Collisional rate coefficients for OH-H$_2$ at high temperatures

Abstract

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. We provide a new set of collisional rate coefficients for OH with H$_2$, expanding the existing data to $j$ levels up to $j=15/2$ and temperatures up to 750 K. These rate coefficients are obtained from state-to-state collision cross sections calculated by means of well-converged close-coupling quantum scattering calculations for collisions of OH with para- and ortho-H$_2$ with energies up to 1700 cm$^{-1}$ ($\simeq 2450$ K). We reproduce the rate coefficients computed by Klos et al. (2017) and extend their results to higher temperatures and higher rotational levels of OH. The de-excitation rate coefficients are lower in collisions with para-H$_2$ ($j_{\rm H_2} = 0$) 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 extrapolation to higher OH rotational states $j_{\rm OH}$. As a first application, we show that under astrophysical conditions typical of warm and dense gas around nascent stars, the populations of low-$j_{\rm OH}$ states are dominated by collisions, even when chemical pumping is included. The full set of rate coefficients is made available in the LAMDA database.