The excitation of OH by H2 revisited - I: fine-structure resolved rate coefficients

Observations of OH in molecular clouds provide crucial constraints on both the physical conditions and the oxygen and water chemistry in these clouds. Accurate modelling of the OH emission spectra requires the calculation of rate coefficients for excitation of OH by collisions with the most abundant collisional partner in the molecular clouds, namely the H-2 molecule. We report here theoretical calculations for the fine-structure excitation of OH by H-2 (both para-and ortho-H-2) using a recently developed highly accurate potential energy surface. Full quantum close coupling rate coefficients are provided for temperatures ranging from 10 to 150 K. Propensity rules are discussed and the new OH-H-2 rate coefficients are compared to the earlier values that are currently used in astrophysical modelling. Significant differences were found: the new rate coefficients are significantly larger. As a first application, we simulate the excitation of OH in typical cold molecular clouds and star-forming regions. The new rate coefficients predict substantially larger line intensities. As a consequence, OH abundances derived from observations will be reduced from the values predicted by the earlier rate coefficients.