Comet classification with new methods for gas and dust spectroscopy

Abstract

We present the results of a program of comet long-slit spectroscopy with the Kast Dual Spectrograph on the 3-m Shane Telescope at Lick Observatory. A total of 26 comets, from a variety of dynamical families, were observed on 39 different nights from 1996 to 2007. A new statistical method extracted the twilight sky from comet frames, because traditional sky subtraction techniques were inadequate. Because previously published Haser model parent and daughter scale lengths did not fit the data well, unbiased ranges of scale lengths were searched for the best-fitting pairs. Coma gas production rates for OH, CN, C 2, C 3, NH, NH 2, and OH confirmed the widely reported carbon-chain depletion for a sub-class of comets, most notably high-perihelion Jupiter-family comets observed at r h > 1.5 AU, with different behaviors for C 2 and C 3. Our long-slit spectroscopy data was also adapted for the A( θ) fρ dust production parameter. The assumption that A( θ) fρ is constant throughout the nucleus was not upheld. High dust-to-gas ratios for comets with large perihelia were not a selection effect, and suggest that the dust was released earlier in the formation of the coma than the gas. The dust-to-gas ratio did not exhibit any evolutionary traces between different comet dynamical families. The comet survey illuminates the diversity among comets, including the unusually carbon poor Comet 96P/Machholz.