An Infrared Investigation of Volatiles in Comet 21P/Giacobini–Zinner

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∼2–3× 1028 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∼2.7× 1026 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 3.28μm, 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