ALMA Autocorrelation Spectroscopy of Comets: The HCN/H13CN Ratio in C/2012 S1 (ISON)

The Atacama Large Millimeter/submillimeter Array (ALMA) is a powerful tool for high-resolution mapping of comets, but the main interferometer (comprised of 50 × 12 m antennas) is insensitive to the largest coma scales due to a lack of very short baselines. In this Letter, we present a new technique...

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Bibliographic Details
Published in:Astrophysical journal. Letters 2019-01, Vol.870 (2)
Main Authors: Cordiner, M. A., Palmer, M. Y., Val-Borro, M. de, Charnley, S. B., Paganini, L., Villanueva, G., Bockelée-Morvan, D., Biver, N., Remijan, A. J., Kuan, Y.-J., Milam, S. N., Crovisier, J., Lis, D. C., Mumma, M. J.
Format: Article
Language:English
Subjects:
Accretion
Accretion disks
Antennas
astrochemistry
Astronomy
Autocorrelation
Comet formation
Comet heads
Comets
comets: individual (C/2012 S1 (ISON))
Emissions
Fractionation
Interferometric observation
Isotope fractionation
Local thermodynamic equilibrium
Mapping
Optical analysis
Optical thickness
Radiative transfer
Radio telescopes
Spectroscopy
Spectrum analysis
submillimeter: planetary systems
techniques: interferometric
techniques: spectroscopic
Telescopes
Terrestrial environments
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Summary:The Atacama Large Millimeter/submillimeter Array (ALMA) is a powerful tool for high-resolution mapping of comets, but the main interferometer (comprised of 50 × 12 m antennas) is insensitive to the largest coma scales due to a lack of very short baselines. In this Letter, 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 H13CN in this comet. Using a detailed coma radiative transfer model accounting for optical depth and non-local thermodynamic equilibrium excitation effects, we obtained an H12CN/H13CN ratio of 88 18, which matches the terrestrial value of 89. This is 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.
ISSN:2041-8205
2041-8213
DOI:10.3847/2041-8213/aafb05