An infrared investigation of volatiles in Comet 21P/Giacobini-Zinner

We obtained high resolution (lambda/delta lambda similar to 10,00-20,000) i nfrared (IR) spectra of Comet 21P/Giacobini-Zinner (GZ) at five different w avelengths between 1.9 and 5.0 mu m during 25-29 October 1998 using CSHELL at the NASA Infrared Telescope Facility (IRTF) on Mauna Kea. We also obtain ed a moderate resolution (lambda/delta lambda similar to 680) spectrum cove ring the wavelength range from 3.082 to 3.720 mu m on 29 October 1998 using CGS4 at the United Kingdom Infrared Telescope (UKIRT) on Mauna Kea. Five r ovibrational lines in three different vibrational bands of H2O were detecte d in the CSHELL spectra. Assuming that the rotational temperature was simil ar to 50 K, we derive a H2O production rate of similar to 2-3 x 10(28) mole cules s(-1), which is similar to 2 times smaller than the value derived fro m nearly simultaneous radio observations of OH. After continuum subtraction , the CGS4 spectrum displays significant excess flux that we attribute main ly to CH3OH fluorescence, and we derive that the CH3OH production rate was similar to 2.7 x 10(26) molecules s(-1). The corresponding CH2OH/H2O relati ve abundance is similar to 0.9-1.4%, which falls within the range of values observed in other comets, albeit at the low end. The CGS4 spectrum also ha s significant excess flux near 3.43 mu m that is not explained by our CH3OH fluorescence model; a similar feature has been observed in several other c omets, but its origin remains a mystery We did not detect any excess emissi on near 3.28 mu m, 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. Th e 3 sigma upper limits for the abundances relative to H2O are 0.05-0.08% fa r 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 rotatio nal temperature in the coma is 50 K. C2H6 is depleted by a factor of simila r to 15 or more compared to its relative abundance in Comets Hale-Bopp (C/1 995 O1) and Hyakutake (C/1996 B2); this depletion is similar to that observ ed for C-2 and C-3 from optical observations of GZ (A'Hearn et al. 1995) an d suggests that the formation of volatile carbon-chain molecules was inhibi ted in GZ. We are unable to find any clear correlation between the C2H6 and the C-2 and C-3 abundances in a sample of nine other comets, assuming that the residual emission near 3.35 mu m in moderate resolution spectra of sev en of the comets provides an accurate indicator of the C2H6 abundance. Howe ver, this latter assumption is questionable and highlights the need to obta in high spectral resolution data in order to make accurate abundance measur ements of C2H6. (C) 1999 Academic Press.