Hyperfine excitation of CH and OH radicals by He

Abstract

Context. Because of their high reactivity, the CH and OH radicals are of particular interest in astrochemistry. Modeling of CH and OH molecules requires the calculation of accurate radiative Einstein coefficients and rate coefficients for (de)excitation by collisions with the most abundant species such as H2 and He. Aims. The present paper focuses on the calculation of inelastic rate coefficients among the lowest OH/CH hyperfine levels in their ground vibrational state induced by collisions with He atoms. Methods. Calculations of hyperfine (de)excitation of CH/OH by He were performed using the close-coupling and recoupling methods from the most recent ab initio potential energy surfaces. Results. Cross sections for transitions among the 60 and 56 lowest hyperfine levels of CH and OH, respectively, were calculated for collision energies up to 2500 cm−1. These cross-sections were used to calculate the rate coefficients for temperatures between 10 and 300 K. A propensity rule for transitions with ΔF = Δj was observed. Conclusions. The new rate coefficients will help significantly in interpreting the CH/OH spectroscopic data observed with current and future telescopes, and help to accurately describe the OH masers and the hyperfine anomalies in CH emission spectra.