Systematic Observations of Methanol and Other Organics in Comet P/Swift-Tuttle: Discovery of New Spectral Structure at 3.42 μm

Abstract

We present a detailed study of the 3.4-μm "cometary organics feature" in Comet P/Swift-Tuttle (1992 XXVIII). The study is based on spectra acquired from Kitt Peak and Mauna Kea Observatories on UT 1992 November 8, 9, 12, 13, 26, and 27, during which time the heliocentric distance decreased from 1.12 to 0.99 AU. Two distinct components are present, cometary methanol (CH 3OH), and another as yet unidentified organic volatile responsible for an emission feature centered at ∼3.424 μm. We obtain a CH 3OH abundance of 4.3 ± 0.5% relative to H 2O, revising our previously reported value of 7 ± 1%. The revisions result from improved data analysis, inclusion of additional P/Swift-Tuttle observations, and use of corrected CH 3OH g-factors. The unidentified feature consistently accounted for one-half of the flux contained in the 3.4-μm feature and exhibited a heliocentric dependence consistent with a volatile parent species, in agreement with conclusions based on observations of several recent comets. We present the first echelle spectrum of this feature (resolving power λ/Δλ ≈ 8000), which reveals spectral structure in the form of a pronounced peak centered at a rest frequency of(2920.24 ± 0.09) cm -1. We suspect this to be representative of asymmetric CH 2 (or CH-X) vibrational stretching in some as yet unidentified organic compound. Our results indicate the presence of an additional abundant form of volatile carbon in comets. If the unidentified band(s) possess g-factors similar to those of the adjacent methanol bands, the inferred abundance of the unidentified emitting species would be comparable to that of methanol.