The answer may lie in the molecular cloud's turbulence, which helps to provide pressure in the absence of heat, but is also responsible for the gravitational collapse of parts of a molecular cloud. This turbulence is supersonic, often measuring at Mach 10, because the speed of sound in the dense interstellar medium is extremely low. "As a result, the cloud is uneven throughout, with higher concentration of gas in some regions, and, although the turbulence might prevent the global collapse of the cloud, it promotes collapse locally," explains Heitsch.

In addition to providing the pressure necessary to maintain the cloud's stability, turbulence can, however, also increase the ambipolar diffusion rate by mixing the field to small scales. Heitsch simulated a two-dimensional section of a molecular cloud to find out whether turbulence would indeed have such an effect on diffusion and discovered that this was indeed the case. "We can [now] diffuse the flux-to-mass ratio at a rate a factor of 10 or 100 higher than the standard diffusion rate, Heitsch says. "This would explain the weak correlation between magnetic field strength and density in the interstellar medium."

Heitsch has been using an extension of a gas-kinetic flux-splitting method developed by Xu (1999) and Tang and Xu (2000), which is especially useful for investigating MHD problems including diffusive processes. A bit daunted by the prospect of porting his code to a new machine when the Origin2000 neared retirement, he was in for a pleasant surprise when he did his first test run. "I was so impressed that I could take the code and compile it on the IBM p690 and it ran--that has never happened to me. That's the first time a code ran quickly for me, which was very nice."

Heitsch is attempting to make his simulations progressively more and more realistic. "The models so far are restricted because they're only two-dimensional--the magnetic field is perpendicular to the flow plane, which is the easiest, most tractable case." The next step, he says, is to create a three-dimensional model. "I'm looking forward to the time when that will happen, because that will be computationally very expensive--and very exciting."

This research is funded by the National Science Foundation.

Team members
Fabian Heitsch
Adrian Slyz
Ellen Zweibel

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Fabian Heitsch's simulation of a model interstellar medium (ISM). Like dye released into a river, the reddish-orange tracer fluid reveals the complex flow patterns created by ISM turbulence. Heitsch is studying whether these motions can increase the effectiveness of processes like ambipolar diffusion.