• DiSanti, M. A.
;
• Bonev, B. P.
;
• Villanueva, G. L.
;
• Mumma, M. J.

Abstract

We obtained infrared spectra of Comet 21P/Giacobini-Zinner (hereafter 21P/GZ) using NIRSPEC at Keck II on UT 2005 June 3, approximately one month before perihelion, that simultaneously sampled H₂O, C₂H₆, and CH₃OH. Our production rate for H₂O (3.885 ± 0.074 ×ばつ 10²⁸ molecules s^-1) was consistent with that measured during other apparitions of 21P/GZ as retrieved from optical, infrared, and radio observations. Our analysis also provided values for rotational temperature (T _rot = 51 ± 3 K) and the abundance ratio of ortho and para spin populations for water (OPR = 2.99 ± 0.23, implying a spin temperature exceeding 50 K). Six Q-branches in the ν₇ band of C₂H₆ provided a production rate (5.27 ± 0.90 ×ばつ 10²⁵ s^-1) that corresponded to an abundance ratio of 0.136 ± 0.023 ×ばつ 10^-2 relative to H₂O, confirming the previously reported strong depletion of C₂H₆ from IR observations during the 1998 apparition, and in qualitative agreement with the depletion of C₂ known from optical studies of 21P/GZ. For CH₃OH, we applied our recently published quantum model for the ν₃ band to obtain a rotational temperature (48⁺¹⁰/_-7 K) consistent with that obtained for H₂O. In addition, we developed a new empirical model for the CH₃OH ν₂ band, based on observations of Comet 8P/Tuttle with NIRSPEC. The application of our ν₂ model to 21P/GZ yielded a production rate in agreement with that obtained from the ν₃ band. Combining results from both ν₂ and ν₃ bands provided a production rate (47.5 ± 4.4 ×ばつ10²⁵ s^-1) that corresponded to an abundance ratio of 1.22 ± 0.11 ×ばつ 10^-2 relative to H₂O in 21P/GZ, indicating mild depletion of CH₃OH. Together with observations of 21P/GZ in 1998, our study provides a measure of primary volatile production rates for this Jupiter family comet over multiple apparitions using high-resolution IR spectroscopy.