ANGULAR MOMENTUM TRANSPORT AND VARIABILITY IN BOUNDARY LAYERS OF ACCRETION DISKS DRIVEN BY GLOBAL ACOUSTIC MODES

Disk accretion onto a weakly magnetized central object, e.g., a star, is inevitably accompanied by the formation of a boundary layer near the surface, in which matter slows down from the highly supersonic orbital velocity of the disk to the rotational velocity of the star. We perform high-resolution...

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Bibliographic Details
Published in:The Astrophysical journal 2012-11, Vol.760 (1), p.1-20
Main Authors: BELYAEV, Mikhail A, RAFIKOV, Roman R, STONE, James M
Format: Article
Language:English
Subjects:
ACCRETION DISKS
Acoustics
ANGULAR MOMENTUM
ASTRONOMY
ASTROPHYSICS
ASTROPHYSICS, COSMOLOGY AND ASTRONOMY
AXIAL SYMMETRY
Boundary layer
BOUNDARY LAYERS
COMPUTERIZED SIMULATION
Disks
Earth, ocean, space
Exact sciences and technology
HYDRODYNAMICS
Lindblad resonance
MASS
RESOLUTION
RESONANCE
Rotation
SHEAR
STARS
TRAPPING
VELOCITY
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Summary:Disk accretion onto a weakly magnetized central object, e.g., a star, is inevitably accompanied by the formation of a boundary layer near the surface, in which matter slows down from the highly supersonic orbital velocity of the disk to the rotational velocity of the star. We perform high-resolution two-dimensional hydrodynamical simulations in the equatorial plane of an astrophysical boundary layer with the goal of exploring the dynamics of non-axisymmetric structures that form there. We genetically find that the supersonic shear in the boundary layer excites non-axisymmetric quasi-stationary acoustic modes that are trapped between the surface of the star and a Lindblad resonance in the disk. These modes rotate in a prograde fashion, are stable for hundreds of orbital periods, and have a pattern speed that is less than and of the order of the rotational velocity at the inner edge of the disk. The origin of these intrinsically global modes is intimately related to the operation of a corotation amplifier in the system. Dissipation of acoustic modes in weak shocks provides a universal mechanism for angular momentum and mass transport even in purely hydrodynamic (i.e., non-magnetized) boundary layers. We discuss the possible implications of these trapped modes for explaining the variability seen in accreting compact objects.
ISSN:0004-637X
1538-4357
DOI:10.1088/0004-637X/760/1/22