By Eugenie Samuel Reich
They are more than a 100 times the mass of the Sun, glow more than 10 million times as brightly, and, over the course of their lives, spew out more than half their mass in the form of a relentless stellar wind. Yet the origin of the young massive stars in the Large Magellanic Cloud (LMC) galaxy has been a mystery for astronomers for decades.
Vasilii Gvaramadze, an astronomer at the Sternberg Astronomical Institute at Moscow State University, and his colleagues have now located the birthplace of one of the group, and have shown that it is a "runaway star," bolting across the LMC at more than 130 kilometers per second after being ejected from its home cluster. The discovery implies that other young massive stars in the LMC might also be runaways, and casts light on the violent processes that can cause giant stars to be ejected from the stellar clusters in which they first form. A paper on the findings has been accepted for publication by Astronomy & Astrophysics and is available on the arXiv preprint database.
Massive stars were observed in the LMC as far back as 1982. Their brightness and blue color, which indicate that they are extremely hot, suggest to astronomers that the stars are relatively young--about 2 million years old. Yet they are located hundreds of light years from known star-forming regions. Computer simulations suggest that massive stars cannot form on their own without producing clusters of smaller stars around them. "It's like a king without a kingdom," says Mordecai-Mark Mac Low, an astrophysicist at the American Museum of Natural History in New York.
The problem had astronomers scratching their heads, and some began to consider an alternative possibility--that the stars formed elsewhere, and were then ejected from their birthplace to travel across the LMC. Runaway stars had previously been seen in the Milky Way and traveling away from clusters in Orion. However, at 160,000 light years away from Earth, the LMC was too distant for astronomers to get a direct measure of the young stars' velocity to confirm whether they were also runaways.
Gvaramadze approached the challenge in a different way, by looking for bow shocks--parabolic shapes produced as the stellar wind from runaway stars impinges on the interstellar gas ahead of them. Searching through Spitzer Space Telescope archival images of six stars that might be runaways, he found an infrared image of a bow shock for one, called BI 237.
The shape of the bow shock revealed the direction of the star's motion, and by tracing that backwards Gvaramadze concluded that BI 237 had formed in the LH 82 cluster in the LMC's Tarantula Nebula (also known as 30 Doradus) around 2 million years ago.
The star was probably ejected from LH 82 when it gained speed by traveling around other massive stars in the center of the cluster three or four times before shooting past them and out of the cluster altogether. Gvaramadze and his colleagues argue that in the early days of a star cluster, the most massive stars group together in the centre, where they can easily interact and produce this effect.
"The claim that such clusters form very early is realistic," says Douglas Gies, an astrophysicist at Georgia State University. "Bow shocks are an interesting way to get further information" on runaway stars, he adds.
Mac Low agrees the results make sense. "It's a nice result and it pins down the answer to a question that's been bugging some of us for some time, which is how do you get such massive stars out on their own."