Cold CO in circumstellar disks: On the effects of photodesorption and vertical mixing

TL;DR

This study investigates how photodesorption and turbulent mixing influence the presence of gas-phase CO in cold circumstellar disks, highlighting photodesorption's significant role and the complex, parameter-dependent effects of turbulent mixing.

Contribution

It provides a detailed chemical model including grain surface reactions, assessing the impacts of photodesorption and turbulent mixing on CO abundance in disks.

Findings

Photodesorption enhances gas-phase CO below 5 magnitudes of visual extinction.

The radial column density profile from photodesorption does not match observations, possibly due to grain growth.

Turbulent mixing shows no significant effect on CO column density for the parameters tested.

Abstract

Aims. We attempt to understand the presence of gas phase CO below its freezing temperature in circumstellar disks. We study two promising mechanisms to explain this phenomenon: turbulent mixing and photodesorption. Methods. We compute the chemical evolution of circumstellar disks including grain surface reactions with and without turbulent mixing and CO photodesorption. Results. We show that photodesorption significantly enhances the gas phase CO abundance, by extracting CO from the grains when the visual extinction remains below about 5 magnitudes. However the resulting dependence of column density on radial distance is not consistent with observations so far. We propose that this inconsistency could be the result of grain growth. On the other hand, the influence of turbulent mixing is not found to be straightforward. The efficiency of turbulent mixing depends upon a variety of parameters, including the disk structure. For the set of parameters we chose, turbulent mixing is not found to have any significant influence on the CO column density.