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

## TL;DR

This paper introduces a novel ALMA autocorrelation technique to detect extended comet emissions, leading to the first measurement of HCN isotopic ratios in comet C/2012 S1 (ISON), confirming no isotopic fractionation.

## Contribution

The study presents a new method using ALMA autocorrelation data to recover extended coma emissions, enabling detailed isotopic analysis of comets.

## Key findings

- Extended HCN emission detected with a fourteen-fold brightness increase.
- First detection of H^13CN rotational emission in comet ISON.
- H^12CN/H^13CN ratio measured as 88±18, matching terrestrial values.

## Abstract

The Atacama Large Millimeter/submillimeter Array (ALMA) is a powerful tool for high-resolution mapping of comets, but the main interferometer (comprised of 50x12-m antennas) is insensitive to the largest coma scales due to a lack of very short baselines. In this work, we present a new technique employing ALMA autocorrelation data (obtained simultaneously with the interferometric observations), effectively treating the entire 12-m array as a collection of single-dish telescopes. Using combined autocorrelation spectra from 28 active antennas, we recovered extended HCN coma emission from comet C/2012 S1 (ISON), resulting in a fourteen-fold increase in detected line brightness compared with the interferometer. This resulted in the first detection of rotational emission from H^13CN in this comet. Using a detailed coma radiative transfer model accounting for optical depth and non-LTE excitation effects, we obtained an H^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 formation in the protosolar accretion disk. The possibility of future discoveries in extended sources using autocorrelation spectroscopy from the main ALMA array is thus demonstrated.