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E.L. Gibb (NAS-NRC, NASA's GSFC), M.A. DiSanti, N. Dello Russo (CUA, NASA's GSFC), M.J. Mumma (NASA's GSFC), K. Magee-Sauer (Rowan University)
Deriving molecular production rates, and hence abundances, is important for characterizing the chemistry of cometary volatiles. Of particular interest are organics and hypervolatile species that may be used to determine information about comet formation and evolution once a large enough database of cometary observations exists. Methane is one of these important species. Methane and CO are comparably volatile, so a comparison of their native abundance tests the role of formation temperature as a causal factor for the wide range observed for native CO in comets. As a symmetric hydrocarbon with no permanent dipole moment, methane has no rotational transitions and cannot be observed in the radio or ultraviolet regions. The only observable transitions are ro-vibrational bands in the infrared region.
We used CSHELL at the IRTF and NIRSPEC at the W.M. Keck Observatory to obtain high spectral resolving power (R~2x104) observations of eight Oort Cloud comets since the apparition of comet Hyakutake in 1996. We detected the R0 and R1 lines of the \nu3 vibrational band of CH4 near 3.3 \mum in each of these comets. In order to quantify production rates, a fluorescence model has been developed for the \nu3 vibrational band of CH4. New g-factors for the R0 and R1 transitions at rotational temperatures relevant to our observations are reported and the resulting CH4 production rates and abundances are presented. Abundances are compared among our existing sample.
E. Gibb gratefully acknowledges support from the National Research Council under her Resident Research Associateship.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.