Gas Sources from the Coma and Nucleus of Comet 46P/Wirtanen Observed Using ALMA

TL;DR

This study uses ALMA to map molecular emissions in comet 46P/Wirtanen, revealing the origins of various gases from the nucleus and coma with unprecedented spatial resolution, and comparing their abundances to typical comet values.

Contribution

First high-resolution interferometric mapping of multiple molecules in comet 46P/Wirtanen, distinguishing nucleus and coma sources and quantifying their spatial distributions.

Findings

HCN, CH3OH, CH3CN originate near the nucleus

H2CO, CS, HNC mainly from distributed coma sources

Some molecular abundances are consistent with typical comets, others are lower

Abstract

Gas-phase molecules in cometary atmospheres (comae) originate primarily from (1) outgassing by the nucleus, (2) sublimation of icy grains in the near-nucleus coma, and (3) coma (photo-)chemical processes. However, the majority of cometary gases observed at radio wavelengths have yet to be mapped, so their production/release mechanisms remain uncertain. Here we present observations of six molecular species towards comet 46P/Wirtanen, obtained using the Atacama Large Millimeter/submillimeter Array (ALMA) during the comet's unusually close (~0.1 au) approach to Earth in December 2018. Interferometric maps of HCN, CH3OH, CH3CN, H2CO, CS and HNC were obtained at an unprecedented sky-projected spatial resolution of up to 25 km, enabling the nucleus and coma sources of these molecules to be accurately quantified. The HCN, CH3OH and CH3CN spatial distributions are consistent with production by direct outgassing from (or very near to) the nucleus, with a significant proportion of the observed CH3OH originating from sublimation of icy grains in the near-nucleus coma (at a scale-length $L_p=36\pm7$ km). On the other hand, H2CO, CS and HNC originate primarily from distributed coma sources (with $L_p$ values in the range 550-16,000 km), the identities of which remain to be established. The HCN, CH3OH and HNC abundances in 46P are consistent with the average values previously observed in comets, whereas the H2CO, CH3CN and CS abundances are relatively low.