Quasi-classical rate coefficient calculations for the rotational (de)excitation of H₂O by H₂^*

¹ Laboratoire d'Astrophysique, UMR 5571 CNRS, Université Joseph-Fourier, BP 53, 38041 Grenoble Cedex 09, France e-mail: afaure@obs.ujf-grenoble.fr
² Observatoire de Paris-Meudon, LERMA UMR 8112 CNRS, 5 place Jules Janssen, 92195 Meudon Cedex, France

Received: 19 April 2007
Accepted: 4 June 2007

Abstract

Context.The interpretation of water line emission from existing observations and future HIFI/Herschel data requires a detailed knowledge of collisional rate coefficients. Among all relevant collisional mechanisms, the rotational (de)excitation of H₂O by H₂ molecules is the process of most interest in interstellar space.

Aims.To determine rate coefficients for rotational de-excitation among the lowest 45 para and 45 ortho rotational levels of H₂O colliding with both para and ortho-H₂ in the temperature range 20-2000 K.

Methods.Rate coefficients are calculated on a recent high-accuracy H₂O-H₂ potential energy surface using quasi-classical trajectory calculations. Trajectories are sampled by a canonical Monte-Carlo procedure. H₂ molecules are assumed to be rotationally thermalized at the kinetic temperature.

Results.By comparison with quantum calculations available for low lying levels, classical rates are found to be accurate within a factor of 1-3 for the dominant transitions, that is those with rates larger than a few 10^-12 cm³ s^-1. Large velocity gradient modelling shows that the new rates have a significant impact on emission line fluxes and that they should be adopted in any detailed population model of water in warm and hot environments.