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), discussed in a previous companion paper. For the case of large-scale, azimuthal oscillations in the BL, we find a triplet of unstable "torsional" modes consisting primarily of perturbations of the azimuthal velocity, plus a singlet mode consisting mainly of a pressure perturbation. In the disk, the torsional modes are modified gravity waves, with rise times short enough for significant amplification before entering the BL; in the BL, the frequencies of these modes are close enough in magnitude to produce beating. In the BL region 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 stellar rotation frequency ?*up to an order of magnitude larger than that, i.e., from periods of P~ 20 s to (2-3) × 100 s, with the underlying fundamental oscillations having much smaller periods, ~1 s. These beat-frequency oscillations of the torsional modes have attributes similar to the quasi-periodic oscillations (QPOs) observed in many CVs. Furthermore, if a particular region of the BL is excited, fairly high Q(where Q? |dP/dt|-1) beat-frequency oscillations are produced, which are similar to some observed dwarf nova oscillations (DNOs). We propose two observational tests to determine whether some QPOs can be identified with the modes studied here. The tests require an extension of the search for CV oscillations to the 0.2-2 Hz frequency range.