Abstract
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 H2 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-H2, 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+–H2 system, and adapted to the DCO+–H2 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-H2 were compared with the available data, and a set of rate coefficients for HCO+ + ortho-H2 is also reported. The difference between the collision rates with ortho- and para-H2 is found to be small. These calculations confirm that the use of the rate coefficients for HCO+ + para-H2 for estimating those for HCO+ + ortho-H2 as well as for DCO+ + para-H2 is a good approximation.
Highlights
Molecular cloud temperatures can range from 5 K up to a few tens of K, and densities from 102 up to 106 cm−3, the temperature and densities of most molecular clouds are low, 10–15 K and 102– 104 cm−3, respectively, and the collisions are rare
HCO+ and DCO+ are often used as tracers of ionization in dense cores (Caselli et al 1998) and DCO+ is observed in the molecular layer of protoplanetary discs (e.g. Dutrey et al 2014)
Accurate rate coefficients of these systems with He and H2 are important for a correct description of the physico-chemical conditions in these astrophysical media
Summary
Molecular cloud temperatures can range from 5 K up to a few tens of K, and densities from 102 up to 106 cm−3, the temperature and densities of most molecular clouds are low, 10–15 K and 102– 104 cm−3, respectively, and the collisions are rare. The physico-chemical conditions in the molecular clouds should be determined by nonlocal thermal equilibrium (non-LTE) models. Such models require the knowledge of the Einstein coefficients, usually known, and the state-to-state rate coefficients of the observed molecule with the most common colliders in the interstellar medium (ISM), named H2, He, e, and H. HCO+, discovered by Buhl & Snyder (1970), was the first cation identified in the cold ISM (Kraemer & Diercksen 1976) It has been detected almost in all the regions of the ISM, including dense cores, molecular outflows, and even diffuse clouds (Lucas & Liszt 1996). Accurate rate coefficients of these systems with He and H2 are important for a correct description of the physico-chemical conditions in these astrophysical media
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