Hyperfine excitation of CH and OH radicals by He

Context
Because of their high reactivity, the CH and OH radicals are of particular interest in astrochemistry. Modelling of CH and OH requires the calculation of accurate radiative and collisional 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 in 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 collisions with ∆F = ∆j 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 describe accurately the OH masers and the hyperfine anomalies in CH emission spectra.