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

Abstract

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 H 2 O were detected in the CSHELL spectra. Assuming that the rotational temperature was ∼50 K, we derive a H 2 O production rate of ∼2-3 × 10 28 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 CH 3 OH fluorescence, and we derive that the CH 3 OH production rate was ∼2.7 × 10 26 molecules s −1 . The corresponding CH 3 OH/H 2 O 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 CH 3 OH 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 C 2 H 6 , CO, HCN, C 2 H 2 , and H 2 CO but did not detect any of these molecules. The 3σ upper limits for the abundances relative to H 2 O are 0.05-0.08% for C 2 H 6 , 2-3% for CO, 0.2-0.3% for HCN, 0.3-0.4% for C 2 H 2 , and 0.5-0.8% for H 2 CO, assuming that all species are parent molecules and that their rotational temperature in the coma is 50 K. C 2 H 6 is depleted by a factor of ∼15 or more compared to its relative abundance in Comets Hale-Bopp VOLATILES IN COMET 21P/GZ 483 (C/1995 O1) and Hyakutake (C/1996 B2); this depletion is similar to that observed for C 2 and C 3 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 C 2 H 6 and the C 2 and C 3 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 C 2 H 6 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 C 2 H 6 .

The chemistry of C2and C3in the coma of Comet C/1995 O1 (Hale-Bopp) at heliocentric distancesrh≥2.9 AU

“…Their measurements indicate that this comet was depleted in C 2 H 6 . Weaver et al (1999a) did not detect C 2 H 6 in this comet and derived an upper limit for the production rate of C 2 H 6 which was lower than the production rate derived by Mumma.…”

Remote infrared observations of parent volatiles in comets: A window on the early solar system

“…A direct measurement of acetylene in JFCs is of great interest, and a comparison of acetylene and ethane would also test the production of CzH6 by hydrogenation of CzH2. The CO mixing ratio was either nearly normal (Mumma et al 2000) or strongly depleted (Weaver et al 1999b) in this comet. The reason for this difference is not known; the measurements were separated in time by several weeks so internal heterogeneity could be at work if different active vents were observed.…”

“…The quintessential &-depleted comet (21P/Giacobini-Zinner) is also deficient in ethane (Weaver et al 1999b, Mumma et al 2000. However, Cz can be produced directly from CZHl by photolysis but not from &I&, and the depletion of ethane does not directly test a depletion of acetylene.…”

Behaviour of Comet 21P/Giacobini-Zinner during the 1998 perihelion

“…At the time of the 1998 apparition, to our knowledge, G-Z was only observed by Weaver et al (1999) during October 25-29, and by Mumma et al (2000) during October 2-10 in the IR, and by Biver et al (1999) in radio-wavelengths from Oct. 26 to 31. The IR observations by both groups were made using the same instrument CSHELL at the NASA Infrared Telescope Facility, Mauna Kea, Hawaii.…”