Time-dependent Turbulence in Stars

TL;DR

This paper investigates the time-dependent, bursty turbulence in stellar convection zones through 3D simulations, simplified models, and theoretical analysis, revealing a global instability absent in traditional mixing-length theory.

Contribution

It introduces a new model extending the Lorenz system to explain turbulent fluctuations in stellar convection, highlighting a global instability overlooked by standard methods.

Findings

3D simulations show recurrent turbulent fluctuations due to a global instability.

An extended Lorenz model reproduces bursty behavior similar to simulations.

A multi-cell model suggests luminosity variations akin to irregular variable stars.

Abstract

Three-dimensional (3D) hydrodynamic simulations of shell oxygen burning (Meakin and Arnett 2007) exhibit bursty, recurrent fluctuations in turbulent kinetic energy. These are shown to be due to a global instability in the convective region, which has been suppressed in calculations of stellar evolution which use mixing-length theory (MLT). Quantitatively similar behavior occurs in the model of a convective roll (cell) of Lorenz (1963), which is known to have a strange attractor that gives rise to random fluctuations in time.An extension of the Lorenz model, which includes Kolmogorov damping and nuclear burning, is shown to exhibit bursty, recurrent fluctuations like those seen in the 3D simulations. A simple model of a convective layer (composed of multiple Lorenz cells) gives luminosity fluctuations which are suggestive of irregular variables (red giants and supergiants, Schwarzschild 1975). Apparent inconsistencies between Arnett, Meakin, and Young (2009) and Nordlund, Stein, and Asplund (2009) on the nature of convective driving have been resolved, and are discussed.