Weak 10-micron Silicate Features as a Signature of Dead Zones in Protoplanetary Disks

Details Weak 10-micron Silicate Features as a Signature of Dead Zones in Protoplanetary Disks Weak 10-micron Silicate Features as a Signature of Dead Zones in Protoplanetary Disks

Jan 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009aas...21344106c&link_type=abstract

American Astronomical Society, AAS Meeting #213, #441.06; Bulletin of the American Astronomical Society, Vol. 41, p.319

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Scientific paper

We show that the abundance of crystalline silicate grains in the surface layers of protostellar disks is sensitive to the presence of a dead zone where the low ionization fraction shuts off magneto-rotational turbulence.
In a fully-turbulent Solar nebula analog disk, crystalline grains produced in the hot annealing region near the star are readily mixed out to AU distances, since the mean turbulent diffusivity measured in stratified shearing-box MHD calculations is about twice the mean turbulent effective viscosity. The crystalline particles collect into larger aggregates before reaching 1 AU. The aggregates fall into the disk interior, where they are disrupted by collisions. The secondary particles return to the surface layers in the turbulence, giving significant amounts of fine crystalline dust in the region of formation of the 10-micron silicate emission band. On the other hand, in a disk containing a laminar dead zone, the interior acts as a sink, and particles falling out of the surface layers never return. The growth time for crystalline particles mixed radially outward from the annealing region by the turbulence is less than the time to reach 1 AU, and consequently the surface layers retain few small crystalline grains, leading to a weak 10-micron feature.
We conclude that systems with weak or absent 10-micron features are likely to contain laminar dead zones. More generally, the infrared silicate bands serve as probes of the redistribution of grains during the early stages of planet formation.

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