HIGHLY DEPLETED ETHANE AND MILDLY DEPLETED METHANOL IN COMET 21P/GIACOBINI-ZINNER: APPLICATION OF A NEW EMPIRICAL ν2-BAND MODEL FOR CH3OH NEAR 50 K

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 H 2 O, C 2 H 6 , and CH 3 OH. Our production rate for H 2 O (3.885 ± 0.074 × 10 28 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 ν 7 band of C 2 H 6 provided a production rate (5.27 ± 0.90 × 10 25 s −1 ) that corresponded to an abundance ratio of 0.136 ± 0.023 × 10 −2 relative to H 2 O, confirming the previously reported strong depletion of C 2 H 6 from IR observations during the 1998 apparition, and in qualitative agreement with the depletion of C 2 known from optical studies of 21P/GZ. For CH 3 OH, we applied our recently published quantum model for the ν 3 band to obtain a rotational temperature (48 +10 / −7 K) consistent with that obtained for H 2 O. In addition, we developed a new empirical model for the CH 3 OH ν 2 band, based on observations of Comet 8P/Tuttle with NIRSPEC. The application of our ν 2 model to 21P/GZ yielded a production rate in agreement with that obtained from the ν 3 band. Combining results from both ν 2 and ν 3 bands provided a production rate (47.5 ± 4.4 × 10 25 s −1 ) that corresponded to an abundance ratio of 1.22 ± 0.11 × 10 −2 relative to H 2 O in 21P/GZ, indicating mild depletion of CH 3 OH. 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.