Material Enhancement in Protoplanetary Nebulae by Particle Drift through Evaporation Fronts

Abstract

Solid material in a protoplanetary nebula is subject to vigorous redistribution processes relative to the nebula gas. Meter-sized particles drift rapidly inwards near the nebula midplane, and material evaporates when the particles cross a condensation/evaporation boundary. The material cannot be removed as fast in its vapor form as it is being supplied in solid form, so its concentration increases locally by a large factor (more than an order of magnitude under nominal conditions). As time goes on, the vapor phase enhancement propagates for long distances inside the evaporation boundary (potentially all the way in to the star). Meanwhile, material is enhanced in its solid form over a characteristic lengthscale outside the evaporation boundary. This effect is applicable to any condensible (water, silicates, {\it etc.}). Three distinct radial enhancement/depletion regimes can be discerned by use of a simple model. Meteoritics applications include oxygen fugacity and isotopic variations, as well as isotopic homogenization in silicates. Planetary system applications include more robust enhancement of solids in Jupiter's core formation region than previously suggested. Astrophysical applications include differential, time-dependent enhancement of vapor phase CO and H$_2$O in the terrestrial planet regions of actively accreting protoplanetary disks.