The Dependence of the Circumnuclear Coma Structure on the Properties of the Nucleus

Abstract

The second assumed nucleus fitted to P/W is an inhomogeneous sphere of dusty ice, most of the gas and dust production A new step of development of the 3-D circumnuclear coma being due to four identical active areas separated by a back-(''CNC'') model described in Crifo et al. (1995) is presented:ground area, and covering a fraction g ‫؍‬ 0.43 of the surface. now the gas and dust production are computed from an explicit It is found that, up to at least 2 AU, the structure of the CNC dusty-ice sublimation model and a realistic dust mass spectrum is, in such a case complex. (1) Owing to the difference in solar is used. For the first time, (1) a clear distinction is made between zenith angle, identical active areas produce differing CNC gas nucleus active area fraction g and active surface icy area fraction and dust distributions: therefore, in an inhomogeneous rotating f ; (2) the dependence of the dusty ice sublimation rate on f nucleus, an active region is not characterized by a fixed corotatand that of f on the heliocentric distance r h due to the evolution ing CNC pattern. (2) As with more productive comets, the of the dust cover are explicitly taken into account; (3) the interaction between gas issuing from different active regions 3-D structure of the CNC is described thoroughly instead of leads to the formation of three-dimensional quasi stationary only in one symmetry plane; (4) the 3-D CNC model is quantitashock structures. (3) In particular, the weaker active areas tively fitted to observational data; (5) a detailed comparison is cannot expand freely in the sunward hemisphere, but are surmade between the CNC structures resulting from two alternarounded by a low-altitude concave sheath of shocked gas and tive assumptions concerning the nucleus. deflected dust. (4) Near to the shock surfaces, the dust density The first assumed nucleus is a homogeneous sphere of dusty distribution is characterized, as in more productive comets, by ice ( g ‫؍‬ 1). We show that, in this case: (1) The maximum density patterns that mimic dust jets. (5) Over the background ejectable mass of spherical dust grains has a ؔ f (r h ) cos z/r 2 h areas, and close to the surface, the gas flows transverse to the dependence upon solar zenith angle z and upon r h . (2) The vertical, and exhibits steep increases in density outwards. (6) terminal ejected dust velocities have an approximate The results reveal a fast smoothing-out of the CNC near-surface ؔ ͙cos z dependence on z and a strong ؔ ͙f (r h )/r h depenpatterns with increasing distance to the surface. dence on r h . Fitting-for definiteness-the model to the lightThe implications of the present results for (1) the general curve of the weak Comet P/Wirtanen (P/W), target of the physical characterization of comet nuclei on the basis of their future Rosetta mission, we find, assuming an upper limit nucoma-averaged properties, and (2) the assessment of the envicleus radius of 1.4 km, that: (1) f decreases from about 14% at ronmental parameters of future cometary close encounter misperiheli...

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