Boundary Layers of Accretion Disks: Discovery of Vortex-Driven Modes and Other Waves. (arXiv:2103.12119v2 [astro-ph.HE] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Coleman_M/0/1/0/all/0/1">Matthew S. B. Coleman</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rafikov_R/0/1/0/all/0/1">Roman R. Rafikov</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Philippov_A/0/1/0/all/0/1">Alexander A. Philippov</a>

Disk accretion onto weakly magnetized objects possessing a material surface
must proceed via the so-called boundary layer (BL) – a region at the inner edge
of the disk, in which the velocity of accreting material abruptly decreases
from its Keplerian value. Supersonic shear arising in the BL is known to be
conducive to excitation of acoustic waves that propagate into both the accretor
and the disk, enabling angular momentum and mass transport across the BL. We
carry out a numerical exploration of different wave modes that operate near the
BL, focusing on their morphological characteristics in the innermost parts of
accretion disk. Using a large suite of simulations covering a broad range of
Mach numbers (of the supersonic shear flow in the BL), we provide accurate
characterization of the different types of modes, verifying their properties
against analytical results, when available. We discover new types of modes, in
particular, global spiral density waves launched by vortices forming in the
disk near the BL as a result of the Rossby wave instability; this instability
is triggered by the vortensity production in that region caused by the
nonlinear damping of acoustic waves. Azimuthal wavenumbers of the dominant
modes that we observe appear to increase monotonically with the Mach number of
the runs, but a particular mix of modes found in a simulation is mildly
stochastic. Our results provide a basis for better understanding of the angular
momentum and mass transport across the BL as well as the emission variability
in accreting objects.

Disk accretion onto weakly magnetized objects possessing a material surface
must proceed via the so-called boundary layer (BL) – a region at the inner edge
of the disk, in which the velocity of accreting material abruptly decreases
from its Keplerian value. Supersonic shear arising in the BL is known to be
conducive to excitation of acoustic waves that propagate into both the accretor
and the disk, enabling angular momentum and mass transport across the BL. We
carry out a numerical exploration of different wave modes that operate near the
BL, focusing on their morphological characteristics in the innermost parts of
accretion disk. Using a large suite of simulations covering a broad range of
Mach numbers (of the supersonic shear flow in the BL), we provide accurate
characterization of the different types of modes, verifying their properties
against analytical results, when available. We discover new types of modes, in
particular, global spiral density waves launched by vortices forming in the
disk near the BL as a result of the Rossby wave instability; this instability
is triggered by the vortensity production in that region caused by the
nonlinear damping of acoustic waves. Azimuthal wavenumbers of the dominant
modes that we observe appear to increase monotonically with the Mach number of
the runs, but a particular mix of modes found in a simulation is mildly
stochastic. Our results provide a basis for better understanding of the angular
momentum and mass transport across the BL as well as the emission variability
in accreting objects.

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