Other
Scientific paper
Dec 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999icar..142..482w&link_type=abstract
Icarus, Volume 142, Issue Icarus, pp. 482-497.
Other
25
Scientific paper
We obtained high resolution (λ/δλ~10,000-20,000) infrared (IR) spectra of Comet 21P/Giacobini-Zinner (GZ) at five different wavelengths between 1.9 and 5.0 μm during 25-29 October 1998 using CSHELL at the NASA Infrared Telescope Facility (IRTF) on Mauna Kea. We also obtained a moderate resolution (λ/δλ~680) spectrum covering the wavelength range from 3.082 to 3.720 μm on 29 October 1998 using CGS4 at the United Kingdom Infrared Telescope (UKIRT) on Mauna Kea. Five rovibrational lines in three different vibrational bands of H2O were detected in the CSHELL spectra. Assuming that the rotational temperature was ~50 K, we derive a H2O production rate of ~2-3x1028 molecules s-1, which is ~2 times smaller than the value derived from nearly simultaneous radio observations of OH. After continuum subtraction, the CGS4 spectrum displays significant excess flux that we attribute mainly to CH3OH fluorescence, and we derive that the CH3OH production rate was ~2.7x1026 molecules s-1. The corresponding CH3OH/H2O relative abundance is ~0.9-1.4%, which falls within the range of values observed in other comets, albeit at the low end. The CGS4 spectrum also has significant excess flux near 3.43 μm that is not explained by our CH3OH fluorescence model; a similar feature has been observed in several other comets, but its origin remains a mystery. We did not detect any excess emission near 3.28 μm, where some comets show a feature that may be associated with polycyclic aromatic hydrocarbons. We also searched for emissions from C2H6, CO, HCN, C2H2, and H2CO but did not detect any of these molecules. The 3σ upper limits for the abundances relative to H2O are 0.05-0.08% for C2H6, 2-3% for CO, 0.2-0.3% for HCN, 0.3-0.4% for C2H2, and 0.5-0.8% for H2CO, assuming that all species are parent molecules and that their rotational temperature in the coma is 50 K. C2H6 is depleted by a factor of ~15 or more compared to its relative abundance in Comets Hale-Bopp (C/1995 O1) and Hyakutake (C/1996 B2); this depletion is similar to that observed for C2 and C3 from optical observations of GZ (A'Hearn et al. 1995) and suggests that the formation of volatile carbon-chain molecules was inhibited in GZ. We are unable to find any clear correlation between the C2H6 and the C2 and C3 abundances in a sample of nine other comets, assuming that the residual emission near 3.35 μm in moderate resolution spectra of seven of the comets provides an accurate indicator of the C2H6 abundance. However, this latter assumption is questionable and highlights the need to obtain high spectral resolution data in order to make accurate abundance measurements of C2H6.
Bockelée-Morvan Dominique
Brooke Timothy Young
Chin Gordon
Crovisier Jacques
Cruikshank Dale P.
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