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Special Case Stellar Blast Teaching Astronomers New Lessons About Cosmic Explosions

Expansion of RS Ophiuchi captured by NRAO. Image credit: NRAO (Rupen, Mioduszewski & Sokoloski)/ AUI/NSF
by Staff Writers
Socorro NM (SPX) Jul 21, 2006
A powerful thermonuclear explosion on a dense white-dwarf star last February has given astronomers their best look yet at the early stages of such explosions, called novae, and also is giving them tantalizing new clues about the workings of bigger explosions, called supernovae, that are used to measure the Universe.

Using the National Science Foundation's Very Long Baseline Array and other telescopes, "We have seen structure in the blast earlier than in any other stellar explosion," said Tim O'Brien of the University of Manchester's Jodrell Bank Observatory in the United Kingdom.

"We see evidence that the explosion may be ejecting material in jets, contrary to theoretical models that assumed a spherical shell of ejected material," O'Brien added.

The explosion occurred in a star system called RS Ophiuchi, in the constellation Ophiuchus. RS Ophiuchi consists of a dense white dwarf with a red giant companion whose prolific stellar wind dumps material onto the surface of the white dwarf. When enough of this material has accumulated, theorists say, a gigantic thermonuclear explosion, similar to a hydrogen bomb but much larger, occurs.

Systems such as RS Ophiuchi eventually could produce a vastly more powerful explosion � a supernova - when the white dwarf accumulates enough mass to cause it to collapse and explode violently.

Because such supernova explosions - called Type 1a supernovae by astronomers - are triggered as the white dwarf reaches the same mass, they are thought to be identical in their intrinsic brightness. This makes them extremely valuable as "standard candles" for measuring distances in the Universe.

"We think the white dwarf in RS Ophiuchi is about as massive as a white dwarf can get, and so is close to the point when it will become a supernova," said Jennifer Sokoloski, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

"If astronomers use such supernovae to measure the Universe, it's important to fully understand how these systems evolve prior to the explosion," she added.

RS Ophiuchi is a recurrent nova that experienced such blasts in 1898, 1933, 1958, 1967, and 1985 prior to this year's event. Sokoloski also noted that RS Ophiuchi is "a very special type of system," in which the nova explosions occur inside a gaseous nebula created by the stellar wind coming from the red giant companion to the white dwarf.

"This means that we can track the outward-moving blast wave from the explosion by observing X-rays produced as the blast plows through this nebula," said Sokoloski, who led a team using the Rossi X-Ray Timing Explorer satellite to do so.

"One natural way to produce what we observe is with an explosion that was not spherical," she added.

Another surprise came when the radio waves coming from RS Ophiuchi indicated that a strong magnetic field is present in the material ejected by the explosion.

"This is the first case we've seen that showed signs of such a magnetic field in a recurrent nova," said Michael Rupen who, with Amy Mioduszewski, both of the National Radio Astronomy Observatory, and Sokoloski, conducted another study of the system using the VLBA.

Rupen pointed out the importance of observing the object with both X-ray and radio telescopes. "What we could infer from the X-ray data, we could image with the radio telescopes," he said.

All of the researchers agree their studies show the explosion is more complex than scientists previously thought such blasts to be.

"It's a jet-like explosion, probably shaped by the geometry of the binary-star system at the center," said O'Brien.

Rupen added that RS Ophiuchi showed the "earliest detection ever of such a jet. In fact, we could actually tell -- within a couple of days -- when the jet turned on."

The new information is valuable for understanding not only nova explosions, but also other stellar blasts, the scientists think.

"The physics is analogous to the physics of supernova explosions, so what we're learning from this object can be applied to supernovae and possibly to stellar explosions in general," Sokoloski said.

In addition, she said, "in the early days of this explosion, we saw changes in the blast wave that it would take hundreds of years to see in a supernova explosion."

The teams led by O'Brien and Sokoloski reported their findings in the July 20 edition of the scientific journal Nature.

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NASA Balloon Flight Successful At Esrange Space Center
Esrange, Sweden (SPX) Jul 17, 2006
NASA's high-altitude balloon mission carrying the scientific instrument TRACER has been successfully completed. The balloon was launched July 8 from the Swedish Space Corp. launch facility at Esrange Space Center. It drifted over the Atlantic Ocean toward North America, where it landed over the weekend.

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