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

Institute of Physics, Laboratory of Astrophysics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland

^★ Corresponding author: aris.tritsis@epfl.ch

Received: 7 March 2025
Accepted: 10 July 2025

Abstract

Context. 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 to our theoretical understanding of molecular clouds and cores.

Aims. Our objective is to study both the spatial features and the time evolution of the neutral-ion drift velocity and the mass-to-flux ratio in a 3D chemo-dynamical simulation of a supercritical turbulent collapsing molecular cloud.

Methods. Using our modified version of the FLASH astrophysical code, we performed a 3D nonideal MHD simulation of a turbulent collapsing molecular cloud. The resistivities of the cloud were computed self-consistently from a vast nonequilibrium 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 to measure the true mass-to-flux ratio in 3D simulations.

Results. Despite the cloud’s turbulent nature, at early times, the neutral-ion drift velocity follows the expected structure from 2D axisymmetric nonideal 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 a poor correlation with both the true mass-to-flux ratio and the density structure of the cloud.