A Study of Molecular and Physical Processes in Comets

Citation

Arpigny, Claude (1964) A Study of Molecular and Physical Processes in Comets. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Y86M-JB16. https://resolver.caltech.edu/CaltechETD:etd-09102002-154144

Abstract

The rotational structure of the violet CN (0, 0) band in cometary spectra is investigated from a new point of view. The usual assumption of a Boltzmann distribution of populations of the rotational levels in the ground state is abandoned. These populations are determined by solving the system of steady-state equations which describes the resonance-fluorescence of the CN band. As expected, it is possible in this way to reproduce the observed profile more satisfactorily than in the case when a Boltzmann distribution is used. Furthermore, it is shown that the Swings effect (presence of absorption lines in the solar exciting light) has to be taken into account from the beginning in the resolution of the system of the steady-state equations, to achieve complete agreement between observed and computed profiles. The Greenstein effect (differences in relative intensities of rotational lines in different regions of the comet) has been studied in high-resolution spectrograms of Comet Seki-Lines (1962c). The observations can be interpreted partly by means of either of two simple models for the comet's head (uniform, isotropic expansion, or "fountain model"). In addition there are random motions of the order of a few km/sec. A detailed analysis of spectra of Comet Humason (1961e) has furnished the following results: a) The CO+ comet-tail bands are excited by the same resonance-fluorescence mechanism known to be responsible for the emissions of the neutral molecules observed in the head. b) The continuum is of the pure reflection type: there is no detectable selective scattering. c) Molecular abundances are estimated (at 10[superscript 4] km from the nucleus) N(CO+)=10[superscript 13] cm[superscript -2] N(N2+)=10[superscript 11] cm[superscript -2] N(CN) = 5X10[superscript 10] cm[superscript -2] The probable error on these estimates corresponds to a factor of 2 or 3. d) The infra-red surface brightness produced by the CO+ ions in pure vibration transitions v" = 1 -> v" = 0([lambda] approx.= 4.6 [mu]) is found to be rather large: approx.= 0.001 erg cm[superscript -2] sec[superscript -1]. e) The radial distributions of the various molecules are discussed. While CN follows the usual l/[rho] law ([rho] being the projected distance from the nucleus), it is suggested that the remarkable flatness of the ion distributions might be explained in terms of a "magnetic model." A critical discussion of some physical and chemical characteristics of comets is presented in the last chapter. In particular, the results of a number of abundance determinations are tabulated. (Photographic material on page 46 will not reproduce clearly on Xerox copies).

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