A model for quasi-periodic oscillations in cataclysmic variables based on boundary layer oscillations

We have performed a linearized perturbation analysis of the disk and bounda ry layer (BL) in a cataclysmic variable. In the BL, we find a triplet of la rge-scale, torsional oscillation modes consisting primarily of perturbation s of the azimuthal velocity, together with a singlet mode consisting primar ily of a pressure perturbation. Two of these, which we call the "fast torsi onal modes," have rise times short enough to provide significant amplificat ion of a perturbation carried inward through the BL with the local drift ve locity. These are gravity modes, modified by the large shear and radial acc eleration in the BL. The two fast torsional modes have frequencies close en ough in magnitude to produce beating; the underlying oscillations have very short periods, similar to 1 s, which may be observable. The resulting beat frequency ranges from the local orbital rotation period, 2 pi/Omega, up to an order of magnitude larger than that, i.e., similar to 20 to (2-3) x 100 s, through the region of the boundary layer in which the effective tempera ture reaches a maximum. The beat oscillations between the fast torsional mo des have attributes similar to the quasi-periodic oscillations (QPOs) obser ved in some cataclysmic variables. We suggest that these beat oscillations represent a class of QPOs, and we suggest some tests of this hypothesis.