Rotational collisional excitation of H2O by H2: collisional rate coefficients and pressure broadening coefficients

Details Rotational collisional excitation of H2O by H2: collisional rate coefficients and pressure broadening coefficients Rotational collisional excitation of H2O by H2: collisional rate coefficients and pressure broadening coefficients

Dec 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004sf2a.conf..491d&link_type=abstract

SF2A-2004: Semaine de l'Astrophysique Francaise, meeting held in Paris, France, June 14-18, 2004. Edited by F. Combes, D. Barret

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Scientific paper

Water is a key molecule for the chemistry and the energy balance of the gas in cold clouds and star forming regions, thanks to its relatively large abundance and large dipole moment. Chemical models have predicted for a long time the water abundance to be around 10-7 in cold clouds, i.e. among the most abundant O-bearing molecules. Yet, in warm regions water is expected to become the main reservoir of oxygen, thanks to both evaporation of the icy grain mantles and endothermic reactions that transform all the gaseous oxygen not in CO into H2O. For this reason, water modeling deserves a particular attention. In this respect, 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 study has been motivated not only by the interpretation of the data recently acquired by ISO and SWAS, but also by the future ESA mission, the Herschel Space Observatory (hereinafter HSO): several water lines will be observed with unprecedent sensitivity in different environments, from the interstellar medium to the stellar or planetary atmospheres. Our project is a thorough theoretical study of the collisional excitation rates of H2O by H2 on a wide range of temperature (5K - 1000K) and for all transitions of interest. We use a new potential energy surface averaged on the fundamental vibrational states of both H2O and H2. Preliminary results show that the new rate coefficients might differ by as much as 100% from previously calculated rate coefficients. Our goal is to get the best possible accuracy on the whole range of temperature and for all levels of interest. This is a challenge that we will first tackle through optimization of the codes and careful use of approximations. We will present calculations up to 300K and tests of various approximations up to 1000K. The line parameters calculations at 300K will be compared to experimental results, they will provide a valuable test of the new potential energy surface.

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