Collisional excitation of water in warm astrophysical media ^*

I. Rate coefficients for rovibrationally excited states

¹ Laboratoire d'Astrophysique de Grenoble (LAOG), Université Joseph-Fourier, UMR 5571 CNRS, BP 53, 38041 Grenoble Cedex 09, France e-mail: afaure@obs.ujf-grenoble.fr
² Groupe de Recherche en Astronomie et Astrophysique du Languedoc (GRAAL), Université Montpellier II, UMR 5024 CNRS, 34095 Montpellier Cedex 05, France

Received: 30 July 2008
Accepted: 25 September 2008

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 H₂O colliding with both H₂ 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 H₂O 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 cm³ s^-1). Smaller rates, including the ro-vibrational 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.