$\textit{Herschel}$/SPIRE Observations of Water Production Rates and Ortho-to-Para Ratios in Comets

TL;DR

This study uses Herschel/SPIRE spectroscopy to measure water production rates and ortho-to-para ratios in comets, revealing variations likely caused by post-sublimation nuclear-spin conversion processes.

Contribution

It provides new measurements of water OPR and production rates in multiple comets, linking OPR variations to nuclear-spin conversion after sublimation.

Findings

OPRs in three comets are lower than the statistical value of 3.

Water production rates decrease with increasing nucleocentric offset.

OPR variations are consistent with post-sublimation nuclear-spin conversion evidence.

Abstract

This paper presents $\textit{Herschel}$/SPIRE spectroscopic observations of several fundamental rotational ortho- and para-water transitions seen in three Jupiter-family comets and one Oort-cloud comet. Radiative transfer models that include excitation by collisions with neutrals and electrons, and by solar infrared radiation were used to produce synthetic emission line profiles originating in the cometary coma. Ortho-to-para ratios (OPRs) were determined and used to derived water production rates for all comets. Comparisons are made with the water production rates derived using an OPR of 3. The OPR of three of the comets in this study are much lower than the statistical equilibrium value of 3, however they agree with observations of comets 1P/Halley and C/2001 A2 (LINEAR), and the protoplanetary disc TW Hydrae. These results provide evidence suggesting that OPR variation is caused by post-sublimation gas-phase nuclear-spin conversion processes. The water production rates of all comets agree with previous work and, in general, decrease with increasing nucleocentric offset. This could be due to a temperature profile, additional water source, or OPR variation in the comae, or model inaccuracies.