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How Teardrops And Bagels Are Producing Violent Stellar Outbursts In Space

University of Washington

They can resemble teardrops and bagels in space, and they can produce sudden and violent energy surges. Astronomers call them cataclysmic variables, and the curious antics of these double-star systems are enabling the exploration of a new frontier in the physics of extreme gravity and magnetism.

Researchers have discovered that one such variable, V1159 Ori, goes through regular 45-day cycles of one super-outburst of radiation, followed by eight smaller outbursts. The team studying the double star, which is in the Milky Way galaxy, was led by University of Washington astronomy professor Paula Szkody.

The first report on the Szkody team's observations, the most detailed multi- wavelength analysis yet of a cataclysmic variable star system, is to be presented on Saturday at the annual meeting of the American Association for the Advancement of Science in Seattle.

Szkody's report is part of a review of the first year's results of the Rossi X-ray Timing Explorer satellite, which was launched by NASA in December 1995. The Szkody group used the satellite, in conjuction with the ROSAT X-ray satellite and the IUE (now defunct) and EUVE ultraviolet satellites, together with ground telescope observations, to make the analysis of V1159 Ori. "We hope that, in time, theory will catch up with what we are seeing in these strange systems," says Szkody.

Cataclysimic variable is the name given to a class of objects consisting of two stars that are so close together that hydrogen gas flies from one to the other. The star losing material is a red dwarf, a middle-aged star similar in structure to the sun, but cooler and less massive. It is in very fast orbit around a hot, collapsed star, called a white dwarf, which has a mass of the sun compressed into a volume the size of the Earth (whose diameter is 100 times smaller than that of the sun). In the case of V1159 Ori, the red dwarf is orbiting the white dwarf every 1 1/2 hours at a speed close to 200 kilometers (124 miles) a second. It is squeezed into the shape of a teardrop by the powerful gravitational field of the nearby collapsed star -- about 300,000 times greater than the Earth's gravity.

As the red dwarf spins in space, at a distance from the white dwarf of only 1.4 million kilometers (865,00 miles) -- the diameter of the sun -- its outer layer of gas is stripped away and transferred to the white dwarf. Because of the intense gravity, "as the hydrogen splats down, a tremendous amount of energy is released," says Szkody. This cataclysm is called a nova or dwarf nova outburst, and the energy surge can increase the star's normal output by 10 to 10,000 times. Much of this energy is emitted in X-rays and extreme ultraviolet radiation..

V1159 Ori's 45-day cycle starts with a very bright, long-lasting super-outburst, slowly declines, then goes to a low level called quiescence. The super-outburst, it's theorized, is the result of the hydrogen buildup around the white dwarf suddenly triggering a massive dumping of material, which gives rise to a large energy release. That is somewhat akin to water storing energy before it boils, and then releasing it as steam .

By measuring V1159 Ori's emissions at different wavelengths, the Szkody group was able to calculate when and how the star system went from one state to another. They theorize that as the hydrogen disk around the white dwarf thickens, the material emits energy in the extreme ultraviolet or soft X-rays. However, in the quiescent stage, the disk is much less thick, and produces hard X-rays.

Szkody and her colleagues picked this particular star system because its previously observed cyclical outbursts made it a good subject for the study of what happens when hydrogen-rich material flows onto a white dwarf. "We are now getting information on the time scale of the flow, the temperature of the disk, and the region close to the white dwarf by using the amount and color of the light observed at the different wavelengths," says Szkody. The optical light comes from the cool, outer disk, while the ultraviolet is emitted by the hot, inner disk. The X-rays come from the dumping ground at the white dwarf surface.

Szkody is also reporting on the work of associated teams observing two other cataclysmic variables, XY Ari and EX Hya, using the same X-ray explorer satellite. The two groups, led by astronomers Coel Hellier of Keele University in England and Koji Mukai of Goddard Space Flight Center, found that these two magnetic star systems produce X-rays at a level 10 times greater than V1159 Ori. The energy surges were even higher during an observed outburst of XY Ari.

The two binaries studied by Hellier and Mukai are known as magnetic variables because the strength of their collapsed stars' magnetic fields is one million times that of the sun. Because of this intense field, the hydrogen from the inner part of the white dwarf's disk is drawn towards the poles, until the disk ends up resembling a bagel spinning in space. "We do not have a theory as to exactly what happens to material under such extremely high magnetic field strengths," says Szkody. "We can't predict based on what we know about magnetic fields on Earth what is going to happen under these conditions."

Only about 400 cataclysmic variables have been recorded since the 1960s, even though Szkody guesses there are probably at least a million in the Milky Way. But they are so faint, they can be hard to find. The fast-cycling variables, like V1159 Ori, were first observed only two years ago.

The need to understand the effects of mass transfer in close double stars is important because they are thought to be fairly common in the universe. Compared to single stars, how is stellar evolution speeded up or slowed down in binaries, Szkody asks. And what is actually happening to these stars as a result of all this mass transfer?

Says Szkody: "The behavior of matter in the realm of extreme gravity and magnetic fields, which is common in stellar old age, is pushing the leading edge of our understanding."

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Paula Szkody is at 543-1988, or at szkody@astro.washington.edu

An artist's interpretation of the V1159 Ori variable is available on request. It shows the teardrop-shaped red dwarf with material being drawn onto the white dwarf. The red outer ring indicates a cooler band emitting in the optical, with the hotter, blue center producing ultraviolet radiation and X-rays.


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