Torque Exerted on an Oblique Rotator by a Magnetically Threaded Accretion Disk
Abstract
Simple analytical expressions are derived for the torque exerted by an accretion disk on a star whose magnetic dipole axis is inclined to the rotation axis. Spin-down stresses are transmitted to the star by the field lines that penetrate the disk beyond the corotation radius Rc. As the dipole inclination angle χ increases, the vertical magnetic flux through the disk decreases, and the spin-down contribution to the torque weakens. For inclinations exceeding some limiting value χc in the range of approximately 54°-67°, the braking component is unable to offset the spin-up resulting from the accretion of matter, even when the inner radius of the Keplerian disk, R0, is located very close to Rc, so that the fastness parameter ω ≡ (R0/Rc)3/2 approaches unity. Thus, for large tilt angles, there can be no steady equilibrium state in which the net torque on the accreting star vanishes, unless some of the material is simultaneously expelled by centrifugal forces or accumulates within the disk.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- February 1997
- DOI:
- 10.1086/310481
- Bibcode:
- 1997ApJ...475L.135W
- Keywords:
-
- ACCRETION;
- ACCRETION DISKS;
- STARS: MAGNETIC FIELDS;
- STARS: ROTATION;
- Accretion;
- Accretion Disks;
- Stars: Magnetic Fields;
- Stars: Rotation