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M. Fulle (Osservatorio Astronomico di Trieste, Italy), J.F. Crifo (CNES, Verrieres Les Buissons, France)
Crifo and Rodionov (1997) have developped new hydrodynamical codes able to compute the circumnuclear coma linked to a given nucleus shape and topography. An output of these models is the dust density and vectorial velocity at the terminal surface, out of which the gas drag on the dust becomes negligible. An interesting sampled possibility is the bean-shaped nucleus: it is the simplest shape able to produce jets in the coma even if all its surface is uniformly active, thanks to its surface concavity. The small size of the adopted nucleus (radius of the volume equivalent sphere less than 1 km) and the large fraction of active surface make this nucleus model able to simulate two extremely interesting comets, 46P/Wirtanen, target of the Rosetta ESA space mission, and C/Hyakutake 1996B2. We use the model output to compute the dust coma and tail of these two comets, by means of rigorous keplerian heliocentric dynamics taking into account the proper effects of the solar radiation pressure force. We show that the model necessarily explains a rare dust feature observed in the inner coma of C/1996B2 at the end of March, 1996: a dust spike pointing in the antisolar direction (Desvoivres et al. 1999). We show that this feature is due to the rare combinations of the following factors: (i) images at high resolution (thanks to the low Earth-Comet distance at the observations); (ii) small nucleus size (and therefore low escape velocity); (iii) a dust size distribution so dominated by the largest released grains (as obtained by dust tail models performed by Fulle et al., 1997) to make the coma brightness too dominated by mm-sized dust, i.e. the main component of the observed spike.
The author(s) of this abstract have provided an email address for comments about the abstract: fulle@ts.astro.it