Collisional excitation of water in warm astrophysical media

Abstract

Context. The interpretation of water line emission from infrared and submillimetre observations requires a detailed knowledge of collisional rate coefficients over a wide range of levels and temperatures. Aims. We attempt to determine rotational and rovibrational rate coefficients for H 2 O colliding with both H 2 and electrons in warm, molecular gas. Methods. Pure rotational rates are derived by extrapolating published data using a new method partly based on the information (phase space) theory of Levine and co-workers. Ro-vibrational rates are obtained using vibrational relaxation data available in the literature and by assuming a complete decoupling of rotation and vibration. Results. Rate coefficients were obtained for the lowest 824 ro-vibrational levels of H 2 O in the temperature range 200-5000 K. Our data is expected to be accurate to within a factor of 5 for the highest rates ( > 10 -11 cm 3 s -1 ). Smaller rates, including the rovibrational ones, should be generally accurate to within an order of magnitude. As a first application of this data, we show that vibrationally excited water emission observed in evolved stars is expected to be at least partly excited by means of collisions.