Bonev, Boncho P. and Mumma, Michael J. and Gibb, Erika L. and DiSanti, Michael A. and Villanueva, Geronimo L. and Magee-Sauer, Karen and Ellis, Richard S. (2009) Comet C/2004 Q2 (Machholz): parent volatiles, a search for deuterated methane, and constraint on the CH_4 spin temperature. Astrophysical Journal, 699 (2). pp. 1563-1572. ISSN 0004-637X http://resolver.caltech.edu/CaltechAUTHORS:20090805-141237370
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High-dispersion (λ/δλ ≈ 25,000) infrared spectra of Comet C/2004 Q2 (Machholz) were acquired on 2004 November 28-29, and 2005 January 19 (UT dates) with the Near InfraRed echelle SPECtrograph (NIRSPEC) at the Keck-2 telescope on Mauna Kea. We detected H_(2)O, CH_4, C_2H_2, C_2H_6, CO, H_2CO, CH_(3)OH, HCN, and NH_3 and we conducted a sensitive search for CH_3D. We report rotational temperatures, production rates, and mixing ratios (with respect to H_(2)O) at heliocentric distances of 1.49 AU (2004 November) and 1.21 AU (2005 January). We highlight three principal results. (1) The mixing ratios of parent volatiles measured at 1.49 AU and 1.21 AU agree within confidence limits, consistent with homogeneous composition in the mean volatile release from the nucleus of C/2004 Q2. Notably, the relative abundance of C_2H_6/C_2H_2 is substantially higher than those measured in other comets, while the mixing ratios C_2H_6/H_(2)O, CH_(3)OH/H_(2)O, and HCN/H_(2)O are similar to those observed in comets, referred to as "organics-normal". (2) The spin temperature of CH_4 is >35-38 K, an estimate consistent with the more robust spin temperature found for H_(2)O. (3) We obtained a 3σ upper limit of CH_3D/CH_4 < 0.020 (D/H < 0.005). This limit suggests that methane released from the nucleus of C/2004 Q2 is not dominated by a component formed in extremely cold (near 10 K) environments. Formation pathways of both interstellar and nebular origin consistent with the measured D/H in methane are discussed. Evaluating the relative contributions of these pathways requires further modeling of chemistry including both gas-phase and gas-grain processes in the natal interstellar cloud and in the protoplanetary disk.
|Additional Information:||© 2009 American Astronomical Society. Print publication: Issue 2 (2009 July 10); received 2008 December 22; accepted for publication 2009 May 11; published 2009 June 24. We are grateful to Neil Dello Russo for his thorough reviews that improved the quality of the paper. We thank Hideyo Kawakita for providing an updated CH3D model. We are grateful to the following colleagues for stimulating discussions on various aspects in this work: Yuri Aikawa, Dennis Bodewits, Steven Charnley, Martin Cordiner, Joe Nuth, Diane Wooden, and Charles Woodward. We thank undergraduate student Cara Rahon (Iona College) for assisting in the preliminary reduction of several spectral orders. Various aspects of this research have been supported by the NASA Planetary Astronomy (grants to PIs Mumma and DiSanti), Planetary Atmospheres (PI, DiSanti), and Astrobiology Programs (PI, Mumma), by the NSF Planetary Astronomy Program (grant to PIs Bonev and Gibb) and by the NSF Planetary Astronomy RUI program (grant to PI Magee-Sauer). The data presented herein were obtained at the W. M. Keck Observatory, operated as a scientific partnership among CalTech, UCLA, and NASA. This Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.|
|Subject Keywords:||comets: general; comets: individual (C/2004 Q2 [Machholz]); infrared: solar system; ISM: molecules; solar system: formation|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Jason Perez|
|Deposited On:||05 Aug 2009 23:16|
|Last Modified:||26 Dec 2012 11:08|
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