Institute of Astronomy

New Ideas on Mechanisms of Angular Momentum Transport and Variability in Boundary Layers of Accretion Disks

SpeakerTalk DateTalk Series
Roman Rafikov (Princeton)23 May 2013Institute of Astronomy Colloquia

Abstract

Disk accretion onto a weakly magnetized central object, e.g. a white dwarf or a neutron 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. Here I will describe a novel, robust mechanism of the angular momentum transport inside the astrophysical boundary layers. Using high resolution 2D and 3D hydrodynamical simulations in the equatorial plane of a boundary layer we generically 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 order the rotational velocity at the inner edge of the disk. 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. Periodicity of these trapped modes may be relevant for explaining the variability seen in accreting compact objects.

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