Astronomy and Astrophysics – Astrophysics
Scientific paper
2006-12-19
Astrophys.J.659:729-737,2007
Astronomy and Astrophysics
Astrophysics
to appear in the Astrophysical Journal; 25 pages, 10 figures
Scientific paper
10.1086/512007
We investigate the conditions for the presence of a magnetically inactive dead zone in protostellar disks, using 3-D shearing-box MHD calculations including vertical stratification, Ohmic resistivity and time-dependent ionization chemistry. Activity driven by the magnetorotational instability fills the whole thickness of the disk at 5 AU, provided cosmic ray ionization is present, small grains are absent and the gas-phase metal abundance is sufficiently high. At 1 AU the larger column density of 1700 g/cm^2 means the midplane is shielded from ionizing particles and remains magnetorotationally stable even under the most favorable conditions considered. Nevertheless the dead zone is effectively eliminated. Turbulence mixes free charges into the interior as they recombine, leading to a slight coupling of the midplane gas to the magnetic fields. Weak, large-scale radial fields diffuse to the midplane where they are sheared out to produce stronger azimuthal fields. The resulting midplane accretion stresses are just a few times less than in the surface layers on average.
Dziourkevitch Natalia
Sano Takayoshi
Turner Neal James
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