Induced Core Formation Time in Subcritical Magnetic Clouds by Large-Scale Trans-Alfvénic Flows

Abstract

We clarify the mechanism of accelerated core formation by large-scale nonlinear flows in subcritical magnetic clouds by finding a semi-analytical formula for the core formation time and describing the physical processes that lead to them. Recent numerical simulations show that nonlinear flows induce rapid ambipolar diffusion that leads to localized supercritical regions that can collapse. Here, we employ non-ideal magnetohydrodynamic simulations including ambipolar diffusion for gravitationally stratified sheets threaded by vertical magnetic fields. One of the horizontal dimensions is eliminated, resulting in a simpler two-dimensional simulation that can clarify the basic process of accelerated core formation. A parameter study of simulations shows that the core formation time is inversely proportional to the square of the flow speed when the flow speed is greater than the Alfvén speed. We find a semi-analytical formula that explains this numerical result. The formula also predicts that the core formation time is about three times shorter than that with no turbulence, when the turbulent speed is comparable to the Alfvén speed.

A Magnetic Ribbon Model for Star-Forming Filaments

“…This process has been demonstrated in published simulations of trans-Alfvénic turbulence in a cloud with an initial subcritical mass-to-flux ratio (e.g., Nakamura & Li 2005;Kudoh & Basu 2011). We have extended the semianalytic model of Kudoh & Basu (2014) to estimate their lateral (perpendicular to magnetic field and ribbon long axis) width. This quantity is independent of the density of the ribbon.…”

“…We assume that the cloud is stratified in the z-direction, with compression happening primarily in the x-y plane. Here we simplify the analysis of the compression by limiting it to one direction, the x-axis (Figure 1), as done by Kudoh & Basu (2014). The initial magnetic field strength is B 0 and the field strength increases upon compression until the magnetic pressure within the compressed ribbon balances the external ram pressure and magnetic pressure.…”

“…Assuming that the gas has adequate time to settle into hydrostatic equilibrium along the z-direction, the half thickness of the cloud is Spitzer 1942). Now if the ambipolar diffusion time is longer than the compression time (Kudoh & Basu 2014), flux freezing is valid during compression, i.e.,…”

“…We argue that this is a more natural model for filaments and is based on the interplay between turbulence, strong magnetic fields, and gravitationally driven ambipolar diffusion, rather than pure gravity and thermal pressure. We extend the analytic model of Kudoh & Basu (2014) for the formation of magnetic ribbons that is based on numerical simulations. We derive a lateral width of a magnetic ribbon and use it to calculate a synthetic observed relation between apparent width in projection versus observed column density.…”

“…These regions oscillate about an approximate force-balanced state until ambipolar diffusion creates supercritical pockets that collapse to form stars. We explore the consequences of this scenario by extending a semi-analytic model of Kudoh & Basu (2014) that is based on numerical simulations.…”