Supersonic Ambipolar Diffusion

A cloud of pure molecular gas would form stars extremely fast. However, the magnetic field holds up a collapse. The collapse can then only proceed if the magnetic field can be separated from the gas.....this occurs through a processs called ambipolar diffusion.

Ambipolar diffusion: the magnetic field is not tied to the molecules. Instead it is frozen into the electrically-charged gas - the ions and electrons. Hence, if the ions drift or stream through the neutral molecules, then the field can be released. In dense star-forming clouds this actually occurs and, we suspect, controls the rate of star formation.

The problem: it takes millions of years for ambipolar diffusion to work. So how can we learn about the physics and dynamics?

The answer: SAD - Supersonic Ambipolar Diffusion. We study the process where it has its greatest impact. These are called C-shocks......shocks waves in which ambipolar diffusion determines what we see.

Shock waves are produced where supersonic flow is brought to a halt. This is a violent process, which in C-shocks is observed as infrared and submillimeter radiation from excited molecules. We observe high-speed shocks in jets and outflows. What does ambipolar diffusion predict for these shocks?

Here are some answers (from Smith and Mac Low 1997) .

1. Shock waves in molecular clouds should evolve into continuous or C-type structures due to ambipolar diffusion. We determine whether and how this is achieved through plane-parallel numerical simulations using an extended version of ZEUS. We first describe and test the adapted code against analytical results, laying the necessary foundations for subsequent works on supersonic ambipolar

2. The evolution away from jump shocks toward the numerous steady C-shock sub-types is investigated. The evolution passes through four stages, which possess distinctive observational properties. The time scales and length scales cover broad ranges. Specific results are included for shock types including switch, absorber, neutralised, oblique, transverse and intermediate. Only intermediate Type II shocks and `slow shocks', including switch-off shocks, remain as J-type under the low ion levels assumed. Other shocks transform via a steadily growing neutral precursor to a diminishing jump. For neutralised shocks, this takes the form of an extended long-lived ramp.

3. Molecular hydrogen emission signatures are presented. After the jump speed has dropped to under 25 km/s, a non-dissociative jump section can dominate the spectra for a long period. This produces a high-excitation
spectrum. Once the jump has further weakened, to less than 8 km/s, the fully developed ion front is responsible for brisk progress towards a constant C-type excitation. The time scale for emission-line variations is about 6 n(ion) yr, where n(ion) is the pre-shock ion number density.

And some further answers (from Mac Low and Smith 1997).........

We compute the nonlinear development of the instabilities in C-shocks first described by Wardle, using a version of the ZEUS code modified to include a semi-implicit treatment of ambipolar diffusion. We find that, in three dimensions, thin sheets parallel to the shock velocity and perpendicular to the magnetic field lines form. High resolution, two-dimensional models show that the sheets are confined by the Brandenburg Zweibel ambipolar diffusion singularity, forcing them to numerically unresolvable thinness. Hot and cold regions form around these filaments, disrupting the uniform temperature structure characteristic of a steady-state C-shock. This filamentary region steadily grows as the shock progresses. We compare steady-state to unstable C-shocks, showing excitation diagrams, line ratios, and line profiles for molecular hydrogen lines visible in the K-band, with the Infrared Space Observatory, and with NICMOS on the Hubble Space Telescope.

This is an on-going project. More results are being prepared.

Last Revised: 2009 November 6th