Interface Modes and Their Instabilities in Accretion Disc Boundary Layers

Abstract

We study global non-axisymmetric oscillation modes trapped near the inner boundary of an accretion disc. Observations indicate that some of the quasi-periodic oscillations (QPOs) observed in the luminosities of accreting compact objects (neutron stars, black holes and white dwarfs) are produced in the inner-most regions of accretion discs or boundary layers. Two simple models are considered in this paper: The magnetosphere-disc model consists of a thin Keplerian disc in contact with a uniformly rotating magnetosphere with and low plasma density, while the star-disc model involves a Keplerian disc terminated at the stellar atomosphere with high density and small density scale height. We find that the interface modes at the magnetosphere-disc boundary are generally unstable due to Rayleigh-Taylor and/or Kelvin-Helmholtz instabilities. However, differential rotation of the disc tends to suppress Rayleigh-Taylor instability and a sufficiently high disc sound speed (or temperature) is needed to overcome this suppression and to attain net mode growth. On the other hand, Kelvin-Helmholtz instability may be active at low disc sound speeds. We also find that the interface modes trapped at the boundary between a thin disc and an unmagnetized star do not suffer Rayleigh-Taylor or Kelvin-Helmholtz instability, but can become unstable due to wave leakage to large disc radii and, for sufficiently steep disc density distributions, due to wave absorption at the corotation resonance in the disc. The non-axisymmetric interface modes studied in this paper may be relevant to the high-frequency QPOs observed in some X-ray binaries and in cataclysmic variables.

14 pages, 9 figures, submitted to MNRAS

References in corpus (8)

• X-ray Properties of Black-Hole Binaries
• Accretion to Magnetized Stars through the Rayleigh-Taylor Instability: Global Three-Dimensional Simulations
• Unstable Disk Accretion to Magnetized Stars: First Global 3D MHD Simulations
• Advection of Magnetic Fields in Accretion Disks: Not So Difficult After All
• Corotational Instability of Inertial-Acoustic Modes in Black Hole Accretion Discs and Quasi-Periodic Oscillations
• Effects of Magnetic Fields on the Diskoseismic Modes of Accreting Black Holes
• Super-Reflection in Fluid Discs: Corotation Amplifier, Corotation Resonance, Rossby Waves, and Overstable Modes
• DNOs and QPOs in Cataclysmic Variables

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• Simulations of the Boundary Layer Between a White Dwarf and its Accretion Disk
• Angular Momentum Transport by Acoustic Modes Generated in the Boundary Layer II: MHD Simulations
• Trapping of Low-Mass Planets Outside the Truncated Inner Edges of Protoplanetary Discs
• Kelvin-Helmholtz Instability of the Magnetopause of Disc-Accreting Stars
• Dynamics of the Innermost Accretion Flows Around Compact Objects: Magnetosphere-Disc Interface, Global Oscillations and Instabilities
• Viscous Driving of Global Oscillations in Accretion Discs Around Black Holes
• Incompressible Modes Excited by Supersonic Shear in Boundary Layers: Acoustic CFS Instability
• High-Frequency QPOs and Overstable Oscillations of Black-Hole Accretion Disks
• Corotation resonance and overstable oscillations in black-hole accretion discs: general-relativistic calculations
• Simulations of Overstable Inertial-acoustic Modes in Black-Hole Accretion Discs
• Inertial-Acoustic Oscillations of Black-Hole Accretion Discs with Large-Scale Poloidal Magnetic Fields