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

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 toward comet 46P/Wirtanen, obtained using the Atacama Large Millimeter/submillimeter Array during the comet’s unusually close (∼0.1 au) approach to Earth in 2018 December. Interferometric maps of HCN, CH _3 OH, CH _3 CN, H _2 CO, 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, CH _3 OH, and CH _3 CN spatial distributions are consistent with production by direct outgassing from (or very close to) the nucleus, with a significant proportion of the observed CH _3 OH originating from sublimation of icy grains in the near-nucleus coma (at a scale length L _p = 36 ± 7 km). On the other hand, H _2 CO, 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, CH _3 OH, and HNC abundances in 46P are consistent with the average values previously observed in comets, whereas the H _2 CO, CH _3 CN, and CS abundances are relatively low.

Keywords

• Short period comets
• Comet volatiles
• Interstellar molecules
• Radio interferometry
• Millimeter-wave spectroscopy
• Molecular spectroscopy