ON THE ORIGIN OF THE 3.2- TO 3.6-MU-M EMISSION FEATURES IN COMETS

We investigate the contribution of the nu(2), nu(3), and nu(9) CH-stre tching bands of methanol to the 3.2- to 3.6-mu m emission feature obse rved in seven comets at a range of heliocentric distances and analyze the residual emission spectra. The comets examined are P/Halley, Wilso n 1987 VII, Bradfield 1987 XXIX, P/Brorsen-Metcalf, Okazaki-Levy-Ruden ko 1989 XIX, Austin 1990 V, and Levy 1990 XX. From the fitting of its nu(3) band at 3.52 mu m, we infer that methanol is present in all of t he comets with abundances ranging from 0.6 to 5% with respect to water . We do not see strong evidence for a bimodal distribution of methanol -rich and methanol-poor comets, though Levy appears to be relatively l ow. The methanol fundamental bands are found to contribute up to 60% o f the total 3.2 to 3.6-mu m emission flux. All 7 comets have residual emission with a distinct feature centered at 3.43 mu m. The flux of th is 3.43-mu m feature is correlated with the water production rate, sug gesting a gaseous origin, but the correlation with the methanol produc tion rate is even tighter, suggesting a connection between the 3.43-mu m emitter and methanol. Another residual emission feature at 3.28 mu m is roughly correlated with the water production rate, but is stronge r in dustier comets. If the 3.28-mu m feature is due to ultraviolet-pu mped IR fluorescence of aromatic molecules, abundances between 1.5 and 10 x 10(-6) are estimated. These abundances are at least 100 times sm aller than the value deduced from UV observations of P/Halley (G. More els et al., 1994, Astron. Astrophy. 282, 643-656), but both the UV- an d IR-derived abundances depend on modeling assumptions which are still uncertain. Additional residual emission features between 3.30 and 3.4 0 mu m and near 3.24 mu m vary among the comets examined. In addition to the nu(2), nu(3), and nu(9) fundamental bands, overtones, and combi nation bands of methanol are present in the 3.2 to 3.6-mu m spectral r egion and may be enhanced due to Fermi resonances with the CH stretche s. How this could affect the residual cometary emissions is discussed. Further laboratory work and theoretical modeling of the methanol infr ared spectra are needed to fully understand the contribution of methan ol to the 3.2 to 3.6-mu m spectrum of comets. (C) 1995 Academic Press, Inc.