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arXiv · 1702.03322

ALMA mapping of rapid gas and dust variations in comet C/2012 S1 (ISON): new insights into the origin of cometary HNC

M. A. Cordiner·J. Boissier·S. B. Charnley·A. J. Remijan·M. J. Mumma·G. Villanueva·D. C. Lis·S. N. Milam·L. Paganini·J. Crovisier·D. Bockelee-Morvan·Y. -J. Kuan·N. Biver·I. M. Coulson
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Abstract

Observations of the sungrazing comet C/2012 S1 (ISON) were carried out using the Atacama Large Millimeter/submillimeter Array (ALMA) at a heliocentric distance of 0.58-0.54 AU (pre-perihelion) on 2013 November 16-17. Temporally resolved measurements of the coma distributions of HNC, CH$_3$OH, H$_2$CO and dust were obtained over the course of about an hour on each day. During the period UT 10:10-11:00 on Nov. 16, the comet displayed a remarkable drop in activity, manifested as a $>42$% decline in the molecular line and continuum fluxes. The H$_2$CO observations are consistent with an abrupt, $\approx50$% reduction in the cometary gas production rate soon after the start of our observations. On Nov. 17, the total observed fluxes remained relatively constant during a similar period, but strong variations in the morphology of the HNC distribution were detected as a function of time, indicative of a clumpy, intermittent outflow for this species. Our observations suggest that at least part of the detected HNC originated from degradation of nitrogen-rich organic refractory material, released intermittently from confined regions of the nucleus. By contrast, the distributions of CH$_3$OH and H$_2$CO during the Nov. 17 observations were relatively uniform, consistent with isotropic outflow and stable activity levels for these species. These results highlight a large degree of variability in the production of gas and dust from comet ISON during its pre-perihelion outburst, consistent with repeated disruption of the nucleus interspersed with periods of relative quiescence.

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M. A. Cordiner, J. Boissier, S. B. Charnley, A. J. Remijan, M. J. Mumma, G. Villanueva, D. C. Lis, S. N. Milam, L. Paganini, J. Crovisier, D. Bockelee-Morvan, Y. -J. Kuan, N. Biver, I. M. Coulson. 2017-02-10. ALMA mapping of rapid gas and dust variations in comet C/2012 S1 (ISON): new insights into the origin of cometary HNC. https://doi.org/10.3847/1538-4357%2Faa6590

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