THE DEPENDENCE OF THE CIRCUMNUCLEAR COMA STRUCTURE ON THE PROPERTIES OF THE NUCLEUS .1. COMPARISON BETWEEN A HOMOGENEOUS AND AN INHOMOGENEOUS SPHERICAL NUCLEUS, WITH APPLICATION TO P WIRTANEN/

A new step of development of the 3-D circumnuclear coma (''CNC'') mode l described in Crifo et al. (1995) is presented: now the gas and dust production are computed from an explicit dusty-ice sublimation model a nd a realistic dust mass spectrum is used. For the first time, (1) a c lear distinction is made between nucleus active area fraction g and ac tive surface icy area fraction f; (2) the dependence of the dusty ice sublimation rate on f and that off on the heliocentric distance rh due to the evolution of the dust cover are explicitly taken into account; (3) the 3-D structure of the CNC is described thoroughly instead of o nly in one symmetry plane; (4) the 3-D CNC model is quantitatively fit ted to observational data; (5) a detailed comparison is made between t he CNC structures resulting from two alternative assumptions concernin g the nucleus. The first assumed nucleus is a homogeneous sphere of du sty ice (g = 1). We show that, in this case: (1) The maximum ejectable mass of spherical dust grains has a proportional to f(r(h)) cos z/r(h )(2) dependence upon solar zenith angle z and upon r(h). (2) The termi nal ejected dust velocities have an approximate proportional to root c os z dependence on z and a strong proportional to root f(r(h))/r(h) de pendence on r(h). Fitting-for definiteness-the model to the light curv e of the weak Comet P/Wirtanen (P/W), target of the future Rosetta mis sion, we find, assuming an upper limit nucleus radius of 1.4 km, that: (1) f decreases from about 14% at perihelion to possibly 0.025% at r( h) = 3 AU; (2) for r(h) less than or equal to 2.5 AU all or most of th e sunward circumnuclear CNC is in fluid regime; (3) for 2.5 less than or equal to r(h) less than or equal to 3 AU, most of the sunward CNC i s in the so-called ''transition regime''; (4) due to their dependence upon f, the dust velocities are in absolute value much smaller than ex pected from the usually accepted algorithms (which assume f = 1) and d ecrease strongly with increasing r(h). The physical significance of th ese results is discussed. The second assumed nucleus fitted to P/W is an inhomogeneous sphere of dusty ice, most of the gas and dust product ion being due to four identical active areas separated by a background area, and covering a fraction g = 0.43 of the surface. It is found th at, up to at least 2 AU, the structure of the CNC is, in such a case c omplex. (1) Owing to the difference in solar zenith angle, identical a ctive areas produce differing CNC gas and dust distributions: therefor e, in an inhomogeneous rotating nucleus, an active region is not chara cterized by a fixed corotating CNC pattern. (2) As with more productiv e comets, the interaction between gas issuing from different active re gions leads to the formation of three-dimensional quasi stationary sho ck structures. (3) In particular, the weaker active areas cannot expan d freely in the sunward hemisphere, but are surrounded by a low-altitu de concave sheath of shocked gas and deflected dust. (4) Near to the s hock surfaces, the dust density distribution is characterized, as in m ore productive comets, by density patterns that mimic dust jets. (5) O ver the background areas, and close to the surface, the gas flows tran sverse to the vertical, and exhibits steep increases in density outwar ds. (6) The results reveal a fast smoothing-out of the CNC near-surfac e patterns with increasing distance to the surface. The implications o f the present results for (1) the general physical characterization of comet nuclei on the basis of their coma-averaged properties, and (2) the assessment of the environmental parameters of future cometary clos e encounter missions are discussed. In particular, we show that simple , unidimensional heuristic models based on observations of a comet fro m the Earth are unable to provide relevant predictions concerning the physical conditions near to its nucleus. This conclusion is further en forced by the companion paper-Crifo and Rodionov (1996)-in which an as pherical homogeneous nucleus is assumed. (C) 1997 Academic Press.