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D. Lien (Oklahoma State University)
The results from comparing numerical simulations of cometary dust with observations of a number of recent comets suggests real differences in the dynamical properties of dust ejected from "young" and "old" comets.
Old comets, like P/Encke, emit dust from only one or two active areas on the surface of a rotating nucleus. The dust which is emitted has a relatively small maximum \beta (\beta \le 10-2, where \beta is the ratio of forces from radiation pressure and gravity) and the ejection velocity appears to vary with \beta as vej \propto \sqrt{\beta}. The result is an asymmetrically shaped dust coma usually seen in the form of a sunward fan which extends into the anti-solar direction in the form of a featureless tail.
Young comets, like C/Hale-Bopp and C/Hyakutake, appear to have two dynamically distinct dust components. One component creates a featureless parabolic dust coma, and consists of dust emitted isotropically with vej \propto \sqrt{\beta} and a relatively high maximum \beta (\beta \approx 1). The second component creates the characteristic arcs and jets in the inner coma, and is most successfully modeled with grains ejected from multiple small active areas on the surface of a rotating nucleus. The dust ejection velocity appears to be essentially independent of \beta and the dust must have a finite lifetime. The dust tail of young comets can vary from essentially featureless to exhibiting multiple striations and/or streamers.
These results suggest that solids ejected from young comets are primarily in the form of icy grains, which then sublimate and accelerate the embedded non-volatile grains isotropically. The difference in the functional relationship between \beta and ejection velocities between young and old comets suggests that there may be a real difference in the physical mechanisms which eject the grains from the cometary surface.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.