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Astrophile: Blinged-out stars were born rich

Objects: Ancient stars

Composition: Chock-full of heavy metal

At first glance, CS 22892-052 looks like a run-of-the-mill old star. Born about 13 billion years ago, only 700 million years after the big bang, it is lacking in many of the shinier, more interesting elements. But look closer, and you'll see that this stellar granny is adorned with gold.

Now Terese Hansen of the Neils Bohr Institute in Copenhagen, Denmark, and colleagues think they've figured out why. Long ago, a jet of neutrons could have sprinkled gold into this lucky star's nursery.

Stars usually show their ages in their clothes. Very old stars tend to be austere, sticking to the simpler, lighter elements because they condensed from pristine clouds of hydrogen and helium gas created in the big bang.

Precious metal

Younger stars are threaded with heavier metallic elements, such as copper and zinc. These elements are formed by massive stars, which spew their contents into nearby gas clouds to be incorporated into the next generation.

Yet about 3 to 5 per cent of aged stars are decked out in some of the heavy elements of the young. CS 22892-052 was the first known example of such a blinged-out old star in the Milky Way. Strangely, while they don't have much of the middleweight stuff such as iron or cobalt, they have plenty of the heaviest elements, including gold, platinum and lead.

These elements can only be formed in a type of nuclear reaction called the r-process, thought to happen in supernova explosions, when a powerful blast of neutrons rapidly bulks up atomic nuclei. "That tells us that those stars are probably witnesses to whatever astrophysical process gave us the r-process," says Timothy Beers of Michigan State University in East Lansing.

Dense jerk

One theory was that each of these stars once had a massive companion as part of a binary system. When the companion exploded as a supernova it would have riddled the smaller star with freshly made metal.

To test this idea, Beers and his colleagues used the Nordic Optical Telescope on the island of La Palma in the Canary Islands to observe 17 anomalous old stars a total of 14 times between April 2007 and September 2011. They used techniques developed for exoplanet studies to see if the stars were being jerked around by the dense remnant of an exploded companion.

Just 3 of the 17 showed signs of being members of binary systems, which is no more than the average for normal stars. There is no correlation between gold-spattered stars and a binary history, so the source of the bling can't be material donated by a stellar sibling.

Neutron beam

Having ruled out that scenario, the team turned to the next most likely explanation: that the stars got those r-process elements from other supernovae. But why did only a few stars benefit? And why just the r-process elements and not others?

A beam of neutrons could do the trick, Beers says. If a star is rotating rapidly before it explodes, the material inside it could escape easily along the axis of rotation. That could lead to a beam of high-speed neutrons that ploughs through the star's outer layers and build r-process elements as they pass. That would spray a narrow jet of r-process elements into the local interstellar medium, making a gold-studded cloud in which one of these anomalous stars could later form.

Alternatively, the golden inheritance may have been created when two neutron stars collided, says astrophysicist Stan Woosley of the University of California, Santa Cruz. That would form lots of r-process elements and because these mergers are rare, only a few gas clouds would be seeded, leading to just a few gilded old stars.

CS 22892-052 is still an oddball, though. In addition to all its heavy metals, it is also full of carbon – and astronomers still don't know how it got there.

Journal reference: Astrophysical Journal Letters, DOI: 10.1088/2041-8205/743/1/L1



<I>The jewel-filled heavens of the Carina nebula (Image: NASA)</I>



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