Oscillations of accretion disks and boundary layers in cataclysmic variables. II. A local, linear stability analysis of accretion disk boundary layers

We present the results of a linearized perturbation analysis for the models of cataclysmic variable (CV) disks and their boundary layers (BLs), discus sed in a previous companion paper. For the case of large-scale, azimuthal o scillations in the BL, we find a triplet of unstable "torsional" modes cons isting primarily of perturbations of the azimuthal velocity, plus a singlet mode consisting mainly of a pressure perturbation. In the disk, the torsio nal modes are modified gravity waves, with rise times short enough for sign ificant amplification before entering the BL; in the BL, the frequencies of these modes are close enough in magnitude to produce beating. In the BL re gion where the effective temperature has its maximum, the beat frequencies, for a star rotating at 5% of its breakup rate, span a range from the stell ar rotation frequency Omega* up to an order of magnitude larger than that, i.e., from periods of P similar to 20 s to (2-3) x 100 s, with the underlyi ng fundamental oscillations having much smaller periods, similar to 1 s. Th ese beat-frequency oscillations of the torsional modes have attributes simi lar to the quasi-periodic oscillations (QPOs) observed in many CVs. Further more, if a particular region of the BL is excited, fairly high Q (where Ome ga drop \dP/dt\(-1)) beat-frequency oscillations are produced, which are si milar to some observed dwarf nova oscillations (DNOs). We propose two obser vational tests to determine whether some QPOs can be identified with the mo des studied here. The tests require an extension of the search for CV oscil lations to the 0.2-2 Hz frequency range.