Recurrent Eruption: Explosive stellar saga

8:00pm, July 19, 2006
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Imagine the blast of a nuclear bomb as heavy as Earth and you'll get some idea of the energy unleashed in each of the six thermonuclear explosions that have ripped off the outer layers of a dense, nearby star in the past 108 years.

During the star's most recent outburst, which occurred in February, astronomers obtained their sharpest look yet at this recurrent type of explosion, called a nova, as well as new insights into the most powerful of known stellar eruptions, the supernova.

Indeed, the hyperactive star, which is part of a two-star system called RS Ophiuchi, may be teetering on the brink of going supernova and could obliterate itself in just a few hundred thousand years, suggest Jennifer Sokoloski of the Harvard-Smithsonian Center for Astrophysics and her colleagues. They base this conclusion, reported in the July 20 Nature, on their new estimates of the star's mass.

RS Ophiuchi consists of a compact star, called a white dwarf, and a bloated companion star, called a red giant. A strong wind emanating from the red giant dumps matter onto its smaller partner. When enough material piles up, it triggers a thermonuclear explosion on the white dwarf's surface.

On Feb. 12, Japanese astronomers reported that the star system, which last erupted in 1985, had suddenly zoomed in brightness. Within days, an armada of telescopes trained its sights on the eruption.

Astronomers found that the explosion generated a shock wave that rammed into the red giant. The shock wave exhibits many features of remnant material that's been cast from a supernova and plows into surrounding space, says Michael Bode of the Liverpool John Moores University in Birkenhead, England.

"We are seeing the same phases of evolution that one sees in a supernova remnant, but instead of taking thousands of years, here it takes months, [unfolding] right before our eyes," says Bode. He and his colleagues used the Swift satellite to measure X rays from the shock, which they describe in an upcoming Astrophysical Journal.

Bode and other colleagues also observed the blast with several radio telescopes. Two weeks after the eruption, the radius of the blast was already as large as that of Saturn's orbit. Over the next several months, it changed from the shape of a ring to that of a cigar. That suggests that the explosion wasn't spherical but blasted out as jets, Bode and his colleagues say in the July 20 Nature.

Another team found an uneven distribution of near-infrared emissions, described in an upcoming Astrophysical Journal Letters. That finding could have two interpretations, suggests the report's coauthor Richard Barry of NASA's Goddard Space Flight Center in Greenbelt, Md. If the star system resides at about 5,000 light-years from Earth, then the emissions reflect a mysterious, dense reservoir of material surrounding the two stars. If the system lies at only about one-third that distance, then the emission may for the first time be revealing a short-lived epoch during which the white dwarf, soon after its outburst, becomes as bloated as its red giant partner.

Either way, says Barry, the findings "could send theorists back to the blackboards."

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