SYSTEMATIC OBSERVATIONS OF METHANOL AND OTHER ORGANICS IN COMET P SWIFT-TUTTLE - DISCOVERY OF NEW SPECTRAL STRUCTURE AT 3.42 MU-M/

We present a detailed study of the 3.4-mu m ''cometary organics featur e'' in Comet P/Swift-Tuttle (1992 XXVIII). The study is based on spect ra acquired from Kitt Peak and Mauna Kea Observatories on UT 1992 Nove mber 8, 9, 12, 13, 26, and 27, during which time the heliocentric dist ance decreased from 1.12 to 0.99 AU. Two distinct components are prese nt, cometary methanol (CH3OH), and another as yet unidentified organic volatile responsible for an emission feature centered at similar to 3 .424 mu m. We obtain a CH3OH abundance of 4.3 +/- 0.5% relative to H2O , revising our previously reported value of 7 +/- 1%. The revisions re sult from improved data analysis, inclusion of additional P/Swift-Tutt le observations, and use of corrected CH3OH g-factors. The unidentifie d feature consistently accounted for one-half of the flux contained in the 3.4-mu m feature and exhibited a heliocentric dependence consiste nt with a volatile parent species, in agreement with conclusions based on observations of several recent comets. We present the first echell e spectrum of this feature (resolving power lambda/Delta lambda approx imate to 8000), which reveals spectral structure in the form of a pron ounced peak centered at a rest frequency of (2920.24 +/- 0.09) cm(-1). We suspect this to be representative of asymmetric CH2 (or CH-X) vibr ational stretching in some as yet unidentified organic compound. Our r esults indicate the presence of an additional abundant form of volatil e carbon in comets. If the unidentified band(s) possess g-factors simi lar to those of the adjacent methanol bands, the inferred abundance of the unidentified emitting species would be comparable to that of meth anol. (C) 1995 Academic Press, Inc.