An accurate set of H3O+ − H2 collisional rate coefficients for non-LTE modelling of warm interstellar clouds

Abstract

Hydronium (H3O+) was first detected in 1986 in interstellar molecular clouds. It was reported in many galactic diffuse and dense regions, as well as in extragalactic sources. H3O+ plays a major role both in interstellar oxygen and water chemistry. However, despite the large number of H3O+ observations, its collisional excitation was investigated only partially. In the present work we study the state-to-state rotational de-excitation of ortho- and para-H3O+ in collisions both with ortho- and para-H2. The cross sections are calculated within the close-coupling formalism using a highly accurate potential energy surface developed for this system. The rate coefficients are computed up to 300 K kinetic temperature. Transitions between the lowest 21 rotation-inversion states were studied for para-H3O+, and the lowest 11 states for ortho-H3O+, i.e. all levels with rotational energies below 430 K (∼300 cm−1) are considered (up to j ≤ 5). In order to estimate the impact of the new rate coefficients on the astrophysical models for H3O+, radiative transfer calculations were also carried out. We have examined how the new collisional data affect the line intensities with respect to older data previously used for the interpretation of observations. By analysing all detected transitions we find that our new, accurate rate coefficients have a significant impact (typically within a factor of 2) on radiation temperatures, allowing more accurate estimation of column densities and relative abundances of hydronium, especially in warm molecular clouds, paving the path towards better interpretation of interstellar water and oxygen chemistry.

The role of small molecular cations in the chemical flow of the interstellar environments

“…Predicted to be present in dense molecular clouds by Herbst and Klemperer 6 in 1973, H 3 O + was first observed astronomically in 1986 by Wootten et al 120 and Hollis et al 121 A review of subsequent observations and the characteristics of the species is given in the work of Demes et al 122 It is believed that the observation of H 3 O + in ISM is adequate proof for the existence of H 2 O in the same environment. Dissociative recombination of the former leads to the latter:H 3 O + + e → H 2 O + H…”

“…Demes et al 122,123 reported the results of quantum mechanical close coupling calculations using a 5D ab initio PES 125 in the form of rotational de-excitation cross sections for o -/ p -H 3 O + in collision with o -/ p -H 2 for a range of E trans . They reported many Feshbach and shape resonances, particularly at lower energies.…”

“…Demes et al 122,123 reported the results of quantum mechanical close coupling calculations using a 5D ab initio PES 125 in the form of rotational de-excitation cross sections for o-/p-H 3 O + in collision with o-/p-H 2…”

BASECOL2023 scientific content

“…H 3 O + Four recommended datasets are available for the H 3 O + molecular ion. BASECOL2023 has been updated with four datasets (Demes et al 2022) for the rotational de-excitation of the lowest 11 rotation-inversion levels of o-H 3 O + with o/p-H 2 and of the 21 lowest rotation-inversion levels of p-H 3 O + with o/p-H 2 , for temperatures between 10 K and 300 K. The collisional datasets with p-H 2 include calculations described in Demes et al (2021Demes et al ( , 2022, and all four datasets were calculated with the PES of Demes et al (2020). 9.13.…”