Observational Diagnostics for Two-Fluid Turbulence in Molecular Clouds As Suggested by Simulations

TL;DR

This paper uses high-resolution two-fluid MHD turbulence simulations to explain observed differences in linewidth-size relationships between ions and neutrals in molecular clouds, providing new diagnostics for star formation models.

Contribution

It introduces novel density-based diagnostics derived from two-fluid simulations to interpret molecular cloud observations and reconcile different star formation theories.

Findings

Linewidth-size differences are prominent when the line of sight is orthogonal to the magnetic field.

Density PDFs differ significantly between ions and neutrals depending on the line of sight.

Diagnostics are practical for observational testing under certain assumptions.

Abstract

We present high resolution simulations of two-fluid (ion-neutral) MHD turbulence with resolutions as large as 512^3. The simulations are supersonic and mildly sub-Alfvenic, in keeping with the conditions present in molecular clouds. Such turbulence is thought to influence star formation processes in molecular clouds because typical cores form on length scales that are comparable to the dissipation scales of this turbulence in the ions. The simulations are motivated by the fact that recent studies of isophotologue lines in molecular clouds have found significant differences in the linewidth-size relationship for neutral and ion species. The goals of this paper are to explain those observations using simulations and analytic theory, present a new set of density-based diagnostics by drawing on similar diagnostics that have been obtained by studying single-fluid turbulence, and show that our two-fluid simulations play a vital role in reconciling alternative models of star formation. The velocity-dependent diagnostics display a complementarity with the density-dependent diagnostics. We find that the linewidth-size relationships show a prominent difference between ions and neutrals when the line of sight is orthogonal to the mean field. This is because the MHD waves in the ions differ from hydrodynamic waves in the neutrals. We also find that the density probability distribution functions (PDFs) show prominent differences between the ions and neutrals when the line of sight is parallel to the mean field. This is because the velocity fluctuations in the ions tend to produce column density fluctuations along field lines. When the magnetic field makes an angle to the line of sight, both observable differences should be visible. These diagnostics should be easy for observers to test. This analysis assumes optically thin lines and a mean magnetic field that is uniform in direction within a cloud.