Dead zones and extrasolar planetary properties
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abstract
Most low-mass protostellar disks evolve in clustered environments where they
are affected by external radiation fields, while others evolve in more isolated
star-forming regions. Assuming that the magneto-rotational instability (MRI) is
the main source of viscosity, we calculate the size of a poorly ionized, MRI
inactive, and hence low viscosity region - the "dead zone" - in these
protostellar disks. We include disk ionization by X-rays, cosmic rays,
radioactive elements and thermal collisions, recombination by molecules,
metals, and grains, as well as the effect of turbulence stimulation in the dead
zone by the active layers lying above it. We also calculate the gap-opening
masses of planets, which are determined by a disk's viscosity and a disk aspect
ratio, for disks in these environments and compare them with each other.
We find that the dead zone is a robust feature of the protostellar disks that
is largely independent of their environment, typically stretching out to ~ 15
AU. We analyze the possible effects of dead zones on planet formation,
migration, and eccentricity evolution. We show that the gap-opening mass inside
the dead zone is expected to be of the order of terrestrial and ice giant mass
planets while that outside the dead zone is Jovian or super-Jovian mass
planets, largely independent of the star-forming environment. We show that dead
zones can significantly slow down both type I and type II planetary migration
due to their lower viscosity. We also find that the growth of eccentricity of
massive extrasolar planets is particularly favorable through the planet-disk
interaction inside the dead zones due to the large gaps expected to be opened
by planets.
