Conical Winds from the Disk-Magnetosphere Boundary

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Romanova, M. M.
Ustyugova, G. V.
Koldoba, A. V.
Lovelace, R. V. E.
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Abstract
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A new type of wind - a conical wind - has been discovered in axisymmetric magnetohydrodynamic simulations of the disk-magnetosphere interaction in cases where the magnetic field of the star is bunched into an X-type configuration. Such a configuration arises if the effective viscosity of the disk is larger than the effective diffusivity, or if the accretion rate in the disk is enhanced. Conical outflows flow from the inner edge of the disk into a narrow shell with half-opening angle of 30-45 degrees. The outflow carries about 0.1-0.3 of the disk mass accretion rate and part of the disk's angular momentum. The conical winds are driven by the gradient of the magnetic pressure which exists above the disk due to the winding of the stellar magnetic field. Exploratory 3D simulations show that conical winds are symmetric about rotation axis of the disk even if the magnetic dipole is significantly misaligned with the disk's rotation axis. Conical winds appear around stars of different periods. However, in the case of a star in the "propeller" regime, an additional - much faster component appears: an axial jet, where matter is accelerated up to very high velocities at small distances from the star by magnetic pressure force above the surface of the star. The simulations are done in dimensionless units and are applicable to a variety of the disk-accreting magnetized stars: young stars, white dwarfs, neutron stars, and possibly black holes. For the case of young stars, conical winds and axial jets may appear in different cases, including Class I young stars, classical T Tauri stars, and EXors. In EXors periods of enhanced accretion may lead to the formation of conical winds which correspond to the outflows observed from these stars.
Comment: 14 pages, 14 figures, submitted to the MNRAS. See animations at http://www.astro.cornell.edu/~romanova/conical.htm ; http://www.astro.cornell.edu/~romanova/propeller.htm ; http://www.astro.cornell.edu/us-rus/disk_prop.htm
Keywords
Astrophysics - Solar and Stellar Astrophysics
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