ALMA Synthetic Array Observations of Comet C/2012 F6 (Lemmon): Simultaneous 7 m and 12 m Measurements of HCN and CH₃OH

Comets are among the most primitive remnants of solar system formation, and decoding the composition of their nuclei can reveal important clues to the formation history and evolution of the solar system. State-of-the-art millimeter/sub-millimeter interferometers, such as the Northern Extended Millimeter Array (NOEMA) and the Atacama Large Millimeter/Sub-millimeter Array (ALMA), have enabled sampling the coma composition of comets at millimeter wavelengths with high spatial and spectral resolution, providing insights into coma photochemistry and physics. The high spatial resolution of the main 12 m ALMA array is complemented by the short baselines of the 7 m Atacama Compact Array (ACA), providing sensitivity to extended molecular flux present for many cometary volatiles. Here we report analysis of HCN and CH₃OH emission in comet C/2012 F6 (Lemmon) from ALMA Band 6 Science Verification (SV) observations taken to test ALMA Doppler tracking of ephemeris targets. These observations utilized a “synthetic array” with simultaneous measurements including both 12 m and 7 m antennas. Results from 12 m cross-correlation measurements are reported in the literature (Bøgelund & Hogerheijde 2017, A&A, 604, A131). We provide the first analysis of the 7 m measurements, combining them with corresponding 12 m measurements to provide insights into HCN and CH₃OH production over multiple angular scales. We report interferometric maps, autocorrelation spectra, and production rates as measured with each component of the synthetic array, and place our results into context with interferometric measurements of cometary volatiles to date.