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The nature of angular momentum transport in the boundary layers of accretiondisks has been one of the central and long-standing issues of accretion disktheory. In this work we demonstrate that acoustic waves excited by supersonicshear in the boundary layer serve as an efficient mechanism of mass, momentumand energy transport at the interface between the disk and the accretingobject. We develop the theory of angular momentum transport by acoustic modesin the boundary layer, and support our findings with 3D hydrodynamicalsimulations, using an isothermal equation of state. Our first major result isthe identification of three types of global modes in the boundary layer. Wederive dispersion relations for each of these modes that accurately capture thepattern speeds observed in simulations to within a few percent. Second, we showthat angular momentum transport in the boundary layer is intrinsicallynonlocal, and is driven by radiation of angular momentum away from the boundarylayer into both the star and the disk. The picture of angular momentumtransport in the boundary layer by waves that can travel large distances beforedissipating and redistributing angular momentum and energy to the disk and staris incompatible with the conventional notion of local transport by turbulentstresses. Our results have important implications for semianalytical modelsthat describe the spectral emission from boundary layers.

Angular Momentum Transport by Acoustic Modes Generated in the Boundary Layer I: Hydrodynamical Theory and Simulations