The effect of CO-H2O collisions in the rotational excitation of cometary CO

Abstract

We present the first accurate rate coefficients for the rotational excitation of CO by H2O in the kinetic temperature range 5-100 K. The statistical adiabatic channel method (SACM) is combined with a high-level rigid-rotor CO-H2O intermolecular potential surface. Transitions among the first 11 rotational levels of CO and the first 8 rotational levels of both para-H2O and ortho-H2O are considered. Our rate coefficients are compared to previous data from the literature and they are also incorporated in a simple non-LTE model of cometary coma including collision-induced transitions, solar radiative pumping and radiative decay. We find that the uncertainties in the collision data have significant influence on the CO population distribution for H2O densities in the range 10^3-10^8 cm^-3. We also show that the rotational distribution of H2O plays an important role in CO excitation (owing to correlated energy transfer in both CO and H2O), while the impact of the ortho-to-para ratio of H2O is found to be negligible.

8 pages, 7 figures, accepted for publication in MNRAS (January 20 2020)

References in corpus (5)

• CO mass upper limits in the Fomalhaut ring - the importance of NLTE excitation in debris discs and future prospects with ALMA
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• An efficient statistical method to compute molecular collisional rate coefficients
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• Benchmarking an improved statistical adiabatic channel model for competing inelastic and reactive processes
• The effect of water and electron collisions in the rotational excitation of HF in comets
• Collisional Excitation in Space: Recent Advances and Future Challenges in the JWST Era