Infrared excitation processes of C₂H₆ in comets

抄録

A time-dependent and line-by-line fluorescence model of the ν₇ band of C₂H₆ has been constructed. Collisional (neutrals and electrons) and radiative excitation effects have been considered in the calculations of fluorescence efficiency factors (g-factors) of the C₂H₆ ν₇ band. Since the lifetime of C₂H₆ is -91, 000 seconds at a heliocentric distance of 1 AU, C₂H₆ molecules far from the nucleus approach fluorescence equilibrium, while molecules within the contact surface should have a much colder rotational distribution due to collisional equilibration with the low temperature gases in that region. We would recommend using “single-cycle” fluorescence models for the analysis of ν₇ band spectra taken with small apertures centered on the nucleus. We analyzed a ν₇band spectrum of comet Hale-Bopp (C/1995 O1) obtained at the IRTF with the CSHELL on 2 March, 1997 (R = 1.1 AU, Δ = 1.5 AU) using a square aperture of 1, 000×2, 000 km, and constructed synthetic spectra to compare with the observation. We analyzed spatial brightness profiles of the ^RQ₀ sub-branch and found that the eastward profile is very well matched by the models, but the observed westward profile is clearly broader than the eastward profile suggesting asymmetric outflow and/or extended sources. We derived a C₂H₆ production rate of -1.7 ± 0.9 × 1028 molec s-1 from the inner coma region of the comet at the time of the observation.