Infrared excitation processes of C2H6 in comets

Abstract

A time-dependent and line-by-line fluorescence model of the v7 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 v7 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 v7 band spectra taken with small apertures centered on the nucleus. We analyzed a v7 band 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 R Q0 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.