Collisional excitation of O sub(2) by H sub(2): the validity of LTE models in interpreting O sub(2) observations

Context. Oxygen molecules (O sub(2) are of particular interest because of their crucial role in astrochemisty. Modelling of O sub(2) sub()molecular emission spectra from interstellar clouds requires the calculation of rate coefficients for excitation by collisions with the most abundant species. Aims. Rotational excitation of O sub(2)(X super(3) capital sigma super(-)) by H sub(2) is investigated theoretically and experimentally and we check the validity of the local thermodynamic equilibrium (LTE) approach for interpreting O sub(2) observations. Methods. Using a new ab initio potential energy surface, collisional excitation of O sub(2) is studied using a full close-coupling approach. The theoretical calculations are validated by comparison with crossed beam scattering experiments. We also performed calculations for the excitation of O sub(2) from a large velocity gradient (LVG) radiative transfer code using the new rate coefficients. Results. State-to-state rate coefficients between the 27 lowest levels of O sub(2) were calculated for temperatures ranging from 5 K to 150 K. The critical densities of the O sub(2) lines are found to be at > =10 super(4) cm super(-3) for temperatures higher than 50 K. This value is slightly larger than the one previously determined using previous He rate coefficients. Conclusions. The new rate coefficients will help in interpreting O sub(2) emission lines observed where LTE conditions are not fully fulfilled and enable an accurate determination of the O sub(2) abundance in the interstellar medium.