State-to-state rotational rate coefficients for the OCS+H₂ collision at low temperatures

¹ Departamento de Física, Facultad de Ciencias, Universidad de Chile, Av. Las Palmeras 3425, 7800003 Ñuñoa, Santiago
² Facultad de Ingeniería, Universidad Autónoma de Chile, Av. Pedro de Valdivia 425, 7500912 Providencia, Santiago, Chile

^★ Corresponding authors: otoniel.denis@uchile.cl;rodrigo.urzua@uautonoma.cl

Received: 23 April 2025
Accepted: 21 May 2025

Abstract

Context. The physicochemical conditions of interstellar regions with low densities, (e.g., typical molecular clouds), should be analyzed using non-LTE models. In such models, the collisional rate coefficients of the observed molecules with H₂, He, and H are critical inputs. In the case of OCS, the only set of rate coefficients available for the collision with H₂ was computed in the seventies, using a potential energy surface (PES) based on an electron gas model for the collision with He. Furthermore, in a recent study on OCS+He, a mass-scaled approximation for the rates was considered, and different propensity rules were found.

Aims. The main goal of this study is to compute a new set of rotational de-excitation rate coefficients of OCS in collision with H₂ at low temperatures.

Methods. An averaged PES over the orientation of H₂ is developed from a large grid of ab initio energies computed at the CCSD(T)/aug-cc-pVQZ level of theory. This surface is employed in close-coupling calculations for studying the collision of OCS with para-H₂(j = 0). Furthermore, an available 4D PES was also used in close-coupling calculations to confirm the results of our first approximation.

Results. The agreement between the cross sections for the OCS+para-H₂ computed using the reduced and 4D PES was very good. The state-to-state rotational de-excitation rate coefficients for the lowest 30 rotational states of OCS by para-H₂ are computed from these data. However, the rate coefficients show different behavior with published data; particularly, a different propensity rule, Δj = 1, is found. Furthermore, similarities between the rates with para- and ortho-H₂ are found. Finally, the astrophysical implications of the new rate coefficients are explored from non-LTE radiative transfer calculations.