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Session 79 - Special Objects in the Galaxy.
Oral session, Thursday, June 11

[79.08] A Local, Linear Stability Analysis of Accretion Disk Boundary Layers

T. J. B. Collins, H. L. Helfer, H. M. Van Horn (U Rochester)

We present the results of a linearized perturbation analysis of the disk and boundary layer (BL) in cataclysmic variables. 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 short 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 experience 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 the stellar rotation frequency Ømega_* up to an order of magnitude larger than that, i.e., \sim20 s to (2-3)\times100 s, in the region of the BL where the effective temperature has its maximum. The modulating oscillations of the fast torsional modes have much smaller periods, \sim1 s. The beat oscillations between the fast torsional modes have attributes similar to the quasi-periodic oscillations (QPOs) observed in some cataclysmic variables; we propose two observational tests to determine if they are the same. The most important of these is the extension of searches for oscillations to the 0.2-2 Hz frequency range.

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