EPoS Contribution
EPoS Contribution
Ambipolar Diffusion and Prestellar Core Formation

Che-Yu Chen
Department of Astronomy, University of Maryland, College Park, MD, USA
We studied the dependence of turbulence-enhanced ambipolar diffusion on environment through a numerical parameter study, and obtained the shock thickness mediated by ambipolar diffusion as a function of density, inflow velocity, magnetic field, and ionization fraction in the background cloud. Our formula also agrees with an analytic estimate based on ion-neutral momentum exchange. Using time-dependent numerical simulations, we discovered and characterized a transient stage of ambipolar diffusion during compression of magnetized gas by supersonic turbulence, before the ion-neutral drift reaches equilibrium and 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, creating post-shock regions with relatively high mass-to-flux ratio which may represent the birth sites of prestellar cores. We also examined the transient behavior of ambipolar diffusion in shocked layers using three-dimensional MHD simulations with self-gravity and a perturbed velocity field in the convergent flow. Based on our results to date, we found that with ambipolar diffusion, varying the ionization fraction leads to behavior between pure hydrodynamics and ideal MHD. We are working on a more complete parameter study to investigate how the physical factors can affect the forming process of gravitationally-bounded cores during the transient stage of ambipolar diffusion, which will provide insight into the environment where circumstellar disks form.
E. Ostriker, U. of Maryland, USA
Key publication