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

Abstract

We report the first survey of molecular emission from cometary volatiles using standalone Atacama Compact Array (ACA) observations of the Atacama Large Millimeter/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 continuum emission from dust) probed the inner coma of C/2015 ER61, revealing asymmetric outgassing and discerning parent from daughter/distributed source species. This work presents spectrally integrated flux maps, autocorrelation spectra, production rates, and parent scale lengths for each molecule, and a stringent upper limit for CO. HCN is consistent with direct nucleus release in C/2015 ER61, whereas CS, H${}_2$CO, HNC, and potentially CH${}_3$OH are associated with distributed sources in the coma. Adopting a Haser model, parent scale lengths determined 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 extended source production (L${}_p$ $\sim$ 2000 km) is indicated for CS, suggesting production from an unknown parent in the coma. The continuum presents a point-source distribution, with a flux density implying an excessively large nucleus, inconsistent with other estimates of the nucleus size. It is best explained by the thermal emission of slowly-moving outburst ejectas, with total mass 5--8 $\times$ 10${}^{10}$ kg. These results demonstrate the power of the ACA for revealing the abundances, spatial distributions, and locations of molecular production for volatiles in moderately bright comets such as C/2015 ER61.

References in corpus (19)

• An atomic and molecular database for analysis of submillimetre line observations
• LIME - a flexible, non-LTE line excitation and radiation transfer method for millimeter and far-infrared wavelengths
• Unusually High CO Abundance of the First Active Interstellar Comet
• Thirty Years of Cometary Spectroscopy from McDonald Observatory
• Terrestrial deuterium-to-hydrogen ratio in water in hyperactive comets
• Mapping the release of volatiles in the inner comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON) using the Atacama Large Millimeter/Submillimeter Array
• Interferometric imaging of the sulfur-bearing molecules H2S, SO and CS in comet C/1995 O1 (Hale-Bopp)
• Sulfur Ice Astrochemistry: A Review of Laboratory Studies
• Hydrogen Isocyanide in Comet 73P/Schwassmann-Wachmann (Fragment B)
• Radio observations of Jupiter-family comets
• ALMA mapping of rapid gas and dust variations in comet C/2012 S1 (ISON): new insights into the origin of cometary HNC
• The Sources of HCN and CH3OH and the Rotational Temperature in Comet 103P/Hartley 2 from Time-Resolved Millimeter Spectroscopy
• Gas and dust productions of comet 103P/Hartley 2 from millimetre observations: interpreting rotation-induced time variations
• ALMA Autocorrelation Spectroscopy of Comets: The HCN/H^13CN ratio in C/2012 S1 (ISON)
• Interferometric mapping of the 3.3-mm continuum emission of comet 17P/Holmes after its 2007 outburst
• Thermal physics of the inner coma: ALMA studies of the methanol distribution and excitation in comet C/2012 K1 (PanSTARRS)
• Earth-based detection of the millimetric thermal emission of the nucleus of comet 8P/Tuttle
• Exploring the volatile composition of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON) with ALMA
• Major Outburst and Splitting of Long-Period Comet C/2015 ER61 (Pan-STARRS)