Fine-structure-resolved Rovibrational Excitation of OH due to Collisions with He for Nonequilibrium Emission Modeling

The hydroxyl radical (OH) is a key species in the water chemistry of star-forming regions. State-to-state rate coefficients for transitions induced by abundant perturbers like He are needed for astrophysical models in regions where the molecular internal state populations are not in local thermodynamic equilibrium (LTE). This work presents state-to-state rate coefficients for rovibrational transitions of OH(X ²Π) induced by helium collisions for fine-structure-resolved levels from the first rovibrational band of OH. These were obtained over the temperature range 1–1000 K from quantum-mechanical calculations for the inelastic cross sections, which explicitly included fine structure resolution in the dynamics for the first time. Rate coefficients for vibrational-state-changing transitions can be relatively large owing to small energy gaps that are sometimes present between ground and first-excited rovibrational levels of OH(X ²Π). The current rate coefficients are applied in a non-LTE radiative transfer model, which finds good agreement for OH observations of the planet-forming disk d203-506 obtained by the James Webb Space Telescope.