ON THE TORQUE EXERTED BY A MAGNETICALLY THREADED ACCRETION DISK

摘要

Simple expressions are derived for the torque exerted by an accretion disk on a rotating, magnetized object (which may, for example, be a neutron star, a white dwarf, or a T Tauri star). In the equilibrium state (for which there is no net torque on the star), the inner edge of the Keplerian disk R_0 is located very close to the corotation radius R_c, for physically plausible assumptions about the dissipation time of the wound-up field component; the equilibrium value ω_(crit), of the "fastness parameter" ω ≡ (R_0/R_c)~(3/2) lies in the range 0.875-0.95. Empirical constraints requiring ω_(crit) to be significantly less than unity are thus incompatible with the magnetically threaded disk model.