Stellar explosion revealed in unique detail

David Shiga for NewScientist.com news service

July 19, 2006

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The material collected from the red giant leads to a nuclear explosion on the surface of its companion, a white dwarf star (Artist's impression: David A Hardy/PPARC)

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An unprecedented glimpse of the blast wave from an erupting star has been seen by astronomers.

The new view suggests the binary system observed could be responsible for some of the universe's most powerful explosions, called Type Ia supernovae. These are very important to astronomers as they are used as "standard candles" to measure distances, but their source has been a major mystery in astronomy.

The explosion occurred in a binary star system called RS Ophiuchi. It consists of a red giant star orbited by the dense core of a burned-out star, called a white dwarf. The outbursts occur because the white dwarf slowly collects gas shed by the red giant. When enough gas piles up on the white dwarf, the mounting pressure triggers a tremendous nuclear explosion.

RS Ophiuchi explodes this way every few decades, but not with a regular schedule. Before the latest outburst, it had not exploded since 1985. Astronomers were therefore excited to discover a new explosion in progress on 12 February 2006. They were able to track the blast wave's progress sooner after its onset and in more detail than ever before.

"We really saw much, much more this time," says Jennifer Sokoloski of the Harvard-Smithsonian Center for Astrophysics, in Cambridge, US. Sokoloski led a team that observed the event with the Rossi X-ray Timing Explorer (RXTE), starting the day after the initial detection of the outburst.

Flickering candles

The researchers found evidence that the system was on its way to producing a Type Ia supernova. Although these are used as standard candles, there are in fact slight differences in their brightness. This adds uncertainty to distance measurements.

Part of the problem is that astronomers do not know for sure what causes the supernovae. Evidence strongly suggests that they occur when a white dwarf collects too much mass, triggering a nuclear explosion that completely destroys the white dwarf.

Although astronomers have seen many systems where a white dwarf is collecting matter, none seemed to have the right conditions to lead to a Type Ia supernova. For example, some white dwarfs are collecting matter at too low a rate to get to the critical mass in the universe's lifetime.

The properties of the shock wave observed around RS Ophiuchi allowed Sokoloski's team to calculate the mass of the white dwarf that produced it. They determined it is very close to the critical mass that would trigger a supernova.

That led them to suggest that systems like RS Ophiuchi, called recurrent novae, account for at least some of the Type Ia supernovae. If true, this would help solve the mystery of their origin and could help refine the distance scale they underpin. "It would be very nice to explain why there is this slight variation in supernova brightness," Sokoloski told New Scientist.

Lack of hydrogen

But there is a problem with this idea, argues Sumner Starrfield of Arizona State University in Tempe, US, who is also studying RS Ophiuchi's recent outburst. Type Ia supernovae are distinguished by a lack of hydrogen in their blast waves, he says, and the red giant in the RS Ophiuchi system has shed a lot of hydrogen into the surrounding area. "I think it will explode as a supernova but it's not going to be a Type Ia," Sumner told New Scientist.

Sokoloski argues that the white dwarf's recurrent outbursts have probably removed the hydrogen from the immediate vicinity, so that it would not appear in a future Type Ia blast wave.

A second study released on Wednesday shows that the material from the explosion seen in February was probably spewed out in jets rather than equally in all directions. Tim O'Brien of the Jodrell Bank Observatory in Macclesfield, UK, led the study. It was based on radio data from the UK's Multi-Element Radio Linked Interferometer Network (MERLIN) and the European VLBI Network (EVN).

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

"This suggests that there is much more going on than we believed," says Sumner. He added that it will probably take years to figure all out the implications of the new information.

Journal reference: Nature (vol 442, p 276, 279)

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Mystery of Explosive Star Solved

Ken Thar for space.com

July 19, 2006

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In February, a faint star a few thousand light-years away flared suddenly, beaming so brightly that for a few days it was visible to the naked eye.

The star is a stellar corpse the size of Earth, known as a white dwarf, and it is paired in a binary system with a red giant, a dying, bloated star that once resembled our Sun. The red giant has been dumping gas onto the surface of the white dwarf, and every few years, enough matter accumulates to set off a giant thermonuclear explosion.

It was one of these explosions, called a "nova," that astronomers and stargazers detected earlier this year.

The two-star system, called RS Ophiuchi, is known as a recurrent nova because five similar eruptions have been detected before. The first observation occurred in 1898; the last eruption prior to this latest one happened in 1985.

The new observations, made using advanced radio and X-ray telescopes not available during the last outburst, reveal the explosion to be more complex than was previously assumed.

Standard computer models had predicted a spherical explosion with matter ejected in all directions equally. The latest observations instead showed that the explosion evolved into two lobes, confirming suspicions that the nova outburst produces twin jets of stellar material that spews out from the white dwarf in opposite directions.

"The radio images represent the first time we've ever seen the birth of a jet in a white dwarf system. We literally see the jet 'turn on,'" said Michael Rupen, an astronomer at the National Radio Astronomy Observatory who studied RS Ophiuchi using the Very Long Baseline Array (VLBA).

As impressive as the nova are, they might just be precursors for a more violent supernova explosion that will occur in the future, scientists say.

Like the Sun, Only More Powerful

The white dwarf's thermonuclear blasts are similar to those that occur on the surface of the sun, but they can be over 100,000 times more powerful. During each outburst, an amount of gas equal to the mass of the Earth is flung into space. Some of this ejected matter slams into the extended atmosphere of the inflated red giant, creating blast waves that accelerate electrons to nearly the speed of light. As the electrons travel through the stars' magnetic fields, they emit radio waves that can be detected by telescopes on Earth.

The blast waves move at over four million miles (about 6.4 million km) per hour. For a few weeks during each outburst, the white dwarf becomes a red giant.

"After the [thermonuclear explosion], the white dwarf will puff up into a red giant for a few weeks as the hydrogen that has been blasted into space fuses into helium," explains Richard Barry of the NASA Goddard Space Flight Center in Maryland.

All eyes on Ophiuchi

Japanese astronomers first detected signs of RS Ophiuchi's latest nova on the night of Feb. 12. Follow-up observations by radio telescopes revealed an expanding blast wave whose diameter was already the size of Saturn's orbit around the Sun.

In the weeks following, several radio and X-ray telescopes around the world tracked RS Ophiuchi closely, including the MERLIN array in the UK, the European EVN array, the Very Long Baseline Array (VLBA) and Very Large Array (VLA) in the United States, and NASA's Swift and Rossi X-ray Timing Explorer satellites.

Findings from the Rossi X-ray Timing Explorer and the VLBA/EVN observations are detailed in two separate studies published in the July 20 issue of the journal Nature.

The red giant and white dwarf stars making up RS Ophiuchi are separated by about 1.5 astronomical units, or one and a half times the distance the Earth is from the sun. The binary star system is located in the constellation Ophiuchus, about 5,000 light-years away