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We report the first survey of molecular emission from cometary volatilesusing standalone Atacama Compact Array (ACA) observations of the Atacama LargeMillimeter/Submillimeter Array (ALMA) toward comet C/2015 ER61 (PanSTARRS)carried out on UT 2017 April 11 and 15, shortly after its April 4 outburst.These measurements of HCN, CS, CH$_3$OH, H$_2$CO, and HNC (along with continuumemission from dust) probed the inner coma of C/2015 ER61, revealing asymmetricoutgassing and discerning parent from daughter/distributed source species. Thiswork presents spectrally integrated flux maps, autocorrelation spectra,production rates, and parent scale lengths for each molecule, and a stringentupper limit for CO. HCN is consistent with direct nucleus release in C/2015ER61, whereas CS, H$_2$CO, HNC, and potentially CH$_3$OH are associated withdistributed sources in the coma. Adopting a Haser model, parent scale lengthsdetermined for H$_2$CO (L$_p$ $\sim$ 2200 km) and HNC (L$_p$ $\sim$ 3300 km)are consistent with previous work in comets, whereas significant extendedsource production (L$_p$ $\sim$ 2000 km) is indicated for CS, suggestingproduction from an unknown parent in the coma. The continuum presents apoint-source distribution, with a flux density implying an excessively largenucleus, inconsistent with other estimates of the nucleus size. It is bestexplained by the thermal emission of slowly-moving outburst ejectas, with totalmass 5--8 $\times$ 10$^{10}$ kg. These results demonstrate the power of the ACAfor revealing the abundances, spatial distributions, and locations of molecularproduction for volatiles in moderately bright comets such as C/2015 ER61.

astrophysical journal/the astrophysical journal

Leveraging the ALMA Atacama Compact Array for Cometary Science: An  Interferometric Survey of Comet C/2015 ER61 (PanSTARRS) and Evidence for a  Distributed Source of Carbon Monosulfide