ALMA Autocorrelation Spectroscopy of Comets: The HCN/H^13CN ratio in C/2012 S1 (ISON). (arXiv:1901.08676v1 [astro-ph.IM])
<a href="http://arxiv.org/find/astro-ph/1/au:+Cordiner_M/0/1/0/all/0/1">M. A. Cordiner</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Palmer_M/0/1/0/all/0/1">M. Y. Palmer</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Val_Borro_M/0/1/0/all/0/1">M. de Val-Borro</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Charnley_S/0/1/0/all/0/1">S. B. Charnley</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Paganini_L/0/1/0/all/0/1">L. Paganini</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Villanueva_G/0/1/0/all/0/1">G. Villanueva</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bockelee_Morvan_D/0/1/0/all/0/1">D. Bockelée-Morvan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Biver_N/0/1/0/all/0/1">N. Biver</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Remijan_A/0/1/0/all/0/1">A. J. Remijan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kuan_Y/0/1/0/all/0/1">Y.-J. Kuan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Milam_S/0/1/0/all/0/1">S. N. Milam</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Crovisier_J/0/1/0/all/0/1">J. Crovisier</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lis_D/0/1/0/all/0/1">D. C. Lis</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Mumma_M/0/1/0/all/0/1">M. J. Mumma</a>
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 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.
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 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.
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