One of the Milky Way Galaxy's largest stars may in fact be a double star system, according to recent research by a team of astronomers using NASA's Rossi X-ray Timing Explorer (RXTE) spacecraft. The team's results will be the subject of a presentation Jan. 7 in Washington, DC, at the winter meeting of the American Astronomical Society.
The team, led by Dr. Michael Corcoran of the Universities Space Research Association, Columbia, MD, bases its conclusions on unusual variations in the intensity of X-rays emitted by hot gas near the star, called Eta Carinae, which is located about 7,500 light years from Earth. They believe that the variations are caused by the presence of a massive companion star in orbit around Eta Carinae.
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.
"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 make 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 Rossi Explorer 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 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 massive stellar companion orbiting the star, bound to each other by the force of gravity.
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 (a means of measuring the properties of objects by splitting their light into its component colors) 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.
"We believe the orbit of the companion star is elongated into an ellipse," Corcoran said, "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."
The spacecraft's 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," Davidson said. "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 than 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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