Collisional rate coefficients for OH–H

Springer Link (Chiba Institute of Technology)

Abstract

Context. OH is a cornerstone molecule in the chemistry of interstellar and circumstellar media and is ubiquitously detected in warm gas thanks to its infrared rotational lines. However, the excitation processes of OH remain poorly characterized. Aims. We provide a new set of collisional rate coefficients for OH with H2, expanding the existing data to j levels up to j = 15/2 and temperatures up to 750 K. Methods. These rate coefficients are obtained from state-to-state collision cross sections calculated by means of well-converged closecoupling quantum scattering calculations for collisions of OH with para- and ortho-H2 with energies up to 1700 cm−1 (≃2450 K). Results. We reproduce the rate coefficients computed by Kłos et al. (2017, MNRAS, 471, 4249) and extend their results to higher temperatures and higher rotational levels of OH. The de-excitation rate coefficients are lower in collisions with para-H2 (jH2 = 0) due to the absence of a quadrupole moment, but this difference decreases at higher temperatures. We find that the rate coefficients follow scaling relations with the energy gap between the upper and lower levels of a given transition, which allows for extrapolation to higher OH rotational states jOH. As a first application, we show that under astrophysical conditions typical of warm and dense gas around nascent stars, the populations of low- jOH states are dominated by collisions, even when chemical pumping is included. The full set of rate coefficients is made available in the LAMDA database. Conclusions. OH infrared emission provides a unique probe of local conditions in astrophysical environments. These rate coefficients contribute to developing a complete excitation model of OH under warm conditions, with chemical pumping of OH through the O + H2 reaction now being the main remaining uncertainty in such models.

Authors 5

1. Z. van den Heuvel

   Radboud University Nijmegen

   Affiliation as printed

   Theoretical Chemistry , Institute for Molecules and Materials , Radboud University , Heyendaalseweg 135 , 6525 AJ Nijmegen , The Netherlands

2. Benoît Tabone

   Centre National de la Recherche Scientifique · Université Paris-Saclay · Institut d'Astrophysique Spatiale

   Affiliation as printed

   Université Paris-Saclay , CNRS , Institut d'Astrophysique Spatiale , 91405 Orsay , France

3. Ewine F. van Dishoeck Aachen Leiden Observatory

   Leiden University · Max Planck Institute for Extraterrestrial Physics · Leiden Observatory

   Affiliation as printed

   Leiden Observatory , Leiden University , P.O. Box 9513 , 2300 RA Leiden , The Netherlands

   Max Planck Institut für Extraterrestrische Physik (MPE) , Giessenbachstrasse 1 , 85748 Garching , Germany

4. Gerrit C. Groenenboom

   Radboud University Nijmegen

   Affiliation as printed

   Theoretical Chemistry , Institute for Molecules and Materials , Radboud University , Heyendaalseweg 135 , 6525 AJ Nijmegen , The Netherlands

5. Ad van der Avoird

   Radboud University Nijmegen

   Affiliation as printed

   Theoretical Chemistry , Institute for Molecules and Materials , Radboud University , Heyendaalseweg 135 , 6525 AJ Nijmegen , The Netherlands