Plausible Physical Conditions in the Near-Nucleus Coma of Comet P/Wirtanen at 3 AU from the Sun
J.F. Crifo (S.A.), A. Enzian (L.G.G.E.), J. Klinger (L.G.G.E), A.V. Rodionov (TsNIIMASch)
The 2 1/2 dimension Spherical Nucleus Model of Enzian et al. (1997) is used to predict the gas production rate of Comet P/Wirtanen at distances where it lies below the detection limit of Earth-based instruments. It is found that the production of CO from the surface exceeds that of at heliocentric distances exceeding 2.5 AU. Using the range of predicted CO flux for AU, the Dusty Gasdynamic Model of Crifo and Rodionov (1997 a - b) is used to compute the circumnuclear CO-dominated coma structure, assuming a choice of simple nucleus shapes. The results show that a small fluid region capable of efficiently accelerating dust, will surround the nucleus surface; in case of nearly isotropic CO production, the gas density is greater on the night side, due to a smaller initial temperature and velocity. Any departure from strict isotropy, e.g. under the form of surface temperature gradient, or of changes in surface curvature, is found to give birth to pronounced inhomogeneities (weak shocks) in the CO distribution; the concavities of the surface give birth to stronger shocks, as in the case of solar-driven sublimation studied precedingly. In conclusion, the near-nucleus CO and dust coma of the distant comet P/Wirtanen are predicted to be as inhomogeneous in density and flow velocity as the near-Sun coma. Failure to observe pronounced inhomogeneities in the outer dust coma of the near-Sun comet P/Wirtanen is most likely an indication that its nucleus has an irregular shape, giving birth to a highly disordered dust coma which looks smooth owing to the strong line-of-sight averaging and pixel-averaging implied in all (even HST) observations.
Crifo J.F., Rodionov, A.V. (1997 a - b), The dependence of the circumnuclear coma structure on the properties of the Nucleus, I. & II, Icarus in press
Enzian A. et al. (1997), A 2 1/2 thermodynamic model of cometary nuclei, I. Astron. Astrophys., 319, 995 - 1006 (1997)