Incompressible Modes Excited by Supersonic Shear in Boundary Layers: Acoustic CFS Instability

TL;DR

This paper introduces the acoustic CFS instability, a new mechanism where acoustic waves in accretion disks excite incompressible stellar modes, with potential implications for observed oscillations in compact star systems.

Contribution

The paper proposes the acoustic CFS instability, a novel process for exciting stellar modes via acoustic waves in accretion disks, differing from gravitational wave analogs and applicable even to non-rotating stars.

Findings

Identifies a new instability mechanism for stellar mode excitation.

Explains how acoustic waves carry energy and angular momentum opposite to stellar modes.

Suggests relevance to observed oscillations in astrophysical systems.

Abstract

We present an instability for exciting incompressible modes (e.g. gravity or Rossby modes) at the surface of a star accreting through a boundary layer. The instability excites a stellar mode by sourcing an acoustic wave in the disk at the boundary layer, which carries a flux of energy and angular momentum with the opposite sign as the energy and angular momentum density of the stellar mode. We call this instability the acoustic CFS instability, because of the direct analogy to the Chandrasekhar-Friedman-Schutz instability for exciting modes on a rotating star by emission of energy in the form of gravitational waves. However, the acoustic CFS instability differs from its gravitational wave counterpart in that the fluid medium in which the acoustic wave propagates (i.e.\ the accretion disk) typically rotates faster than the star in which the incompressible mode is sourced. For this reason, the instability can operate even for a non-rotating star in the presence of an accretion disk. We discuss applications of our results to high-frequency quasi-periodic oscillations in accreting black hole and neutron star systems and dwarf nova oscillations in cataclysmic variables.