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S.A. Stern, D.D. Durda (Southwest Research Institute)
We have explored the effects of collisional evolution on putative Vulcanoid ensembles in the region between 0.06 and 0.21 AU from the Sun, in order to constrain the probable population density and population structure of this region today (Stern & Durda, 2000. Icarus, in press). Despite the fact that dynamical studies have shown that the Vulcanoid Zone (VZ) could still be populated today (e.g., Evans & Tabachnik, 1999), we found that the frequency and energetics of collisional evolution this close to the Sun, coupled with the efficient radiation transport of small debris out of this region, together conspire to create an active and highly intensive collisional environment which depletes any very significant population of rocky bodies placed in it. Our simulations revealed that this condition obtains unless the bodies exhibit orbits that are circular to ~ 10-3 or less, or highly lossy mechanical properties that correspond to a fraction of impact energy significantly less than 10 being imparted to ejecta. Our work suggests it is unlikely that more than a few hundred objects with radii larger than 1 km will be found in the entire VZ. Assuming the largest objects have a radius of 30 km, then the total mass of bodies in the VZ down to 0.1 km radii is likely to be no more than ~10-6M\sun, some <10-3 the mass of the asteroid belt; and it is plausible that the entire region is virtually empty of km-scale and larger objects. The difficulty of both forming and maintaining a population of small bodies in this region even for timescales relevant to planetary formation (e.g., ~ 107-108 yrs), and in the adjacent one where Mercury now resides, suggests that Mercury's formation was either: (i) assisted by extremely-lossy collisional properties, or highly-damped orbital eccentricities, or both; or (ii) was initiated by the dynamical injection of 100 km diameter and larger seeds from a gentler collisional environment, e.g., near 1 AU. We speculate that similar results likely apply in extra-solar planetary systems around many G-type and earlier stars across the galaxy.