The driving mode of shock-driven turbulence

TL;DR

This study uses hydrodynamical simulations to analyze how shock waves in the interstellar medium drive turbulence, revealing that shocks predominantly induce strongly compressive turbulence, which significantly impacts star formation processes.

Contribution

The paper demonstrates through simulations that shock-driven turbulence in a multi-phase medium is predominantly compressive, providing new insights into turbulence driving modes in the interstellar medium.

Findings

Shock-driven turbulence is strongly compressive with b ~ 1.

Simulations show shock driving results in high density dispersion.

Implications for star formation rates and density structures.

Abstract

Turbulence in the interstellar medium (ISM) is crucial in the process of star formation. Shocks produced by supernova explosions, jets, radiation from massive stars, or galactic spiral-arm dynamics are amongst the most common drivers of turbulence in the ISM. However, it is not fully understood how shocks drive turbulence, in particular whether shock driving is a more solenoidal(rotational, divergence-free) or a more compressive (potential, curl-free) mode of driving turbulence. The mode of turbulence driving has profound consequences for star formation, with compressive driving producing three times larger density dispersion, and an order of magnitude higher star formation rate than solenoidal driving. Here, we use hydrodynamical simulations of a shock inducing turbulent motions in a structured, multi-phase medium. This is done in the context of a laser-induced shock, propagating into a foam material, in preparation for an experiment to be performed at the National Ignition Facility (NIF). Specifically, we analyse the density and velocity distributions in the shocked turbulent medium, and measure the turbulence driving parameter $b=(\sigma^{2 \Gamma}_{\rho /\langle \rho \rangle}-1)^{1/2} (1-\sigma_{\rho \langle \rho \rangle}^{-2})^{-1/2}\mathcal{M}^{-1}\Gamma^{-1/2}$ with the density dispersion $\sigma_{\rho / \langle \rho \rangle}$, the turbulent Mach number $\mathcal{M}$, and the polytropic exponent $\Gamma$. Purely solenoidal and purely compressive driving correspond to $b \sim 1/3$ and $b \sim 1$, respectively. Using simulations in which a shock is driven into a multi-phase medium with structures of different sizes and $\Gamma < 1$, we find $b \sim 1$ for all cases, showing that shock-driven turbulence is consistent with strongly compressive driving.