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En route to destruction: the evolution in composition of ices in comet D/2012 S1 (ISON) between 1.2 and 0.34 AU from the sun as revealed at infrared wavelengths

DiSanti, M. A. and Bonev, B. P. and Gibb, E. L. and Paganini, L. and Villanueva, G. L. and Mumma, M. J. and Keane, J. V. and Blake, G. A. and Dello Russo, N. and Meech, K. J. and Vervack, R. J., Jr. and McKay, A. J. (2016) En route to destruction: the evolution in composition of ices in comet D/2012 S1 (ISON) between 1.2 and 0.34 AU from the sun as revealed at infrared wavelengths. Astrophysical Journal, 820 (1). Art. No. 34. ISSN 0004-637X.

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We report production rates for H2O and eight trace molecules (CO, C_2H_6, CH_4, CH_3OH, NH_3, H_2CO, HCN, C_2H_2) in the dynamically new, Sun-grazing Comet C/2012 S1 (ISON), using high-resolution spectroscopy at Keck II and the NASA IRTF on 10 pre-perihelion dates encompassing heliocentric distances R_h = 1.21–0.34 AU. Measured water production rates spanned two orders of magnitude, consistent with a long-term heliocentric power law Q(H_2O) ∝ R_h^(-3.1±0.1). Abundance ratios for CO, C_2H_6, and CH_4 with respect to H_2O remained constant with R_h and below their corresponding mean values measured among a dominant sample of Oort Cloud comets. CH_3OH was also depleted for R_h > 0.5 AU, but was closer to its mean value for R_h ≤ 0.5 AU. The remaining four molecules exhibited higher abundance ratios within 0.5 AU: for R_h > 0.8 AU, NH_3 and C_2H_2 were consistent with their mean values while H_2CO and HCN were depleted. For R_h < 0.5 AU, all four were enriched, with NH_3, H_2CO, and HCN increasing most. Spatial profiles of gas emission in ISON consistently peaked sunward of the dust continuum, which was asymmetric antisunward and remained singly peaked for all observations. NH_3 within 0.5 AU showed a broad spatial distribution, possibly indicating its release in the coma provided that optical depth effects were unimportant. The column abundance ratio NH_2/H_2O at 0.83 AU was close to the "typical" NH/OH from optical wavelengths, but was higher within 0.5 AU. Establishing its production rate and testing its parentage (e.g., NH_3) require modeling of coma outflow.

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Blake, G. A.0000-0003-0787-1610
Additional Information:© 2016 The American Astronomical Society. Received 2015 May 5; accepted 2015 December 14; published 2016 March 16. This paper includes data collected at the W. M. Keck Observatory, Maunakea, HI, USA, operated as a scientific partnership among Caltech, UCLA, and NASA, and made possible by the generous financial support of the W.M. Keck Foundation. This work was supported by Research Grant Awards through the NASA Planetary Atmospheres Program (PATM12-0049 to M.A.D., and NNX12AG60G to B.P.B.), Planetary Astronomy Program (PAST11-0045 to M.J.M.), and the NSF Astronomy and Astrophysics Program (AST-1211362 to B.P.B. and E.L. G., and AST-1413736 to J.V.K.). This material is based in part on work supported by the National Aeronautics and Space Administration through the NASA Astrobiology Institute, issued through the Office of Space Science under grant 08NAI5-0005 to M.J.M., and under Cooperative Agreement Number NN09DA77A to K.J.M. We thank the Keck and IRTF for providing dedicated observing time for Comet ISON, and J. Green, K. Fast, and L. Johnson in the Planetary Science Division at NASA-HQ for their encouragement in supporting the overall ground- and space-based ISON observing campaign. We thank an anonymous referee for comments that improved the manuscript. M.A.D. gratefully acknowledges Strategic Science support from the Solar System Exploration Division at NASA-GSFC for his participation as a member of the Comet ISON Observing Campaign team. B.P.B.ʼs contribution to the Comet ISON Observing Campaign is dedicated to the memory of Hristo Hristov for his long distinguished service as a physics teacher in The Foreign Language School “Romain Rolland,” Stara Zagora, Bulgaria. We recognize the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community and feel most fortunate to conduct these observations from this mountain.
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Subject Keywords:astrochemistry; comets: general; comets: individual (C&2012 S1 (ISON) ; molecular processes; Oort Cloud
Record Number:CaltechAUTHORS:20160427-131119558
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Official Citation:M. A. DiSanti et al 2016 ApJ 820 34
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:66514
Deposited By: Ruth Sustaita
Deposited On:27 Apr 2016 22:33
Last Modified:21 Aug 2017 20:44

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