RELEASE NO: 98-001

X-RAYS REVEAL GIANT STAR MAY BE TWINS

X-radiation from what may be the Galaxy's largest star lends strong support to a controversial new idea that the star may in fact be a double star system, according to recent research by a team of astronomers led by Dr. Michael Corcoran of the Universities Space Research Association, Columbia, Md. The team bases its conclusions on unusual variations in the intensity of X-rays emitted by hot gas near the star, called Eta Carinae.

They surmise that the variations are caused by the presence of a massive companion star in orbit around Eta Carinae. The observations, made using NASA's Rossi X-ray Timing Explorer (RXTE) spacecraft, will be the subject of a presentation Jan. 7 in Washington, D.C., at the winter meeting of the American Astronomical Society. The new work offers insight into the origin and evolution of a class of stars called luminous blue variables, which are the most massive stars known and include the galactic giant P Cygni and the now-famous "pistol star" observed in infrared light using the Hubble Space Telescope last October.

"Stars such as these shine so intensely that sometimes they become unstable and blow their outer layers off," said Corcoran. "That's what happened to Eta Carinae; during the mid-1800's it blasted an amount of material equivalent to the mass of our entire solar system into space. The gas and dust that makes up this material formed a shell that surrounds the star and now blocks it from direct view. We have taken what amounts to an X-ray of this shell and found that what's inside may really be two stars."

While using the RXTE to monitor the X-ray emission from Eta Carinae every week for a period of two years, the team found that X-rays emitted by hot gas near the star initially increased over a period of months and then rapidly diminished in intensity in a matter of days. Such

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variability is highly unusual and has never before been observed for Eta Carinae. The simplest explanation is that the variability of the X-ray emission is due to the presence of a gravitationally-bound massive stellar companion orbiting the star. The presence of such a companion has recently been claimed based on variations in near-infrared and optical spectra by Dr. Augusto Damineli and collaborators at the University of Colorado at Boulder. However, the presence of the "companion" star remained controversial since the spectrum of Eta Carinae is notoriously variable and since the spectral features originate in a very complex medium. As a result, the "binary model" for Eta Carinae has not yet been generally accepted by the astronomical community. The X-ray variations may help change this situation.

"Like all stars, the surface of Eta Carinae shines brightly. However, because the star is so unbelievably large, it's more than a million times brighter than our sun," said Corcoran. "The light generated by Eta Carinae is so intense that it drives hot gas away from the star’s surface, forming a fast moving 'stellar wind' flowing into interstellar space. If Damineli's 'binary model' is correct, then the same thing is happening to the companion star orbiting Eta Carinae. We believe the orbit of the companion star is elongated into an ellipse, which alternately moves it closer to and further away from Eta Carinae over the five and a half year orbital period. When the stars are close, the two stellar winds slam together which creates a shock wave that heats the gas tremendously, to about 60 million degrees, and it emits large amounts of X-rays. When they are further away, this shock wave diminishes, along with the X-rays. This agrees pretty well with our RXTE X-ray measurements."

RXTE will continue monitoring X-ray emission from Eta Carinae through 1998, and team member Dr. Kris Davidson of the University of Minnesota, Minneapolis, highlighted the need for further research. "Some aspects of the double star scenario seem technically awkward, and some kind of strange single star model still seems possible."

The RXTE data may have helped close one mystery, but at the same time, they have opened another. "Strange peaks in the X-ray emission intensity seem to occur every 85 days," notes Kazunori Ishibashi of the University of Minnesota. "While the first peaks detected were relatively weak, their strength has recently risen as the overall X-ray emission from Eta Carinae has brightened."

"The most puzzling unknown is what causes the 85 day X-ray period," adds Davidson. "It may be the rotation of the star, or the star may pulsate in that time, or it might even be the orbit period of a third object in the system, a possibility that makes some astronomers uncomfortable."

With at least 50 times more mass that the Sun, luminous blue variable stars like Eta Carinae are the most massive known. If Eta Carinae is really a double star system, each is estimated to be 70 times more massive than the Sun, according to Damineli's binary star model.

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"These stars are the flamboyant 'big shots' of the universe, living lives that are both violent and short. They shine so brightly that they literally tear themselves apart, and they burn their fuel

at prodigious rates, ending their lives in a catastrophic supernova explosion. They are the cosmic 'pressure cookers' where elements more complex than hydrogen and helium are made, so they are an important class of objects. Eta Carinae is probably the most well-studied example of this class, and the more we look at it the stranger it seems," says Corcoran.

Note to Editors: Images to support this story are available at the following sites:

http://www.gsfc.nasa.gov/ftp/newsmedia/JAN_AAS/TWINS

http://oposite.stsci.edu/pubinfo/PR/96/23.html

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