Water production in comet 81P/Wild 2 as determined by Herschel/HIFI

TL;DR

This study uses Herschel/HIFI to detect water lines in comet 81P/Wild 2, accurately measuring water production rates and physical conditions of the comet's coma.

Contribution

First high-resolution Herschel/HIFI observations of water in comet 81P/Wild 2, providing detailed water production rates and coma physical parameters.

Findings

Water production rate of 1.0 x 10^28 molecules/sec at 1.61 AU

Constrained electron density and gas temperature profiles

Estimated coma expansion velocity from line shape fitting

Abstract

The high spectral resolution and sensitivity of Herschel/HIFI allows for the detection of multiple rotational water lines and accurate determinations of water production rates in comets. In this letter we present HIFI observations of the fundamental 110-101 (557 GHz) ortho and 111-000 (1113 GHz) para rotational transitions of water in comet 81P/Wild 2 acquired in February 2010. We mapped the extent of the water line emission with five point scans. Line profiles are computed using excitation models which include excitation by collisions with electrons and neutrals and solar infrared radiation. We derive a mean water production rate of $1.0 \times 10^{28}$ molecules s$^{-1}$ at a heliocentric distance of 1.61 AU about 20 days before perihelion, in agreement with production rates measured from the ground using observations of the 18-cm OH lines. Furthermore, we constrain the electron density profile and gas kinetic temperature, and estimate the coma expansion velocity by fitting the water line shapes.