Herschel/SPIRE observations of water production rates and ortho-to-para ratios in comets

Abstract

This paper presents 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.

Fitting the Light Curve of 1I/‘Oumuamua with a Nonprincipal Axis Rotational Model and Outgassing Torques

“…Separately, the constant torque imposed by Mashchenko (2019) necessarily induces a spin-up in 'Oumuamua's rotation. While this is in agreement with the known spin-up effect of outgassing (Jewitt et al 2003;Drahus et al 2011;Gicquel et al 2012;Maquet et al 2012;Fernández et al 2013;Eisner et al 2017;Wilson et al 2017;Roth et al 2018;Kokotanekova et al 2018;Biver et al 2019;Combi et al 2020;Jewitt 2021Jewitt , 2022, the model used would lead to 'Oumuamua's destruction due to centrifugal forces. For example, Jewitt & Luu (2019a, 2019b predicted that the second interstellar object 2I/Borisov would suffer disintegration events due to the spin-up timescale implied by the nucleus size and production rates, which were later observed Jewitt et al (2020).…”

“…While the outgassing behavior of 'Oumuamua is unknown, the substellar model is similar to the outgassing of at least one comet for which in situ measurements exist (Taylor et al 2017). Despite the relevance of the Sun-tracking jet to 67P, empirical results for larger samples of comets have demonstrated that outgassing torques are generally responsible for nucleus spin-up (Jewitt et al 2003;Drahus et al 2011;Gicquel et al 2012;Maquet et al 2012;Fernández et al 2013;Eisner et al 2017;Wilson et al 2017;Kokotanekova et al 2018;Roth et al 2018;Biver et al 2019;Combi et al 2020;Jewitt 2021Jewitt , 2022 or spin-down (Eisner et al 2017;Bodewits et al 2018;Farnham et al 2021).…”