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

Abstract

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 H 2 O and H 2 18 O 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 À1 ) and heliocentric distances (1-3 AU) and demonstrate that water line emission can be easily detected with Herschel.

Submillimeter Wave Astronomy SatelliteMonitoring of the Postperihelion Water Production Rate of Comet C/1999 T1 (McNaught‐Hartley)

“…For the February 2 observations, independent model calculations using a Monte Carlo radiation transfer code ( Bensch & Bergin 2004) give a water production rate of Q H 2 O ¼ (4:53 AE 0:80) ; 10 28 s À1 for x ne ¼ 1:0 and Q H 2 O ¼ (6:71 AE 1:20) ; 10 28 s À1 for x ne ¼ 0:2. These are 12% and 18%, respectively, larger than the water production rate derived here.…”

A study of the distant activity of comet C/2006 W3 (Christensen) withHerscheland ground-based radio telescopes

“…We used the profile derived from the 1P/Halley in-situ measurements and scaled it to the heliocentric distance and activity of comet Christensen, as detailed in e.g. Bensch & Bergin (2004). The electron density was then multiplied by a factor x ne = 0.2, constrained from observations of the 557 GHz water line in other comets (Biver et al 2007;Hartogh et al 2010).…”

Cine: Line excitation by infrared fluorescence in cometary atmospheres

“…Excitation by solar radiation of vibrational bands followed by radiative decay to the ground vibrational state is one of the main mechanisms for molecular excitation in comets. This code calculates the effective pumping rates for rotational levels in the ground vibrational state scaled by the heliocentric distance of the comet as detailed in Bensch and Bergin (2004) and Crovisier and Encrenaz (1983). Line transitions are queried from the latest version of the HITRAN spectroscopic repository (Rothman et al 2013) using the astroquery affiliated package of astropy (Ginsburg et al 2016).…”