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 (H2O)-O-18 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 (10(27)-10(29) s(-1)) and heliocentric distances (1-3 AU) and demonstrate that water line emission can be easily detected with Herschel.