20 October 2011

Stars that shouldn’t exist explained

Cannibalistic companions are responsible for impossibly young, hot stars known as blue-stragglers, say scientists.
Blue stragglers
Blue stragglers

This image of the open star cluster NGC 188 was taken at the WIYN 0.9m in V, B and I. The blue stragglers discussed in this paper are circled. Credit: K. Garmany, F. Haas NOAO/AURA

ESSEX: Cannibalistic companions are responsible for impossibly young, hot stars known as blue-stragglers, say scientists.

For the last ten years, observations have been made on a group of a few thousand stars by the 3.5-metre WIYN Telescope on Kitt Peak, Arizona. Called open cluster NGC 188, these stars were formed from the same giant molecular cloud and have roughly the same age.

They reveal the existence of stars that are brighter and more massive than other stars that are roughly the same age, which means that these stars should, theoretically, have already become giants that have swelled in size towards the end of their life, or burnt-out stellar remnants.

The debate over the origin of such stars has been going on for some time and the results of this study, published in Nature today, may have finally put an end to the mystery of these seemingly young stars in an otherwise ancient galaxy.

“People have been trying to explain the origin of blue stragglers since their discovery in 1953, and now we have the detailed observations needed to identify how they were created,” said co-author Aaron Geller from Northwestern University in the U.S. “I’ve always enjoyed trying to get to the bottom of a mystery.”

The mass transfer debate

There are two main arguments for the formation of blue stragglers – interstellar collisions, or that they’re made from consumed matter from a companion star in a binary system, known as mass transfer origin.

The data revealed in this new study show that nearly 80% of the blue stragglers in NGC 188 are part of a binary system, which helps support the mass transfer origin theory.

Mass transfer occurs when a higher mass companion of a main-sequence star evolves into a giant star. The giant grows in radius and eventually loses hold of its outer envelope, the material of this is then consumed by the proto-blue-straggler and this extends the life of the star and forms the blue straggler.

Confirming the origin

Although the companion stars cannot be observed directly due to the light from the blue stragglers the researchers used the ‘wobble’ caused by the gravitational orbit of the companion to the main star to detect the masses of their companions.

Using binaries with long-orbital periods (of order 1,000 days), the researchers found that their mass distribution peaked around a mean value of 0.55 solar masses.

Collision products predict a much higher distribution, which would be common of a main-sequence companion, whereas the mass transfer origin stars would be accompanied by smaller white dwarfs, consistent with the study, confirming a mass transfer origin for the blue stragglers seen in NGC 188 at “a confidence level of greater than 99%,” said co-author Robert Mathieu from the University of Wisconsin-Madison in the U.S.

“Our discovery that the majority of the blue stragglers in NGC 188 have binary companions with masses consistent with the typical carbon-oxygen white dwarf mass (of about 0.5 solar masses) supports an origin through mass transfer,” said Geller.

Not a happy ending

“The origin of blue stragglers has long been debated for decades,” said Tim O’Brien from the University of Manchester’s Jodrell Bank Centre for Astrophysics, who wasn’t involved in the study. “This paper provides significant evidence in favour of the suggestion that they are formed from mass transfer in a close binary system.

“The smoking gun is that most of the blue stragglers in this star cluster have been found to have white dwarf companions whose properties are consistent with predictions of the mass transfer theory,” said O’Brien.
“In each case the white dwarf, a dead star the size of the Earth, is the tell-tale remnant of a companion star which was cannibalised by the blue straggler, extending its own life.”

But it isn’t a happy ending for these stars once they’ve attracted the material from their giant companions. “The apparent youth is short lived,” commented Christopher Tout from Cambridge University in Britain. “Its increased mass means it exhausts its fuel more quickly and it dies young for its original mass.”


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