Comparing the statistics of interstellar turbulence in simulations and observations: Solenoidal versus compressive turbulence forcing

TL;DR

This study compares solenoidal and compressive turbulence forcing in simulations and observations, revealing how forcing type influences density distributions and star formation predictions in interstellar turbulence.

Contribution

It provides a detailed quantitative analysis of how different turbulence forcing types affect density PDFs and turbulence properties, linking simulation results to observations.

Findings

Compressive forcing produces larger density fluctuations than solenoidal forcing.

Velocity dispersion--size relations are consistent across forcing types and with observations.

Density PDFs are strongly dependent on the forcing type, impacting star formation theories.

Abstract

We study two limiting cases of turbulence forcing in numerical experiments: solenoidal (divergence-free) forcing, and compressive (curl-free) forcing, and compare our results to observations reported in the literature. We solve the equations of hydrodynamics on grids with up to 1024^3 cells for purely solenoidal and purely compressive forcing. Eleven lower-resolution models with mixtures of both forcings are also analysed. We find velocity dispersion--size relations consistent with observations and independent numerical simulations, irrespective of the type of forcing. However, compressive forcing yields stronger turbulent compression at the same RMS Mach number than solenoidal forcing, resulting in a three times larger standard deviation of volumetric and column density probability distributions (PDFs). We conclude that the strong dependence of the density PDF on the type of forcing must be taken into account in any theory using the PDF to predict properties of star formation. We supply a quantitative description of this dependence. We find that different observed regions show evidence of different mixtures of compressive and solenoidal forcing, with more compressive forcing occurring primarily in swept-up shells.