Ambipolar Diffusion in Action: Transient C shock Structure and Prestellar Core Formation

TL;DR

This paper investigates the structure and evolution of C shocks in molecular clouds, revealing a transient phase that can lead to rapid formation of supercritical prestellar cores due to magnetic flux loss.

Contribution

It provides a detailed analysis of steady and time-dependent C shocks, including a new understanding of the transient stage and its role in core formation in molecular clouds.

Findings

Transient C shock stage lasts 0.1-1 Myr.

Transient enhances mass-to-flux ratio significantly.

Supercritical cores can form during shock transient.

Abstract

We analyze the properties of steady and time-dependent C shocks under conditions prevailing in giant molecular clouds. For steady C shocks, we show that ionization equilibrium holds and use numerical integrations to obtain the shock thickness mediated by ambipolar diffusion as a function of density, inflow velocity, magnetic field, and ionization fraction in the cloud. Our formula also agrees with an analytic estimate based on ion-neutral momentum exchange. Using time-dependent numerical simulations, we show that C shocks have a transient stage when the neutrals are compressed much more strongly than the magnetic field. The transient stage has a duration set by the neutral-ion collision time, t_AD ~ L_shock/v_drift ~ 0.1 - 1 Myr. This transient creates a strong enhancement in the mass-to-magnetic flux ratio. Under favorable conditions, supercritical prestellar cores may form and collapse promptly as a result of magnetic flux loss during the transient stage of C shocks.