Stellar Explosion Portends Bigger Blast To Come

July 19, 2006
Stellar Explosion Portends Bigger BlStellar Explosion Portends Bigger Blast To Comeast To Come
On February 12, the recurrent nova RS Ophiuchi underwent a powerful eruption that caused it to brighten briefly to naked-eye visibility. These radio images show RS Ophiuchi at 21 (top) and 27 (bottom) days after the outburst, as observed with the Very Long Baseline Array at a wavelength of 18 centimeters. The expanding shell shows radii roughly 27 and 30 times the Earth-Sun distance in these images, matching that expected from X-ray observations of the same object, assuming a distance of about 5200 light-years. The "jet" to the east (left), clearly present on day 27, is not seen on day 21. Such jets have been seen in white dwarf systems in the past, but never so clearly and so early. In particular, these images give the most precise measurement yet of how soon a jet appears after an outburst. Observations showed that the white dwarf source of the outburst is nearly as massive as it can be, weighing about 1.4 times as much as the Sun. In a few hundred thousand years, the white dwarf may accumulate enough mass to explode as a Type Ia supernova. Such supernovae allow astronomers to measure the size and expansion of the universe. Credit: NRAO/CfA

On February 12, 2006, skygazers spotted a nova that appeared when a faint star brightened dramatically, becoming visible to the unaided eye. The cause of the brightening was a thermonuclear explosion that blasted off a white dwarf star's outer layers while leaving the core unscathed.

"This nova was more exciting to astronomers than any fireworks display," said Jennifer Sokoloski (Harvard-Smithsonian Center for Astrophysics), lead author on a paper appearing tomorrow in the journal Nature.

Yet the eruption was minuscule compared to what will come. Astronomers predict that the star in question may eventually explode violently as a supernova in the distant future, ripping itself apart and scattering its gaseous remains across space. Similar explosions are bright enough to be seen across billions of light-years of space. This nearby system within the Milky Way offers astronomers a unique opportunity to refine their physical understanding of one type of rare star system that can generate such powerful blasts.

"Astronomers use such supernovae to measure the expansion of the universe, so it's important for us to understand how the star systems that generate those explosions evolve prior to their demise," said Sokoloski.

The star system under study, RS Ophiuchi, is located about 5,000 light-years from Earth in the direction of the constellation Ophiuchus. RS Ophiuchi consists of a dense, white dwarf star (a stellar core about the size of the Earth but containing more mass than the Sun) and a bloated red giant star. The red giant companion emits a stellar wind that spills material onto the white dwarf. When enough of that material has accumulated, theorists say, a gigantic thermonuclear explosion occurs.

Interestingly, the white dwarf star orbits inside the extended gaseous envelope of its companion. The material ejected from the white dwarf during the nova plows into this surrounding material, creating a shock wave that both heats gas to emit energetic X-rays and accelerates electrons to emit radio waves.

"What we could infer from the X-ray data, we could image with the radio telescopes," explained Michael Rupen (National Radio Astronomy Observatory), who studied RS Ophiuchi using the National Science Foundation's Very Long Baseline Array.

Using satellites and ground-based telescopes, independent teams studied RS Ophiuchi at multiple wavelengths. Their observations showed that the explosion was more complex than scientists generally assumed. Standard computer models presume a spherical explosion with matter ejected in all directions equally. Observations of RS Ophiuchi showed evidence for two opposing jets of matter and a possible ring-like structure.

"The radio images represent the first time we've ever seen the birth of a jet in a white dwarf system," said Rupen. "We literally see the jet 'turn on.'"

Systems such as RS Ophiuchi eventually may 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) all are triggered as the white dwarf reaches the same mass, they are thought to be nearly identical in their intrinsic brightness. This makes them extremely valuable as "standard candles" for measuring distances in the universe.

With the Rossi X-ray Timing Explorer, the scientists calculated the mass of the white dwarf to be close to 1.4 times that of the Sun - nearly as massive as a white dwarf can become before collapsing.

"One day, RS Ophiuchi will explode. What happened this February was just a little hiccup-a precursor of greater things to come," said Koji Mukai (NASA Goddard Space Flight Center), co-author on the Nature report.

Authors on the Nature paper were Sokoloski, Gerardo Luna of the Harvard-Smithsonian Center for Astrophysics, Mukai, and Scott Kenyon of the Center.

The Rossi X-ray Timing Explorer is managed by NASA Goddard. The Very Long Baseline Array is an instrument of the National Radio Astronomy Observatory, which is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

Source: Harvard-Smithsonian Center for Astrophysics

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