Laser Re-creates Stellar Jet

At the University of Rochester’s Laboratory for Laser Energetics in New York state, a team led by Rice University’s Patrick Hartigan used a powerful laser beam to re-create, on a small scale, the highly supersonic velocities at work in newborn stars, simulating the fiery jets that burst from their poles.

What the researchers got was confirmation that it is possible to re-create analogs of these stellar jets here on Earth and to use them to help understand how stars form.

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

Hartigan, a professor of physics and astronomy at Rice, wanted to know how stellar jets affect their surroundings and whether the series of experiments carried out in the lab would match computer simulations of the jet affecting an obstacle along its path. He also was 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 4-m 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,” he said, 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 obtained images of the jet driven into a ball of foam as the laser destroyed the target. It compared the images with those of a jet driven from the young star.

The team, which included Rice graduate student Robert Carver and a host of researchers and technicians, 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.)

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 so the researchers could see how the jet would be deflected. (Image: Jeff Fitlow)

Simulating a stellar jet requires a highly supersonic velocity, and launching it requires a lot of pressure. 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-size cone. On the far side was a minuscule 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 UK’s Atomic Weapons Establishment and to Hartigan’s observations of Herbig-Haro 110 and other stellar jets.

The laser experiment also gave Hartigan data that no single telescope ever could – and 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 left, 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 below.

“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 re-create 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. Besides collaborators at Los Alamos and the Atomic Weapons Establishment, the team also included researchers from the University of Rochester and, in California, Lawrence Livermore National Laboratory in Berkeley and General Atomics in San Diego.

For more information, visit: www.rice.edu.