Local Magnetohydrodynamical Models of Layered Accretion Disks

Details Local Magnetohydrodynamical Models of Layered Accretion Disks Local Magnetohydrodynamical Models of Layered Accretion Disks

2002-10-24

arxiv.org/abs/astro-ph/0210541v1

Astrophys.J. 585 (2003) 908-920

Astronomy and Astrophysics

Astrophysics

Accepted by Ap. J

Scientific paper

10.1086/345848

Using numerical MHD simulations, we have studied the evolution of the magnetorotational instability in stratified accretion disks in which the ionization fraction (and therefore resistivity) varies substantially with height. This model is appropriate to dense, cold disks around protostars or dwarf nova systems which are ionized by external irradiation of cosmic rays or high-energy photons. We find the growth and saturation of the MRI occurs only in the upper layers of the disk where the magnetic Reynolds number exceeds a critical value; in the midplane the disk remains queiscent. The vertical Poynting flux into the "dead", central zone is small, however velocity fluctuations in the dead zone driven by the turbulence in the active layers generate a significant Reynolds stress in the midplane. When normalized by the thermal pressure, the Reynolds stress in the midplane never drops below about 10% of the value of the Maxwell stress in the active layers, even though the Maxwell stress in the dead zone may be orders of magnitude smaller than this. Significant mass mixing occurs between the dead zone and active layers. Fluctuations in the magnetic energy in the active layers can drive vertical oscillations of the disk in models where the ratio of the column density in the dead zone to that in the active layers is <10. These results have important implications for the global evolution of a layered disk, in particular there may be residual mass inflow in the dead layer. We discuss the effects that dust in the disk may have on our results.

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