THE DEPENDENCE OF THE CIRCUMNUCLEAR COMA STRUCTURE ON THE PROPERTIES OF THE NUCLEUS .2. FIRST INVESTIGATION OF THE COMA SURROUNDING A HOMOGENEOUS, ASPHERICAL NUCLEUS

This work is the continuation of a program aimed at a physically and g eometrically realistic modeling of the gas and dust distribution in th e circumnuclear coma (CNC) of comets. Here, we consider two different homogeneous, but aspherical nuclei. Thus, this is the first time that non-spherical nucleus surfaces are taken into account in a coma model. For definiteness, and as in Crifo and Rodionov (1997 Icarus, in press ). The parameters of the model are chosen to be applicable to the fain t Comet P/Wirtanen. As was the case in Paper I, the results reveal bas ic properties of the CNC that are artificially concealed when the nucl eus is assumed spherical: 1. Standing shock structures appear in the g as CNC when a sufficiently deep concave region of the surface is sunli t. 2. Even a shallow concavity gives birth to sharp dust density enhan cements that, on CNC images, might look like pseudo-jet structures. Si nce similar effects have been obtained for inhomogeneous, spherical nu clei, it can be concluded that standing shocks and pseudo-jets are inh erent characteristics of the CNC of all comets. This fact suggests a r e-examination of the previous interpretations of the appearance of P/H alley's CNC in terms of intrinsic ''active areas'' of the nucleus. 3. Sublimation from a subsolar icy region can occur either sonically or s ubsonically, depending on its surroundings: thus, instead of being loc ally determined by the properties of the nucleus surface, as currently assumed, the distribution of the CNC gas and dust are global characte ristics of large areas of the nucleus surface. 4. Nearly horizontal fl ow of gas can occur near the surface of the day side, even near noon. 5. During the limited nucleus rotation of 45 degrees considered in thi s study, the gas shock structures were found to co-rotate, but with si gnificant distortion; the narrow dust pseudo-jets were found to rotate by differing amounts for the two nuclei; the background gas and dust distributions remained approximately fixed in a solar-ecliptic frame, but also underwent significant spatial distortion. These results indic ate that the current practice of discussing the CNC processes on the b asis of spherically symmetric assumptions must be abandoned: the asses sment of the basic physical properties of the CNC, the interpretation of in situ or remote-sensing data concerning this region, in terms of nucleus surface properties and/or nucleus rotational state definitely require tridimensional simulations of the kind presented here. (C) 199 7 Academic Press.