Violent Collision of Stellar Winds Detected

For the first time, astronomers have pinpointed the spot where the intense winds of two massive stars in a binary system violently collide and detected the production of high-energy X-rays there.

The monstrously large Eta Carinae binary contains between 100 and 150 times the mass of the sun and glows more brightly than four million suns together. The so-called hypergiant contains two massive stars, the second of which was not discovered until 2005.

Astronomers have long suspected that the stellar pair should give off high-energy X-rays, but until now, they didn't have the instruments to detect the radiation. But the European Space Agency's Integral telescope, launched to detect some of the most violent events in the universe, has conclusively detected such X-rays emanating from Eta Carinae, more or less as astronomers had thought.

"The intensity of the X-rays is a little lower than we expected, but given that this is the first-ever conclusive observation, that's OK," said Jean-Christophe Leyder of the University of Liege in Belgium, one of the astronomers who made the discovery.

Stellar wind shockwave

The intense X-rays are generated by the collision of the massive stars' stellar winds, flows of charged gas ejected from the stars' upper atmospheres.

The light and other radiation that "blows" off these particles is so strong that the stellar winds of Eta Carinae can reach speeds of 900 to 1200 miles per second (1,500 to 2,000 kilometers per second). Because the two stars are in such close proximity, the winds collide in a ferocious shockwave where temperatures reach several thousand million degrees Kelvin.

"It's a very tough environment," Leyder said.

Electrons get caught in the magnetic environment of the shockwaves and are bounced back and forth, accelerating to huge energies. When they finally burst out of the shockwave, they collide with low-frequency photons and give them an energy boost, creating the high-energy X-ray emissions spied by Integral.

Search for other emitters

Finding other examples of colliding-wind binaries, as astronomers call them, is tough because massive stars are rare; finding two in a binary system is rarer still.

"In our galaxy, there are probably only 30 to 50 colliding-wind binaries that display a clear signature of wind-wind collision," Leyder said. This meager number of stars is just a tiny fraction of the galaxy's stellar population. So to have an example such as Eta Carinae on our cosmic doorstep is a stroke of luck.

Astronomers did detect X-rays emitted from another colliding-wind binary, HD 5980, in our galactic neighbor the Small Magellanic Cloud last year.

The X-rays detected from Eta Carinae are of a much higher energy though. Astronomers estimate that the Eta Carinae system loses one Earth mass per day of ejected material, roughly 140 times higher than the mass loss rate of HD 5980.

Understanding the X-ray emissions from stellar winds is important because stellar winds affect the evolution of stars and the chemical evolution of the universe, while also acting as a source of energy in the galaxy.

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