31st Annual Meeting of the DPS, October 1999
Session 17. Comet Comae Posters
Poster Group I, Monday-Wednesday, October 11, 1999, , Kursaal Center

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[17.14] A Model of the Neutral Coma of Comet Hale-Bopp (1995 O1) from the Surface of the Nucleus to One Million Kilometers

M.R. Combi, K. Kabin, T.I. Gombosi, D.L. DeZeeuw (SPRL/U. of Michigan), K.G. Powell (Dept. of Aerospace Engineering/U. of Michigan)

Because of its phenomenally large gas/dust production rate, Comet Hale-Bopp has been observed throughout the time period from its discovery in the middle of 1995 to the present, covering an unprecedented range of heliocentric distance. Naturally, intensive observational coverage was concentrated during the two months surrounding perihelion on 1 April 1997 when the comet was between 0.9 and 1.1 AU because of the cometís phenomenal brightness. These include runs of continuous temporal coverage and many one-of-a-kind observations of high spatial and spectral resolution. The physical environment of the coma was greatly effected by the large production rate to such an extent that the many published Halley-type comet conditions are not applicable to Hale-Bopp. The interpretations of most data sets require an accurate model for the coma. The model must be consistent with the observations that do exist and must also contain a more detailed picture than can directly extracted from observations. We present here an axisymmetric model for the coma of the comet at a heliocentric distance of 1 AU having one major active region (a jet) superimposed on a weaker uniform background distribution. Such past calculations have only dealt with the conditions in the immediate vicinity of the nucleus and are not extended to see their effect on the observable coma, such as the distributions of daughter species. Our calculations cover (1) the dusty-gas dynamic region from the nucleus to 103 km, (2) the collisional/photochemical region out to 105 km, and (3) the nearly free expansion region out to 106 km. A dusty-gas hydrodynamic calculation with adaptive mesh refinement is used from the nucleus surface (r \approx 28 km) out to \approx 103 km. A fully-kinetic Direct Simulation Monte Carlo calculation which includes water photochemistry and kinetics is used from 103 to 106 km.


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