Star power
Rice astronomer and team recreate stellar jet with laser blast

BY MIKE WILLIAMS
Rice News staff

“… Three … two … one … and suddenly it disappeared.”

With the trillions of watts contained in one brief pop of a powerful laser, the universe became a bit less mysterious.

	Patrick Hartigan set out to discover what causes the deflections seen in stellar jets like Herbig-Haro 110, seen here as photographed by Hartigan at the Kitt Peak National Observatory.

Rice University Professor Patrick Hartigan and a team of laser scientists, physicists, astronomers and technicians used the beams at the University of Rochester’s Laboratory for Laser Energetics to recreate, on a small scale, the highly supersonic velocities at work in newborn stars and simulated the fiery jets that burst from their poles.

What they got was confirmation that it’s possible to recreate analogs of these stellar jets here on Earth and the ability to use these to help understand how stars form.

Hartigan, a professor of physics and astronomy, wanted to know how stellar jets affect their surroundings and to see if the series of experiments would match computer simulations as the jet impacts an obstacle along its path. He was also eager to compare the laboratory images to his infrared photos of Herbig-Haro 110, a supersonic jet of material driven from an active young star he observed last year using the four-meter telescope at Kitt Peak National Observatory in Arizona.

If Hartigan went to Rochester anticipating a bit of “Star Wars”-style dazzle, he was in for a disappointment. “You expect the lights to dim or something,” said the enthusiastic professor, recalling one of a number of trips to the big laser. “But what happens is you watch the image of the target on a TV screen, they count down to zero and suddenly the target disappears. It happens too fast to see the laser beams vaporize the target.”

But by using the equivalent of flash photography timed to a precision of a few billionths of a second, Hartigan’s team was able to obtain images of the jet driven into a ball of foam as the laser destroyed the target and compare these with images of a jet driven from the young star. The team, which was led by Hartigan and included Rice graduate student Robert Carver and a host of researchers and technicians from the other institutions, reported its results last month in a paper published in The Astrophysical Journal. (Hartigan has a second paper in the journal this month on the forces that launch stellar jets.)

JEFF FITLOW
Rice astronomer Patrick Hartigan displays a souvenir, one of the targets from his series of experiments to simulate stellar jets. Powerful lasers blasted a tiny plug of titanium inside the gold-coated cone, shooting the atomized material into a ball of foam-covered plastic on the other side to see how the jet would be deflected.

Simulating a stellar jet requires a highly supersonic velocity, and that requires a lot of pressure to launch. One of the few ways to create a scale model of such a jet is to heat something very fast, atomize it and direct the plasma that results. The laser at Rochester filled that bill.

For Hartigan’s experiment, the Omega laser, one of the most powerful in the world, was focused into a dozen fine beams at a target containing a plug of titanium sitting in the center of a gold-covered, half-dollar-sized cone. On the far side was a miniscule ball of foam-covered plastic representing a cloud of interstellar material.

Dozens of firings over several years gave Hartigan’s team a stunning series of images of the shock waves, with the atomized titanium blasting into the foam and deflecting from the plastic ball, creating swirling clouds that look remarkably like the streamers of shocked gas strewn about space by collimated stellar winds. They also bear close similarities to 3-D computer models of deflected jets developed by researchers at Los Alamos National Laboratory and the United Kingdom’s Atomic Weapons Establishment and to Hartigan’s observations of Herbig-Haro 110 and other stellar jets.

The laser experiment also gave Hartigan data no single telescope ever could – a three-dimensional look at what happens when a stellar jet slams into something.

A jet traveling hundreds of miles per second should move in a straight line forever. “But the astronomical images we took at Kitt Peak showed something different,” said Hartigan, one of the first to use the observatory’s Extremely Wide-Field Infrared Imager, which went online in early 2007. His images show material being dragged out from a dense obstacle along the path of the jet as well as a series of shock waves that, with the help of the laser experiment, the team determined arose from pulses of high velocity material ejected by the young star.

	The images at top, taken in a few billionths of a second, detail experiments at the Laboratory for Laser Energetics meant to simulate stellar jets and their effects on interstellar materials, as seen in the image above.

“It was apparent that the jet was impacting a dense cloud and deflecting from it, and we realized we could construct an experiment with the laser that would do the same thing,” he said. “Now something that takes hundreds of years to unfold in space we can recreate in less than a millionth of a second on Earth. With repeated experiments we can study how jets behave at different times, with various collision distances from the obstacle, viewed from a variety of angles, and follow how the jet mixes with material in the obstacle.

“This phenomenon is a primary way that young stars affect their surroundings, which in turn determines whether or not other stars may form in the same region.”

That the computer and laser simulations match up so well reinforces their value to astrophysicists like Hartigan, who strive to understand the dynamics of these complex flows.

“So now, that’s in the back of my head,” Hartigan said. “Whenever I have an image of an object, like a nebula, I can think about using this technique to analyze it.”

The Department of Energy, National Science Foundation and NASA funded the research. In addition to collaborators at Los Alamos and the Atomic Weapons Establishment, the team also included researchers from the University of Rochester, Lawrence Livermore National Laboratory and General Atomics in San Diego.