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Title: Dynamics of Thin Astrophysical Boundary Layers
Authors: Belyaev, Mikhail
Advisors: Rafikov, Roman R
Stone, James M
Contributors: Astrophysical Sciences Department
Keywords: accretion
angular momentum
Subjects: Astrophysics
Issue Date: 2013
Publisher: Princeton, NJ : Princeton University
Abstract: We study the modal structure and angular momentum transport mechanisms of astrophysical boundary layers. We focus on the case where the accretion disk extends all the way to surface of the star and the boundary layer is thin in comparison with the stellar radius. Such a scenario is applicable, for example, to weakly magnetized neutron stars and white dwarfs, for which the strength of the magnetic field outside the star is too small to disrupt the disk and funnel matter to the poles. Within the boundary layer, material rotating at the Keplerian velocity within the disk slows down to the rotational velocity inside the star. This generates intense velocity gradients and makes the boundary layer susceptible to shear instabilities. By performing a linear stability analysis for the simplified case of a plane-parallel, compressible shear layer, we argue that astrophysical boundary layers are unstable to the sonic instability. This instability is part of a more general class of acoustic instabilities that includes the Papaloizou-Pringle instability. We confirm the predictions of our linear stability analysis by running a suite of simulations in 2D and 3D, with and without stratification, and with and without magnetic field. In our numerical experiments, we find that acoustic modes excited by the sonic instability persist even in the nonlinear regime. We explain the morphological properties and derive analytic formulas for the pattern speed of these acoustic modes. Our work has significant implications for semianalytic models describing the structure and spectral emission from boundary layers. Typically, these models adopt a local, effective viscosity prescription for the angular momentum transport. However, in our simulations we find that angular momentum transport in the boundary layer is facilitated by acoustic modes. In this scenario, accreting material inside the boundary layer loses angular momentum to sound waves that propagate into both the star and the disk. Since transport of angular momentum by waves is inherently nonlocal, our work invites the construction of new phenomenological models of the boundary layer in which angular momentum is transported by waves rather than by an anomalous viscosity.
Alternate format: The Mudd Manuscript Library retains one bound copy of each dissertation. Search for these copies in the library's main catalog
Type of Material: Academic dissertations (Ph.D.)
Language: en
Appears in Collections:Astrophysical Sciences

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