Dust Transport in Protostellar Disks Through Turbulence and Settling

We apply ionization balance and MHD calculations to investigate whether magnetic activity moderated by recombination on dust can account for the mass accretion rates and the mid-infrared spectra and variability of protostellar disks. The MHD calculations use the stratified shearing-box approach and include grain settling and the feedback from the changing dust abundance on the resistivity of the gas. The two-decade spread in accretion rates among T Tauri stars is too large to result solely from variety in the grain size and stellar X-ray luminosity, but can be produced by varying these together with the disk magnetic flux. The diversity in the silicate bands can come from the coupling of grain settling to the distribution of the magneto-rotational turbulence, through three effects: (1) Recombination on grains yields a magnetically inactive dead zone extending above two scale heights, while turbulence in the magnetically active disk atmosphere overshoots the dead zone boundary by only about one scale height. (2) Grains deep in the dead zone oscillate vertically in waves driven by the turbulent layer above, but on average settle at the laminar rates, so the interior of the dead zone is a particle sink and the disk atmosphere becomes dust-depleted. (3) With sufficient depletion, the dead zone is thinner and mixing dredges grains off the midplane. The MHD results also show that the magnetic activity intermittently lifts clouds of dust into the atmosphere. The photosphere height changes by up to one-third over a few orbits, while the extinction along lines of sight grazing the disk surface varies by factors of two over times down to 0.1 orbit. We suggest that the changing shadows cast by the dust clouds on the outer disk are a cause of the daily to monthly mid-infrared variability in some young stars. (Abridged.)Comment: ApJ in pres

Accretion of dust by chondrules in a MHD-turbulent solar nebula

(Abridged) Numerical magnetohydrodynamic (MHD) simulations of a turbulent solar nebula are used to study the growth of dust mantles swept up by chondrules. A small neighborhood of the solar nebula is represented by an orbiting patch of gas at a radius of 3 AU, and includes vertical stratification of the gas density. The differential rotation of the nebular gas is replaced by a shear flow. Turbulence is driven by destabilization of the flow as a result of the magnetorotational instability (MRI), whereby magnetic field lines anchored to the gas are continuously stretched by the shearing motion. A passive contaminant mimics small dust grains that are aerodynamically well coupled to the gas, and chondrules are modeled by Lagrangian particles that interact with the gas through drag. Whenever a chondrule enters a region permeated by dust, its radius grows at a rate that depends on the local dust density and the relative velocity between itself and the dust. The local dust abundance decreases accordingly. Different chondrule volume densities lead to varying depletion and rimmed-chondrule size growth times. Most of the dust sweep-up occurs within 1 gas scale height of the nebula midplane. Chondrules can reach their asymptotic radius in 10 to 800 years. The vertical variation of nebula turbulent intensity results in a moderate dependence of mean rimmed-chondrule size with nebula height. The technique used here could be combined with Monte Carlo (MC) methods that include the physics of dust compaction, in a self-consistent MHD-MC model of dust rim growth around chondrules in the solar nebula.Comment: 33 pages, 9 figures. Icarus, in pres