks. In our models we directly calculate the changing propagation and penetration of a high energy radiation field with Lyman alpha radiation included. We also explore the effect on our models of including dust grain settling. We find that, in agreement with earlier studies, the evolution of dust grains plays a large role in determining how deep the UV radiation penetrates into the disk. Significant grain settling at the midplane leads to much smaller freeze-out regions and a correspondingly larger molecular layer, which leads to an increase in column density for molecular species such as CO, CN and SO. The inclusion of Lyman alpha radiation impacts the disk chemistry through specific species that have large photodissociation cross sections at 1216 A. These include HCN, NH3 and CH4, for which the column densities are decreased by an order of magnitude or more due to the presence of Lyman alpha radiation in the UV spectrum. A few species, such as CO2 and SO, are enhanced by the presence of Lyman alpha radiation, but rarely by more than a factor of a few.