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Session 5 - Accretion and Outflows in YSOs.
Display session, Wednesday, January 07
The magnetohydrodynamical evolution of the interaction region between the inner edge of an accretion disk and the magnetosphere of the central object is studied by means of time-dependent numerical simulations. The disk is adiabatic, axisymmetric, has non-zero resistivity, and is initially in Keplerian rotation. The magnetosphere corotates with the central star and is threaded by one of three initial magnetic field topologies: 1) a dipole field which threads the disk continuously, 2) a dipole field excluded from the disk by surface currents, and 3) a dipole field continuously threading a disk superposed with a uniform axial magnetic field. A number of exploratory simulations are performed by varying the field strength, the disk density and inner radius, the magnitude of the resistivity, and the stellar rotation rate.
Generally, we find rapid evolution of the disk occurs due to angular momentum transport by either the Balbus-Hawley instability or magnetic braking effects. Equatorial accretion results on a dynamical timescale unless the magnetic pressure of the magnetosphere exceeds the ram pressure of the accreting disk plasma; the latter we find to be a highly time dependent quantity. In the case of a pure dipole magnetospheric field, however, rapid stellar rotation can result in a field geometry which inhibits polar accretion even when ram and magnetic pressures balance. In contrast, we find that polar accretion can occur regardless of the stellar rotation rate when strong global disk fields combine with stellar fields to create a favorable net field topology.
Highly time dependent winds are evident in the evolution of all field topologies. The winds are generally channeled along field lines which have been opened via reconnection. The speed and variability of the outflows is dependent on the magnetic field strength and accretion topology. Net torque on the star during accretion is measured to be positive.
Program listing for Wednesday