The photodissociation of CO in circumstellar 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.

arXiv commentA&A in press

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• The morpho-kinematics of the circumstellar envelope around the AGB star EP Aqr
• Observation of narrow polar jets in the nascent wind of oxygen-rich AGB star EP Aqr
• Circumstellar envelopes of semi-regular long-period variables: mass-loss rate estimates and general model-fitting of the molecular gas
• On the detection of CO and mass loss of Bulge OH/IR stars
• Sensitivity study of chemistry in AGB outflows using chemical kinetics
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• Reflections on the photodissociation of CO in circumstellar envelopes
• [C I] and [C II] emission in the circumstellar envelope of IRC +10216 I. Observational data and NLTE modeling of the [C I] emission
• Impact of a binary companion in AGB outflows on CO spectral lines
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• The Nearby Evolved Stars Survey II: Constructing a volume-limited sample and first results from the James Clerk Maxwell Telescope