Results

Figure 2 shows the variation of the forced inclination of dust particles originating in the Eos family over a range of diameter spanning from approximately 1-100 microns. It is clear that the dust band material is dispersed into the background cloud as it passes through secular resonances located near the inner edge of the asteroid belt. A similar dispersal is found for eccentricity, making asteroidal particles appear more cometary in nature. The dust bands therefore have a natural inner edge, and explains why previous attempts to model the dust bands have been most successful when confining the dust band material to the asteroid belt.

Figure 3. IRAS dust band profiles (solid lines) in three wavebands are compared with models with a size-frequency index q=1.43. All profiles were made at 90° solar elongation angle in a direction either leading (L) or trailing (T) the Earth in its orbit. The low value of q implies that the dominant particles are large (100 micron diameter or greater).

Figure 3. IRAS dust band profiles (solid lines) in three wavebands are compared with models with a size-frequency index q=1.43. All profiles were made at 90° solar elongation angle in a direction either leading (L) or trailing (T) the Earth in its orbit. The low value of q implies that the dominant particles are large (100 micron diameter or greater).

Once the distributions of the various particle sizes have been calculated, they can be combined to investigate the nature of the dust band size-frequency distribution. A system in collisional equilibrium has a size-frequency distribution index q = 11/6, in which particles at the small end of the distribution dominate. As q is reduced, more large particles are introduced until q < 5/3 at which point large particles dominate. We find that a q of approximately 1.4 produces the best simultaneous fit to the 12, 25 and 60 micron IRAS observations (Figure 3), providing further evidence of the preponderance of large particles in the zodiacal cloud. However, whereas most existing information on the size-frequency distribution of the cloud is based on evidence near 1 AU, our results are valid in the main-belt region. The results shown correspond to astronomical silicate material, but are largely insensitive to reasonable choices for the particle composition. Themis and Koronis dust comprises the central band, whereas the ten degree band is best modeled from material originating from the inner edge of the inclination distribution of the Eos family (9.35°) with relatively high dispersion (1.5°). This is perhaps suggestive of a catastrophic origin for the ten degree band, rather than being an equilibrium feature associated with the grinding down of the Eos family as a whole. However, the mean inclination of the model dust band is dependent on the nature of the dispersion of the orbital inclinations, and further work is required to characterize the dispersion of particles larger than 100 microns. This is an ongoing effort, and early results we have obtained using a modified symplectic algorithm are given elsewhere in these proceedings [9].

A direct result of the dust band modeling is the fraction of dust band material in the cloud. These models suggest a fraction of around 25%. Further work needs to be done to relate this number to the total asteroidal contribution to the cloud, but assuming that collision rates in family and non-family asteroids are similar, and that the ratio of non-family to family members is about 3:1, our estimate for the asteroidal contribution is approximately 75%. However this will change as we obtain more information on the large particle dynamics and refine our models.

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