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T. J. B. Collins (LLE, U Rochester), H. L. Helfer (U Rochester), H. M. Van Horn (NSF)
We present results from a linearized perturbation analysis of the disk and BL in cataclysmic variables. We build on Collins, Helfer & Van Horn (1998; {\em ApJ} 508, L159) and discuss the detailed character and observational signatures of the modes which propagate in the BL.
For the case of large-scale, azimuthal oscillations, we found a triplet of ``torsional'' modes (consisting primarily of a perturbation of the azimuthal velocity), and a singlet pressure mode. Two of these, which we call the ``fast torsional modes,'' have rise times short enough for significant amplification of a perturbation entering the BL. These are gravity modes, modified by the large shear and radial acceleration in the BL.
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 \Omega* up to an order of magnitude larger than that, {\em i.e.}, ~0 s to (2-3) \times100 s, with the underlying fundamental oscillations having much smaller periods, ~ 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 beat frequency oscillations are produced, which are similar to some observed dwarf-novae oscillations (DNOs). We propose two observational tests to determine if 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. We also present a simple model of the power spectrum produced by BL beating of the fast torsional modes.
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