Formation of collapsing cores in subcritical magnetic clouds: three-dimensional MHD simulations with ambipolar diffusion

TL;DR

This study uses 3D MHD simulations with ambipolar diffusion to explore how subcritical molecular clouds form collapsing cores, revealing that supersonic flows significantly accelerate core formation without heavily depending on initial magnetic field strength.

Contribution

It provides new insights into the core formation process in subcritical clouds, emphasizing the role of supersonic flows and quantifying formation timescales with detailed simulation data.

Findings

Core formation time is about a few million years with large-scale supersonic flows.

Supersonic flows accelerate core formation, with times inversely proportional to the square root of density enhancement.

Core structures are similar regardless of initial velocity fluctuation strength, with subsonic infall velocities.

Abstract

We employ the three-dimensional magnetohydrodynamic simulation including ambipolar diffusion to study the gravitationally-driven fragmentation of subcritical molecular clouds, in which the gravitational fragmentation is stabilized as long as magnetic flux-freezing applies. The simulations show that the cores in an initially subcritical cloud generally develop gradually over an ambipolar diffusion time, which is about a few $\times 10^7$ years in a typical molecular cloud. On the other hand, the formation of collapsing cores in subcritical clouds is accelerated by supersonic nonlinear flows. Our parameter study demonstrates that core formation occurs faster as the strength of the initial flow speed in the cloud increases. We found that the core formation time is roughly proportional to the inverse of the square root of the enhanced density created by the supersonic nonlinear flows. The density dependence is similar to that derived in quasistatically contracting magnetically supported clouds, although the core formation conditions are created by the nonlinear flows in our simulations. We have also found that the accelerated formation time is not strongly dependent on the initial strength of the magnetic field if the cloud is highly subcritical. Our simulation shows that the core formation time in our model subcritical clouds is several $\times 10^6$ years, due to the presence of large-scale supersonic flows ($\sim 3$ times sound speed). Once a collapsing core forms, the density, velocity, and magnetic field structure of the core does not strongly depend on the initial strength of the velocity fluctuation. The infall velocities of the cores are subsonic and the magnetic field lines show weak hourglass shapes.