H 2 O‐H 2 O Collision Rate Coefficients

Abstract

collision cross sections are calculated by adapting the method developed by Anderson and H 2 O-H 2 O Tsao and Cornette for the calculation of collision broadening of molecular spectral lines. Cross sections are computed with a cuto limit at 20 for the rotational levels of water with J ¹ 10 and E \ 2000 A Ž 2 cm~1 in the temperature range 100È800 K. The accuracy of the calculated values is estimated to be 10%È20%. Subject headings : ISM : molecules È methods : numerical È molecular processes 1. INTRODUCTION Molecular collision rates are essential for describing energy exchange processes responsible for the thermal balance, spectral features, and line formation dynamics in interstellar objects and comets. Thermal balance within such objects is determined by the radiative cooling rates due to collisionally excited molecules. Spectral features often are produced under conditions far from thermodynamic equi-librium. In order to interpret the observed spectra, it is necessary to know the rates of all microscopic processes that determine the populations of the emitting and absorb-ing levels. Finally, line shape and line formation are pheno-mena that involve collisions between the molecules and the medium. Despite some progress in laboratory measure-ments of state-to-state collision rates in recent years, still the astrophysical models heavily rely on theoretical estimates. Water is one of the abundant species in molecular clouds, in particular in star-forming regions, in circumstellar envelopes, and in comets. Many lines of water are detected in interstellar objects with both ground-based and airborne telescopes. In regions of high densities inside molecular clouds and circumstellar envelopes, water shows maser e ect through its rotational transition at 22.2 GHz. 6 16 È5 23 Water has been observed in the comets C/1996 B2 Hyaku-take and C/1995 O1 Hale-Bopp (Crovisier 1998) and in the impact of the Shoemaker-Levy 9 with Jupiter (Bjoraker et al. 1996 ; Cosmovici et al. 1996). In conditions where is much more abundant than any H 2 other species, most of the collision excitation of water is induced by However, many of the calculations of cross H 2 . sections for interstellar collision processes have been carried out with He as the collision partner rather than In fact, H 2 . in its ground-state rotational level J \ 0 behaves para-H 2 similarly to the corresponding inert gas, when the mass di erence is taken into account. This analogy holds only for in the J \ 0 level and not for whose para-H 2 ortho-H 2 , ground state is J \ 1, nor for in levels with para-H 2 J D 0.