On the Origin of the 3.2- to 3.6-μm Emission Features in Comets

We investigate the contribution of the v 2, v 3 , and v 9 CH-stretching bands of methanol to the 3.2- to 3.6-μm emission feature observed in seven comets at a range of heliocentric distances and analyze the residual emission spectra. The comets examined are P/Halley, Wilson 1987 VII, Bradfield 1987 XXIX, P/Brorsen-Metcalf, Okazaki-Levy-Rudenko 1989 XIX, Austin 1990 V, and Levy 1990 XX. From the fitting of its v 3 band at 3.52 μm, we infer that methanol is present in all of the 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 low. The methanol fundamental bands are found to contribute up to 60% of the total 3.2 to 3.6-μm emission flux. All 7 comets have residual emission with a distinct feature centered at 3.43 μm. The flux of this 3.43-μm feature is correlated with the water production rate, suggesting a gaseous origin, but the correlation with the methanol production rate is even tighter, suggesting a connection between the 3.43-μm emitter and methanol. Another residual emission feature at 3.28 μm is roughly correlated with the water production rate, but is stronger in dustier comets. If the 3.28-μm feature is due to ultraviolet-pumped IR fluorescence of aromatic molecules, abundances between 1.5 and 10 × 10 -6 are estimated. These abundances are at least 100 times smaller than the value deduced from UV observations of P/Halley (G. Moreels et al., 1994, Astron. Astrophy. 282, 643-656), but both the UV- and IR-derived abundances depend on modeling assumptions which are still uncertain. Additional residual emission features between 3.30 and 3.40 μm and near 3.24 μm vary among the comets examined. In addition to the v 2, v 3, and v 9 fundamental bands, overtones, and combination bands of methanol are present in the 3.2 to 3.6-μm spectral region and may be enhanced due to Fermi resonances with the CH stretches. How this could affect the residual cometary emissions is discussed. Further laboratory work and theoretical modeling of the methanol infrared spectra are needed to fully understand the contribution of methanol to the 3.2 to 3.6-μm spectrum of comets.