Rotational quenching rate coefficients for H_2 in collisions with H_2 from 2 to 10,000 K

TL;DR

This paper presents new quantum mechanical calculations of rotational quenching rate coefficients for H_2 molecules colliding with each other over a broad temperature range, improving the reliability of data used in astrophysical models.

Contribution

The study provides the first extensive set of rate coefficients based on a more reliable potential energy surface, covering temperatures from 2 to 10,000 K, extending previous work.

Findings

Good agreement with previous data at intermediate temperatures

Discrepancies observed at the lowest and highest temperatures

New cooling functions for molecular regions derived from the data

Abstract

Rate coefficients for rotational transitions in H_2 induced by H_2 impact are presented. Extensive quantum mechanical coupled-channel calculations based on a recently published (H_2)_2 potential energy surface were performed. The potential energy surface used here is presumed to be more reliable than surfaces used in previous work. Rotational transition cross sections with initial levels J <= 8 were computed for collision energies ranging between 0.0001 and 2.5 eV, and the corresponding rate coefficients were calculated for the temperature range 2 < T <10,000 K. In general, agreement with earlier calculations, which were limited to 100-6000 K, is good though discrepancies are found at the lowest and highest temperatures. Low-density-limit cooling functions due to para- and ortho-H_2 collisions are obtained from the collisional rate coefficients. Implications of the new results for non-thermal H_2 rotational distributions in molecular regions are also investigated.