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[12.12] How anisotropic was the gas coma of comet C/Hale-Bopp?

M.C. Festou (Toulouse Observatory, France), M. Womack (St Cloud University, St Cloud, MN, USA), D. Pinnick (St Cloud University, St Cloud, MN? USA), J. Mangum (NRAO, Tucson, AZ, USA)

Comet C/Hale-Bopp exhibited in February-April 1997 a highly asymmetric dust coma. It is clear that most of the solid material was released from the sunward side of its nucleus (obviously, outgassing from the night side can only represent a small component of the total outgassing power of comets situated near the sun). At large scales, one could see a strong, often double, dust jet that remained visible throughout March and early April 1997. Many teams processed their images to reveal dust structures. Images of the gas component did not show the same degree of asymmetry and many images had to be heavily processed to reveal the presence of spiral-like structures. We use our On-the-Fly maps of the HCN emission to fully characterize the dominant, spherically symmetric, component of the source of nuclear material that has to be removed from the total signal for the asymmetric component to appear. This component is symmetric only when seen in projection on the sky. However, in 3D, its real structure appears axisymmetric. We determine the outgassing pattern of the HCN molecules at the nucleus, i.e. the angular distribution of the HCN flux, from a simultaneous analysis of the line shape throughout the coma and the spatial map. We determine the relative weights of the two main coma gas components. We determine which fraction of HCN can be produced in an extended region of the coma and thus which fraction of CN comes from HCN when CN "jets" are observed. Using HCN as a proxy for the spatial distribution of water molecules, the coma of comet C/H-B is highly collisional up to distances in excess of 5 10⁴ km from the nucleus, we determine the degree of coma asymmetry for the OH coma. We will demonstrate that the OH coma (and by inference that of most secondary species) is not as asymmetric as that of the nuclear species. Finally, we will evaluate the validity of using spherically symmetric models to determine gas production rates when the coma shows no obvious signs of an anisotropic ejection of parent molecules and dust particles.

MW benefited from the NSF CAREER and RUI Programs, and obtained support from the NASA Planetary Atmospheres.

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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.