Feb. 16, 2001

Lab closer to understanding nebula



Scientists may be closer to understanding the formation of the famous Eagle Nebula columns of galactic gases and dust, thanks to
the recent analysis of Lab physicists Jave Kane, Dmitri Ryutov and Bruce Remington.

"In computer simulations, our revised Rayleigh-Taylor model shows it has a very good chance of explaining the pillars of the Eagle Nebula," Kane said.

This nebula is known for its spectacular towering columns called the Pillars of Creation, which reach almost 6 trillion miles high. Formally known as M16, the Eagle Nebula is located in the constellation Serpens, about 5,700 light years from Earth, and became well known after the Hubble Space Telescope captured a dramatic image of the pillars in November 1995.

"Ours is a modified version of Rayleigh-Taylor," said Kane. "It explains detailed observations made in the last few years with the Hubble telescope and other instruments."

Astrophysicists have been seeking an alternative explanation for the celestial cloud columns since 1998, when Marc Pound of the University of Maryland, using the Berkeley-Illinois-Maryland Array (BIMA) in Hat Creek, Calif., measured the velocity and density of the gas in the pillars. Pound found that his measurements were inconsistent with the classic Rayleigh-Taylor explanation put forward by Lyman Spitzer in 1954.

The Rayleigh-Taylor instability occurs when a light material supports a dense one in a gravitational field — like light salad oil trying to support a layer of dense vinegar. The dense material will fall in "spikes" and the light material rises in "bubbles" until the fluids swap places.

Specifically in the Eagle Nebula, which is a gaseous cloud of largely hydrogen and helium, the building blocks for new stars, intense ultra-violet light from nearby stars heats and evaporates the surface layer of the cloud. Like rocket thrust, the hot evaporated gas accelerates the cold, dense cloud. By reaction, the dense cloud feels an effective gravitational tug directed toward the hot evaporated gas (similar to the feeling of being pushed back into the seat of an accelerating car.) Thus, the accelerating cloud is Rayleigh-Taylor-unstable.

The new theory considers possible variations in the stellar flux irradiating the clouds, and also the finite cloud thickness. Finally, unlike the classic Rayleigh-Taylor model, which assumes incompressible matter, the new model takes into account the compressibility of the heated gases.

With these modifications in place, the new simulations of the Livermore team do in fact reproduce Pound’s detailed observations of density and velocity.

Ryutov, a theoretical physicist with Magnetic Fusion Energy, reviewed Spitzer’s original 1954 paper and Pound’s recent results, and realized that a more modern view of the RT instability, including time dependence, might be the missing link. Ryutov suggested to Kane and Remington, ICF Hydrodynamics group leader, that perhaps Rayleigh-Taylor could apply, with revisions, to the recent observations, despite "the general opinion of the astrophysical community that it did not work." Ryutov is well known for developing models for simulation of astrophysical phenomena.

Kane, Remington and Ryutov, in collaboration with Pound, hope to take their revised theory beyond simulation, to actual experimentation with the Omega laser system at the University of Rochester. This will be the true test of the theory’s update, for as Remington noted, "simulations are good up to a certain point. Experiment is the ultimate test of everything."

However, said Kane, "We first need to find funding and laser shots," as is often the case with such large-scale experiments.