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The Atacama Large Millimeter/submillimeter Array (ALMA) is a powerful toolfor high-resolution mapping of comets, but the main interferometer (comprisedof 50x12-m antennas) is insensitive to the largest coma scales due to a lack ofvery short baselines. In this work, we present a new technique employing ALMAautocorrelation data (obtained simultaneously with the interferometricobservations), effectively treating the entire 12-m array as a collection ofsingle-dish telescopes. Using combined autocorrelation spectra from 28 activeantennas, we recovered extended HCN coma emission from comet C/2012 S1 (ISON),resulting in a fourteen-fold increase in detected line brightness compared withthe interferometer. This resulted in the first detection of rotational emissionfrom H^13CN in this comet. Using a detailed coma radiative transfer modelaccounting for optical depth and non-LTE excitation effects, we obtained anH^12CN/H^13CN ratio of 88+-18, which matches the terrestrial value of 89,consistent with a lack of isotopic fractionation in HCN during comet formationin the protosolar accretion disk. The possibility of future discoveries inextended sources using autocorrelation spectroscopy from the main ALMA array isthus demonstrated.

ALMA Autocorrelation Spectroscopy of Comets: The HCN/H^13CN ratio in C/2012 S1 (ISON)