martes, 29 de mayo de 2007

Battle of the Winds Rages at Crashing Stellar Pair

An artist depicts the binary system LH54-425, which consists of two very massive stars. The larger star’s powerful wind overpowers the smaller star’s wind, creating a region of hot gas where the outflows collide. LH54-425 is in the Large Magellanic Cloud, a satellite galaxy of the Milky Way about 165,000 light-years from Earth. Credit: NASA/Casey Reed

A pair of stellar titans, whipping around each other as a close-knit orbital team, reveals a sort of "battle of the winds" as the stars blow off their mass in speedy streams.

Astronomers describe for the first time what happens when winds from the massive binary members crash into each other.

The binary system, dubbed LH54-425, resides in the Large Magellanic Cloud, a satellite galaxy of the Milky Way about 165,000 light-years from Earth. Containing the heftiest and brightest of stars, called O-stars, the binary has a combined mass of about 100 Suns and is considered one of the most extreme binaries ever spotted.

Its stars, weighing 62 and 37 solar masses, jet around each other at a whopping 1.1 million miles per hour (1.8 million kilometers per hour). The stars are probably less than 3 million years old, and thus in an early stage of their lives.

"The stars are so close to each other-about one-sixth the average Earth-Sun distance-that they orbit around a common center of mass every 2.25 days," said one of the study team members Douglas Gies of Georgia State University.

George Sonneborn of NASA Goddard Space Flight Center in Greenbelt, Md., presented the results here at a meeting of the American Astronomical Society. "These results expand our understanding of the nature of very massive binaries, which was not well understood," Sonneborn said.

Sonneborn and his colleagues relied on NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) instruments to characterize properties of the rare system, particularly the stellar wind blowing from each star.

The close-knit couple blows off a stellar wind at rates commensurate with each star's size.

The heaviest of the pair ejects material at a speed of 5.4 million miles per hour (8.7 million kph), tossing off 500 trillion tons of mass every second. That's about 400 times the rate that the Sun sheds its mass through solar wind. The smaller star is losing material at about one-tenth the rate of its companion.

Sonneborn and his colleagues discovered the larger star's whipping wind overpowers its sibling's "breeze," creating an area of hot gas where the outflows collide. This wind-collision zone, they found, envelops the smaller star and forms a curved layer of superheated gases that emit X-rays and far-ultraviolet radiation.

In a million years or so, the aging stars will start to dump substantial amounts of mass onto the other. With such a close orbit, the stars will likely merge into a single massive star rivaling the more massive member of the binary Eta Carinae binary that weighs in at about 100 solar masses.

"The merger of two massive stars to make a single super star of over 80 Suns could lead to an object like Eta Carinae, which might have looked like LH54-425 one million years ago," Sonneborn said. "Finding stars this massive so early in their life is very rare."