The Pure Rotational Line Emission of Ortho-Water Vapor in Comets. I. Radiative Transfer Model

We present a numerical model for the simulation of water line emission in cometary coma. The model is based on a spherically symmetric density distribution with a constant expansion velocity (Haser model) and the Monte Carlo radiative transfer code published by Hogerheijde & van der Tak. It includes the seven lowest rotational levels of ortho-water, which are the primarily populated levels in the rotationally cold gas of the coma. We discuss the main excitation mechanisms for ortho-water in the coma and study their relative contribution as a function of distance from the comet nucleus. The model is used to derive the water production rate from observations made with the Submillimeter Wave Astronomy Satellite toward comet C/1999 T1 (McNaught-Hartley). They differ from the water production rates derived with an independent model by less than 20% and thus agree within the larger uncertainty due to the limited signal-to-noise ratio of the observations. We give predictions for spectral line observations of H2O and H218O in comets with present and future airborne and space observatories, including ESA's Herschel Space Observatory and the Stratospheric Observatory for Infrared Astronomy (SOFIA). These models cover a range of water vapor production rates (1027-1029 s-1) and heliocentric distances (1-3 AU) and demonstrate that water line emission can be easily detected with Herschel.

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