Collisional evolution in the Vulcanoid region: Implications for present-day population constraints

We explore the effects of collisional evolution on putative Vulcanoid ensem bles in the region between 0.06 and 0.21 AU from the Sun in order to constr ain the probable population density and population structure of this region today. Dynamical studies have shown that the Vulcanoid Zone (VZ) could be populated. However, we find that the frequency and energetics of collisiona l evolution this close to the Sun, coupled with the efficient radiation tra nsport of small debris out of this region, together conspire to create an a ctive and highly intensive collisional environment that depletes any very s ignificant population of rocky bodies placed in it, unless the bodies exhib it orbits that are circular to similar to 10(-3) or less or highly lossy me chanical properties that correspond to a fraction of impact energy signific antly less than 10% being imparted to ejecta. The most favorable locale for residual bodies to survive in this region is in highly circular orbits nea r the outer edge of the dynamically stable Vulcanoid Zone (i.e., near 0.2 A U), where collisional evolution and radiation transport of small bodies and debris proceed most slowly. If the mean random orbital eccentricity in thi s region exceeds similar to 10(-3), then our work suggests it is unlikely t hat more than a few hundred objects with radii larger than 1 km will be fou nd in the entire VZ; assuming the largest objects have a radius of 30 km, t hen the total mass of bodies in the VZ down to 0.1 km radii is likely to be no more than similar to 10(-6) M+, <10(-3) the mass of the asteroid belt. A 0.01-AU-wide ring near the outer stability boundary of the VZ at 0.2 AU w ould likely not contain over a few tens of objects with radii larger than 1 km. Despite the dynamical stability of large objects in this region (Evans , N. W., and S. Tabachnik, 1999, Nature 399, 41-43), it is plausible that t he entire region is virtually empty of kilometer-scale and larger objects. (C) 2000 Academic Press.