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SPIN-UP/SPIN-DOWN
OF MAGNETIZED STARS WITH ACCRETION DISKS AND OUTFLOWS
Authors:
R.V.E. Lovelace, M.M. Romanova, and G.S. Bisnovatyi-Kogan
An investigation is made of disc accretion of matter on to a rotating
star with an aligned dipole magnetic field. A new aspect of this work is that we argue
that, when the angular velocities of the star and disc differ
substantially, the B
field linking the star and disc rapidli inflates to give regions of open field lines
extending from the polar caps of the star and from the disc. The open field line region of
the disk leads to the possibility of magnetically driven outflows. An analysis is made of
the outflows and their back effect on the disk structure, assuming an a turbulent viscosity model for the disc and a
magnetic diffusivity comparable to this viscosity. The outflows are found to extend over a
range of radial distances inward to a distance close to rto , which is the distance of the maximum of the angular rotation rate
of the disc. We find that rto depends on the star's magnetic
moment, the accretion rate,
and the disc's magnetic diffusivity. The outflow regime is accompanied in general by a
spin-up of the rotation rate of the star. When rto exceeds the star's
corotation radius, we argue that outflow solutions do not occure, but instead that
`magnetic braking' of the star by the disc due to field-line twisting occurs in the
vicinity of rcr . The magnetic braking siolutions can give spin-up or spin-down
(or no spin change) of the star, depending mainly on the star's magnetic moment and the
mass accretion rate. For a system with rto comparable to rcr
bimodal behaviour is possible where extraneous perturbations (for
example,
intermittency of a, B field flux introduced from the companion star, or variations in the mass accretion
rate) cause the system to flip between spin-up (with outflows,
rto< rcr and spin-down (or spin-up) (with no
outflows, rto
> rcr).
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