A taxonomic survey of comet composition 1985-2004 using CCD spectroscopy

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Abstract

A summary is presented of our spectroscopic survey of comets extending for roughly 19 years from 1985 to 2004 comprising data for 92 comets of which 50 showed good emissions. All data were re-analyzed using consistent reduction techniques. Our observations of comets over several apparitions and comets observed over an extended period indicate no major changes in compositional classification. To our regret, no major unidentified cometary features were found in our surveyed spectral region of 5200-10400 Å. Absolute production rates for the dominant parent molecule H2O and the daughter species C2, NH2 and CN are determined within the limits of the Haser model as are values for the dust continuum, Afρ. From these data, production rate ratios are calculated for C2/H2O, NH2/H2O, CN/H2O and Afρ/H2O. Excluding the odd Comets Yanaka (1988r), 43P/Wolf-Harrington and 19P/Borrelly, with unusual spectra, our set of comets exhibited relatively uniform composition. Detailed analyses of our data resulted in four taxonomic classes:-Comets of typical composition (∼70%); exhibiting typical ratios with respect to water of C2, NH2, and CN.-Tempel 1 type (∼22%); having a deficiency in C2 but normal NH2 abundance.-G-Z type (∼6%); having both low C2 and NH2 ratios.-The unusual object Yanaka (1988r) (∼2%?); no detectable C2 or CN emission but normal NH2. It is uncertain whether there is a clear separation between the comets of typical composition and those with C2 depletion, or whether the latter consists of a group showing a continuum of decreasing C2/CN ratios. Our spectroscopic investigations result in a visual record of the various compositional classes, which are illustrated in a number of figures. Production rate comparisons with the comet photometry program of Schleicher and A'Hearn [A'Hearn, M.F., and 4 colleagues, 1995. Icarus 118, 223-270] for 13 comets in common yielded good agreement once the different scale lengths are taken into account. An investigation into the possible origin of our compositional groups with respect to dynamical families of comets shows that the Halley family exhibits essentially no C2 depletion. These objects were presumably formed in the region of Saturn and Uranus and scattered into the Oort cloud. Comets formed in the space near Neptune, responsible for the scattered Kuiper Belt show a mixture of "typical" and C2 depleted objects, while we associate comets formed in-situ in the classical Kuiper belt with our C2 depleted group.

Original languageEnglish (US)
Pages (from-to)311-334
Number of pages24
JournalIcarus
Volume201
Issue number1
DOIs
StatePublished - May 1 2009

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Keywords

  • Comets
  • coma
  • composition
  • dust

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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