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Title:
A Search for Variation in the H2O Ortho-Para Ratio and Rotational Temperature in the Inner Coma of Comet C/2004 Q2 (Machholz)
Authors:
Bonev, Boncho P.; Mumma, Michael J.; Villanueva, Geronimo L.; Disanti, Michael A.; Ellis, Richard S.; Magee-Sauer, Karen; Dello Russo, Neil
Affiliation:
AA(Department of Physics, The Catholic University of America, Washington, DC 20064; . Work was begun while a graduate student at the University of Toledo, Toledo, OH 43606.; Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771.), AB(Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771.), AC(Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771.; NASA Postdoctoral Fellow.), AD(Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771.), AE(Department of Astronomy, California Institute of Technology, Pasadena, CA 91125.), AF(Department of Physics and Astronomy, Rowan University, Glassboro, NJ 08028-1701.), AG(Space Department, Planetary Exploration Group, Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20023-6099.)
Publication:
The Astrophysical Journal, Volume 661, Issue 1, pp. L97-L100. (ApJL Homepage)
Publication Date:
05/2007
Origin:
UCP
ApJ Keywords:
Comets: General, comets: individual (C/2004 Q2 (Machholz)), Infrared: Solar System
DOI:
10.1086/518419
Bibliographic Code:
2007ApJ...661L..97B

Abstract

We present spatially resolved measurements of the rotational temperature and ortho-para ratio for H2O in the inner coma of the Oort Cloud comet C/2004 Q2 (Machholz). Our results are based on direct simultaneous detections of ortho-H2O and para-H2O via ``hot-band'' fluorescence near 2.9 μm. We find a well-defined decline in rotational temperature with increasing nucleocentric distance (up to ~1000 km). The ortho-para ratio remains constant (within stochastic uncertainty) with increasing nucleocentric distance and is close to the statistical equilibrium value of 3.0 (2.86+/-0.06 [0.17], including, respectively, stochastic [systematic] uncertainty), resulting in spin temperature Tspin>=34 K. We compare the present results with those reported for other comets and discuss the difficulties in interpreting spin temperatures deduced from measured ortho-para ratios. Improved understanding of the special conditions that enable nuclear spin conversion would test the extent to which derived spin temperatures reflect the formative history or the processing record of cometary ices.
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