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Rotational relaxation of HCO+ and DCO+ by collision with H2

Abstract : The HCO + and DCO + molecules are commonly used as tracers in the interstellar medium. Therefore, accurate rotational rate coefficients of these systems with He and H 2 are crucial in non-local thermal equilibrium models. We determine in this work the rotational de-excitation rate coefficients of HCO + in collision with both para-and ortho-H 2 , and also analyse the isotopic effects by studying the case of DCO +. A new four-dimensional potential energy surface from ab initio calculations was developed for the HCO +-H 2 system, and adapted to the DCO +-H 2 case. These surfaces are then employed in close-coupling calculations to determine the rotational de-excitation cross-sections and rate coefficients for the lower rotational states of HCO + and DCO +. The new rate coefficients for HCO + + para-H 2 were compared with the available data, and a set of rate coefficients for HCO + + ortho-H 2 is also reported. The difference between the collision rates with ortho-and para-H 2 is found to be small. These calculations confirm that the use of the rate coefficients for HCO + + para-H 2 for estimating those for HCO + + ortho-H 2 as well as for DCO + + para-H 2 is a good approximation.
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https://hal-cnrs.archives-ouvertes.fr/hal-03044185
Contributor : Thierry Stoecklin <>
Submitted on : Wednesday, December 16, 2020 - 6:13:26 PM
Last modification on : Thursday, December 17, 2020 - 11:41:42 AM
Long-term archiving on: : Wednesday, March 17, 2021 - 6:11:38 PM

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Otoniel Denis-Alpizar, Thierry Stoecklin, Anne Dutrey, Stéphane Guilloteau. Rotational relaxation of HCO+ and DCO+ by collision with H2. Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP): Policy P - Oxford Open Option A, 2020, 497 (4), pp.4276 - 4281. ⟨10.1093/mnras/staa2308⟩. ⟨hal-03044185⟩

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