On the possible turbulence mechanism in accretion disks in non-magnetic binary stars

TL;DR

This paper investigates how precessional density waves caused by tidal interactions in binary star accretion disks can induce instabilities, leading to turbulence consistent with observations, despite the disks' expected stability.

Contribution

It demonstrates that precessional density waves in non-magnetic accretion disks can generate instabilities that lead to turbulence, providing a potential explanation for observed disk turbulence.

Findings

Precessional waves induce radial velocity gradients causing instabilities.

Instabilities develop in the inner disk and propagate outward.

Resulting turbulence parameters match observational data.

Abstract

The arising of turbulence in gas-dynamic (non-magnetic) accretion disks is a major issue of modern astrophysics. Such accretion disks should be stable against the turbulence generation, in contradiction to observations. Searching for possible instabilities leading to the turbulization of gas-dynamic disks is one of the challenging astrophysical problems. In 2004, we showed that in accretion disks in binary stars the so-called precessional density wave forms and induces additional density and velocity gradients in the disk. Linear analysis of the fluid instability of an accretion disk in a binary system revealed that the presence of the precessional wave in the disk due to tidal interaction with the binary companion gives rise to instability of radial modes with the characteristic growth time of tenths and hundredths of the binary orbital period. The radial velocity gradient in the precessional wave is shown to be responsible for the instability. A perturbation becomes unstable if the velocity variation the perturbation wavelength scale is about or higher than the sound speed. Unstable perturbations arise in the inner part of the disk and, by propagating towards its outer edge, lead to the disk turbulence with parameters corresponding to observations (the Shakura-Sunyaev parameter $\alpha \lesssim 0.01$).