The photodissociation of CO in circumstellar envelopes

TL;DR

This paper updates the classical model of CO photodissociation in circumstellar envelopes by incorporating recent data, improved methods, and the impact of interstellar radiation, providing a more comprehensive understanding of CO envelope sizes.

Contribution

It extends the Mamon et al. (1988) model with enhanced numerical techniques, updated shielding functions, and a broader parameter space including ISRF effects.

Findings

Updated CO abundance profiles for various parameters

Demonstrated impact of photodissociation radius on CO line intensities

Provided a larger grid for modeling circumstellar CO envelopes

Abstract

Carbon monoxide is the most abundant molecule after H$_2$ and is important for chemistry in circumstellar envelopes around late-type stars. The size of the envelope is important when modelling low-J transition lines and deriving mass-loss rates from such lines. Now that ALMA is coming to full power the extent of the CO emitting region can be measured directly for nearby asymptotic giant branch (AGB) stars. In parallel, it has become obvious in the past few years that the strength of the interstellar radiation field (ISRF) can have a significant impact on the interpretation of the emission lines. In this paper an update and extension of the classical Mamon et al. (1988; ApJ 328, 797) paper is presented; these authors provided the CO abundance profile, described by two parameters, as a function of mass-loss rate and expansion velocity. Following recent work an improved numerical method and updated H$_2$ and CO shielding functions are used and a larger grid is calculated that covers more parameter space, including the strength of the ISRF. The effect of changing the photodissociation radius on the low-J CO line intensities is illustrated in two cases.