Feb. 15, 2002

Lasers point way to astronomical phenomena

BOSTON, Mass. — Large lasers and Z-pinch generators in laboratories are recreating conditions relevant to astrophysical phenomena of the universe such as supernovae, black holes, molecular clouds like the Eagle Nebula, and the creation of new planets.

Laboratory physicist Bruce Remington will discuss this and other findings this weekend in a presentation entitled: "Laboratory Astrophysics Using High-Power Lasers," during the "Visualizing and Looking Beyond Earth" track of the annual meeting of the American Association for the
Advancement of Science in Boston.

"Modern laser and magnetic pinch facilities offer new opportunities for pursuing experimental science under extreme conditions of temperature and density," Remington said. "The potential of this new experimental capability is phenomenal when it comes to applications in astrophysics and planetary physics."

An understanding of supernovae, the cataclysmic death of a massive star, relies on an understanding of macroscopic matter under extreme conditions. Recreating aspects of the dynamics and energetics of supernova explosions has already been achieved on lasers and z-pinch facilities in laboratory experiments, thereby providing direct measurements under relevant and controlled conditions.

Remington said the next generation "mega lasers" — the National Ignition Facility at the Lab and the Laser Mega Joule in France — may be used to experimentally address questions of thermonuclear ignition and rare nuclear reaction measurements that are common occurrences in supernovae.

Supernova remnants (SNRs) are the vestiges of supernovae explosions. These remnants evolve for centuries, producing glowing filamentary structures in the galaxy and are widely believed to produce most of the cosmic rays that irradiate the Earth. Lab experiments can help improve the understanding of several of the mechanisms present in SNRs and can test aspects of the computational models developed to interpret their behavior. Experiments on high-energy density facilities can address uncertainties in remnant evolution such as the radiative hydrodynamics of SNRs. In the future, experiments may be able to address issues of collisionless shocks and particle acceleration (the first step in cosmic ray generation).

Galactic and extragalactic jets present some of the most visually captivating images encountered in modern astronomy. Using lasers and pulsed power facilities, Remington said scientists have already demonstrated how high Mach number hydrodynamic jets and radiatively collapsing jets evolve and interact.

Cold, dense molecular clouds illuminated by bright, young nearby massive starts serve as the stellar incubators of the universe. The radiation on the cloud creates high pressure at the surface by ablation and ram pressure from the stellar wind. Such systems are thought to be "cosmic nurseries" where active star formation takes place. Through lab experiments with large lasers and pulsed power facilities, scientists can recreate the dominant ablation-front hydrodynamics of radiatively driven molecular clouds, the best known example being the Eagle Nebula.

"With driven molecular clouds, the possibility to form an integrated program of theory, modeling, test-bed laboratory experiments, and astronomical observations is very real," Remington said. "Such an integrated program would have been undreamed of just a decade ago."

Scientists have moved one step closer to understanding the immediate vicinity of black holes, where matter is relentlessly swept down a singular gravitational abyss. Astronomers use high-resolution spectroscopy of the X-ray emissions from heated matter as it is tugged down the black hole to conditions close to the black hole. Laboratory scientists have demonstrated on Z-pinch facilities that scaled versions of these accretion disk conditions can be recreated for close-up scrutiny.

As for new planets, experimental techniques are being developed on pulsed power facilities, lasers, gas guns and diamond anvil cells to probe the properties of matter under the relevant extreme conditions of pressure and compression. These laboratory conditions reproduce those of the interiors of terrestrial and giant gas planets, brown dwarfs, low mass stars and the envelopes of white dwarfs Gamma-ray bursts — which are detected at a rate of more than one per day from random directions in the sky — have typical burst durations of a few seconds, but exhibit fluctuations as short as a millisecond. Laboratory experiments using ultra-intense, short-pulse lasers offer the most promising means for accessing the relativistic plasma dynamics and directed flow of gamma-ray bursts"Lab experiments and computer simulations are helping to identify and understand some of the dominant physics that are taking place in the vast universe that, until now, we’ve only been able to observe from afar from an astronomical viewpoint," Remington said.