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Waves Said To Churn Across a Sea of Nuclear Fuel on Star

March 22, 2000

Greenbelt, Md. -- Researchers at the University of Chicago have seen the surface of an exploding neutron star, and it ain't pretty. Waves of gaseous metals, the billion-degree nuclear ash of helium fusion, churn across a sea of nuclear fuel at supersonic speeds, while sheets of super-heated material that dwarf Vesuvius' fury spew 15 miles high.

This vision of a nuclear detonation igniting a star from pole to pole in a fraction of a second is part of a visual simulation that illustrates the details of nuclear burning on neutron stars. Drawn from observations made by the Rossi X-ray Timing Explorer (RXTE) satellite, the University of Chicago visual simulation is a result of weeks of number-crunching on supercomputers from the Department of Energy.

Michael Zingale, a doctoral candidate in Astronomy & Astrophysics at the University of Chicago, will present video demonstrations and an explanation of the findings at 10:30 a.m. EST Thurday, March 23 at the Rossi 2000 meeting at NASA Goddard Space Flight Center in Greenbelt, Md. Zingale's colleagues include Frank Timmes, Bruce Fryxell, Don Lamb and members of University of Chicago's Accelerated Strategic Computing Initiative Flash Center.

"This simulation is the first step in creating a model of what really is happening during a burst on the surface of a neutron star, for telescopes are not powerful enough to image such a tiny region," said Zingale. "Although there may be other ways to explain an outburst, this detonation model gives the correct timescale for the event-an explosion racing from pole to pole in three milliseconds. Future simulations will hopefully allow us to connect more closely with the Rossi results."

A neutron star is the skeletal remains of a star once several times more massive than the sun that exhausted its nuclear fuel and subsequently exploded its outer shell. The remaining core, still possessing about a sun's worth of mass, collapses to a sphere no larger than Chicago, about 10 miles in diameter.

Yet the star is far from retirement. Its million-degree surface is visible as faint X-ray light, and the occasional nuclear bursts from the surface are seen as even brighter X-rays. Now, the University of Chicago computer simulations show nuclear explosions detonated in material from a nearby star that has crashed onto the surface of a neutron star.

The transfer of matter from one 'living' star to a neutron star is called accretion. The neutron star, being so dense, exerts an extreme gravitational pull on its surroundings. Gas from a nearby star can literally be torn away by the neutron star if it ventures too close. Under the influence of gravity, this gas heats to temperatures even hotter than when it was part of a star and accelerates to one-third the speed of light. The gas often glows as X-ray light at this point.

The accretion process is well known. What Zingale and his colleagues have simulated are the underlying mechanics of the nuclear explosions on the neutron star surface.

What has stymied theorists for so long is how nuclear burning appears almost simultaneously at both poles of a neutron star. The simulation shows that a supersonic detonation, not a slow-moving flame front, ignites everything in its path.

An example of a slow-moving flame front is a trail of gasoline lit at one end. The flame spreads down the trail more slowly than the speed of sound. A detonation is what happens when a flame encounters a closed barrel of gasoline. Pressure builds in the barrel until the barrel ruptures. The burning front now travels faster than the speed of sound, and all the fuel explodes seemingly simultaneously.

Back on a neutron star, accretion dumps a steady flow of matter at one-third light speed onto the surface, and gravity quickly spreads it evenly across the sphere. The pressure at the base of this accreted material builds until it is high enough to start a nuclear reaction.

The nuclear fusion creates a chain reaction, detonating across a surface of evenly spread accreted gas. Within milliseconds, the polar regions erupt in X-ray light. The detonation travels through the pool of fuel. The computer simulations show surface waves on the hot ash that move much like ocean waves.

"When the surface wave moves ahead of the detonation front, it breaks just like a wave at the beach," said Timmes. "The 'beach' here is the unburned portion of the neutron star. This feature is displayed quite visibly, and beautifully, in the movies."

Adding to the fury, the neutron star surface that is revealed, a section called the photosphere, is thrown violently upward about 15 miles before the neutron star's extreme gravity yanks this material back down to the surface.

Zingale and his colleagues have created a number of simulations, each one demonstrating a particular physical property, such as the temperature, pressure or density of the explosion. QuickTime versions of the simulations are available at http://flash.uchicago.edu.

Rossi 2000 is the first meeting to bring together the diverse pool of observational astronomers and theorists utilizing RXTE, which was launched by NASA in December, 1995. Over 150 researchers will attend the meeting, which is scheduled to be held March 22-24, 2000.

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