Ambipolar diffusion and the mass-to-flux ratio in a turbulent collapsing cloud

TL;DR

This study uses advanced 3D nonideal MHD chemo-dynamical simulations to analyze the evolution of the mass-to-flux ratio and neutral-ion drift velocity in turbulent collapsing molecular clouds, revealing complex behaviors and limitations of observational estimates.

Contribution

It introduces a new method for measuring the true mass-to-flux ratio in 3D simulations and provides detailed insights into the spatial and temporal evolution of drift velocities in turbulent clouds.

Findings

Neutral-ion drift velocity follows expected patterns early on

Drift velocity becomes complex with outward vectors at later times

True mass-to-flux ratio increases over time, decreases with radius

Abstract

The formation of stars is governed by the intricate interplay of nonideal magnetohydrodynamic (MHD) effects, gravity, and turbulence. Computational challenges have hindered a comprehensive 3D exploration of this interplay, posing a longstanding challenge in our understanding of clouds and cores. Our objective was to study the spatial features and time evolution of the neutral-ion drift velocity and the mass-to-flux ratio in a 3D nonideal MHD chemo-dynamical simulation of a supercritical turbulent collapsing molecular cloud. The resistivities of the cloud were computed self-consistently from a vast non-equilibrium chemical network containing 115 species. To compute the resistivities we used different mean collisional rates for each charged species in our network. We additionally developed a new generalized method for measuring the true mass-to-flux ratio in 3D simulations. Despite the cloud's turbulent nature, at early times, the neutral-ion drift velocity follows the expected structure from 2D axisymmetric non-ideal MHD simulations with an hourglass magnetic field. At later times, however, the neutral-ion drift velocity becomes increasingly complex, with many vectors pointing outward from the cloud's center. Specifically, we find that the drift velocity above and below the cloud's ``midplane'' is in ``antiphase''. We explain these features on the basis of magnetic helical loops and the correlation of the drift velocity with the magnetic tension force per unit volume. Despite the complex structure of the neutral-ion drift velocity, we demonstrate that, when averaged over a region, the true mass-to-flux ratio monotonically increases as a function of time and decreases as a function of the radius from the center of the cloud. In contrast, the ``observed'' mass-to-flux ratio shows poor correlation with the true mass-to-flux ratio and the density structure of the cloud.