Rotationally inelastic collisions of SiO with H2

The silicon monoxide (SiO) molecule is a key species for the study of the interstellar medium as it is used to trace warm shocked gas. A large number of transitions, including high rotational levels, are observed, and the modelling of these emission lines can provide valuable information on the chemical and physical conditions of the observed regions. In these environments, where the local thermodynamical equilibrium approximation is not valid, an accurate modelling requires collisional rate coefficients with the most abundant species. We focus on the calculation of rate coefficients of SiO in its ground vibrational state in collision with para-and ortho-H-2 using a new high accurate 4D potential energy surface. Dynamical calculations of pure rotational (de) excitation of SiO were performed for the lowest 21 rotational levels using the close-coupling (CC) approach, while the coupled-state (CS) approximation was used to derive rate coefficients among the first 30 rotational levels. State-to-state rate coefficients were obtained for temperatures ranging from 5 to 300 K in the CC calculations and for temperatures up to 1000 K in the CS approximation. Propensity rules show that rate coefficients for Delta j(1) = 1 transitions are dominant for both para-and ortho-H-2 colliders. The rotational rate coefficients are compared with recent results obtained for j(1) <= 5 levels in a full dimensionality approach. These new data will help to model emission lines in warm environments such as shocked layers of molecular outflows in star-forming regions.