Planet Formation and the Dead Zone
Neal Turner
JPL
October 22, 2007, 4PM, Steward N210
Solar nebula turbulence is a missing link in planet formation: it
drives the inspiral flow of gas through the nebula onto the young Sun,
brings the primordial dust grains together to build up larger bodies,
mixes material between the hot central and cold outer regions, and
regulates the orbital migration of the protoplanets, yet its origins
are mysterious. Magneto-rotational instability can produce turbulence
from the differential rotation of the nebula, but the weak ionisation
of the gas shuts down the instability in a midplane dead zone
overlapping the modern locations of the planets. The dead zone
shrinks as the dust is removed and recombination on the grain surfaces
becomes less important. We made the first 3-D MHD calculations of the
nebula to include time-dependent recombination chemistry, allowing a
treatment of the turbulent mixing and settling of the grains. The
results show that the dead zone is effectively eliminated under
favorable conditions as the grains are lost. Magnetic fields
diffusing into the disk interior drive accretion even where there is
no turbulence. The revived "undead zone" develops magnetic fields
strong enough to halt the migration of Earth-mass protoplanets.
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