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T. J. B. Collins, H. L. Helfer, H. M. Van Horn (U. Rochester)
We have performed a linearized perturbation analysis of the disk and boundary layer (BL). In the BL, for the case of large-scale, azimuthal oscillations, we find a triplet of torsional modes consisting primarily of a perturbation of the azimuthal velocity, and a singlet mode consisting primarily of a pressure perturbation. Two of these, which we call the ``fast torsional modes,'' have rise times of enough for significant amplification of a perturbation entering the BL. They are gravity modes, modified by the large shear and radial acceleration in the BL. The instability is caused by the increase in the inertial force experienced by a parcel of accreting material with a perturbed velocity.
The two fast torsional modes have frequencies close enough in magnitude to produce beating. The beat frequency spans a range from \Omega up to an order of magnitude larger than that, i.e., ~ 20 s to 200-300 s, in the region of the boundary layer where the effective temperature has its maximum; The modulating oscillations of the fast torsional modes have much smaller periods, ~ 1 s. The beat oscillations between the fast torsional modes have attributes similar to the quasi-periodic oscillations observed in some cataclysmic variables.
The author(s) of this abstract have provided an email address for comments about the abstract: tcol@lle.rochester.edu