Reflections on the photodissociation of CO in circumstellar envelopes

TL;DR

This paper examines the accuracy of different models for CO photodissociation in circumstellar envelopes, finding that previous approximations are less accurate than claimed, but resulting in only small to moderate differences in CO envelope size.

Contribution

It provides an improved formalism for calculating CO photodissociation radii, correcting earlier approximation methods used in astrophysical models.

Findings

Previous approximations are less accurate than claimed.

Changes in CO envelope size are generally less than 2%.

Maximum change in size is about 7% for very low mass-loss rates.

Abstract

Carbon monoxide (CO) is the most abundant molecule after molecular hydrogen and is important for the chemistry in circumstellar envelopes around evolved stars. When modelling the strength and shape of molecular lines, the size of the CO envelope is an input parameter and influences the derived mass-loss rates. In particular the low-J transition CO lines are sensitive to the CO photodissociation radius. Recently, new CO photodissociation radii have been published using different formalisms that differ considerably. One set of calculations is based on an escape-probability formalisms that uses numerical approximations derived in the early-eighties. The accuracy of these approximations is investigated and it is shown that they are less accurate than claimed. Improved formalism are derived. Nevertheless, the changes in CO envelope size are small to moderate, less than 2\% for models with $10^{-7}< \dot{M}< 10^{-4}$ \msolyr\ and at most 7\% for model with $\dot{M} = 10^{-8}$ \msolyr.