Science & Environment

Cannibal star consumes neighbour

BP Psc
Image caption An artist's impression (on the right) shows the red giant BP Psc with an unusual dusty orbiting disc

A star that may have gobbled up its neighbour - either a star or a giant planet - has been found with the help of Nasa's Chandra X-ray observatory.

The find may shed light on the complex interaction between planets and stars.

In Astrophysical Journal Letters, researchers say that the star had appeared to be quite young, but may in fact be much older.

It could be a billion-year-old red giant that "ate" a young companion, whose remnants are still visible today.

BP Psc is a more evolved version of our Sun, located about 1,000 light years away.

Scientists first started studying the star some 15 years ago and were puzzled by its unusual look.

Old or young?

It did not look like a typical red giant: it possessed a dusty orbiting disc. These discs are the kind that give rise to planets around young stars - not the kind of feature one would expect around a star this old.

In addition, most young stars form in clusters, and BP Psc was isolated, noted the team led by Joel Kastner of the Rochester Institute of Technology, New York.

"As hard as people have looked, they have not been able to find a young star near BP Psc," said Professor Kastner.

"That was one of several things that made Ben [Zuckerman] and me suspect that it wasn't actually young."

At the same time, the star does not fit the image of a typical "elderly" star, either.

Besides the orbiting disc, there are jets that blast out from BP Psc in opposite directions, mimicking the way young stars get rid of debris from the disc when it falls on to the star's surface.

As is the case with older stars, there is very little of the element lithium in BP Psc's atmosphere. The surface gravity is also rather weak.

X-ray evidence

Recent data from the orbiting Chandra X-ray observatory also show that the star is not young as some indicators had suggested.

"The last piece of evidence, which, to me, is the nail in the coffin that BP Psc is old rather than young, is that its rate of X-ray production is very similar to old, yet rapidly spinning, giant stars that have surface temperatures similar to BP Psc," Kastner says.

The astronomer noted that if the star were a young one, it would have been a much stronger X-ray source in the night sky.

"We stared at BP Psc for one day with Chandra and only detected about 18 X-rays. We could almost name them," he said.

Image caption Scientists believe that BP Psc may provide clues to the formation of exoplanets

Having considered all the evidence, the team decided that they were witnessing a rare case of "stellar cannibalism".

The scientists believe that sometime after BP Psc started expanding into its "red giant" phase - the late stage of stellar evolution - it "consumed" its unlucky neighbour.

"It appears that BP Psc represents a star-eat-star Universe, or maybe a star-eat-planet one," said Professor Kastner.

"Either way, it just shows it's not always friendly out there."

He explained that the giant star's companion has most probably "fallen" inside it and been "digested".

"We've never actually caught one in the act. I think BP Psc is an example of such an interaction," he explained.

"Our working speculation is that we are observing the star right at the point at which it has swallowed its companion and hence formed a disc. Some of the material that used to be its companion has fallen on to the star and some has been shot out at high speeds, and that's what we're seeing."

Another scientist working on the study noted that the "cannibal" also showed magnetic activity that might have been generated by a "dynamo". This situation might have come about through the star's rapid rotation, caused in turn by the engulfment process.

"It seems that BP Psc has been energised by its meal," said Rodolfo Montez Jr, also from the Rochester Institute of Technology.

New planets

Astronomers believe that what happened to BP Pcs's unfortunate neighbour may at some point happen to our Earth as well.

"BP Psc shows us that stars like our Sun may live quietly for billions of years, but when they go, they just might take a star or planet or two with them," said another co-author, David Rodriguez from University of California, Los Angeles.

Astronomers are now studying the star's dusty orbiting disc in search of new planets that may be forming there.

"In order to understand the extrasolar planets that are now being discovered by the dozen, we need to figure out how planets might be forming and therefore where we should go look for them," said Professor Kastner.

"I think this object is especially interesting because it gives us a good shot at finding young planets around an old star."

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