1 Introduction

This paper is the first part of a thorough theoretical study of the collisional excitation rates of H₂O by H₂, which will be used to interpret data from HIFI (Herschel Mission). Many transitions of the H₂O molecule will be observed by HIFI in different environments such as the interstellar medium and stellar or planetary atmospheres. These observations will complete the wealth of data already obtained by the Infrared Space Observatory (see for example Spinoglio et al. 2001; Tsuji 2001; Wright et al. 2000), and the Submillimeter Wave Astronomy Satellite (Melnick et al. 2000). Collisional rotational and ro-vibrational excitation rates of H₂O by H₂ are essential for the interpretation of excitation conditions and for the determination of chemical composition in the different media.

The only available excitation rates of H₂O by H₂ are those calculated by Phillips et al. (1995, 1996), using the close-coupling CC and the coupled states (CS) methods with a potential energy surface (PES) calculated by Phillips et al. (1994). Those authors provided data between 20 K and 140 K for a number of ortho/para H₂O - ortho/para H₂ pure rotational transitions. A previous study by Green et al. (1993) provided the excitation rotational rates of H₂O by He for temperature between 20 K and 2000 K using the improved PES of Maluendes et al. (1992).

The main objective of the present paper is the determination of pure rotational excitation rates of ortho/para H₂O by para H₂, at very low temperatures ($T\leq20$ K) where no data have yet been calculated. We use the same PES (Phillips et al. 1994) and the same description of the water molecule as Phillips et al. (1995), which allows us to compare the collision rates we obtain at 20 K with those of Phillips et al. (1995, 1996). Furthermore we examine the validity of a fitted function that is sometimes used in the interpretation of observed spectra or in modelling the ISM.

Both the description of the quantum calculations and the convergence of the calculated collisional cross sections are presented in Sect. 2. The analysis of the rotational excitation rates is considered in Sect. 3.