An Alternative Look To Precession In Accretion Disks

Abstract

We have considered precession in accretion disks in which a second moment of inertia relative to an axis perpendicular to the axis of rotation may be very important. This formalism, that takes into account the precession contribution to the angular momentum, is based on the existence of a parameter $\it p$ which determines three characteristic densities resulting from the averaging process and imposes constraints on the actual disk density. It is shown that the precession velocity will lie in a three branch solution, and depends on how large is the disk actual density as compared to the characteristic densities. Besides the large spread on the solution for the precession velocity, depending on the density strength, it may be prograde and retrograde. It is shown that the keplerian thin disk, with very large density values compared to characteristic ones, only precesses very far away from the primary object, which implies very large precession periods. For other models, the disk will thicken, with large deviations from the keplerian approximation. Constraints on the density only will be effective for very large values of the ratio ${{\dot M} \over M_{p}}$, respectively, accretion rate and mass of the primary. Under this condition, the structure of the precessing region is found. Lower bounds on the precession period are found for not so large values of this ratio. Deviations from the mean precessional motion are considered. It is shown that these deviations result in periodic motions as long as the time scales associated to them are comparable to the remaining time scales. Otherwise, they result in misalignment motions, forcing the plane of the disk to become normal to the orbital plane of the secondary.