1 September 2011

‘Impossible’ ancient star defies models

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The discovery of a rare ancient star with an extremely primitive chemical composition could force scientists to rethink star formation models.
 SDSS J102915+172927

At the centre of this picture is a very unremarkable looking faint star, too faint to be seen through all but the largest amateur telescopes. Credit: ESO/Digitised Sky Survey 2

The distribution of the light of different colours coming from SDSS J102915+172927 after it has been split up by the X-Shooter instrument on the ESO VLT. Astronomers can use this data to find the chemical signals from different elements within the star, which show up as dark interruptions of the curved lines. The only evidence of elements heavier than helium is the two dark lines from the element calcium. Credit: ESO/E. Caffau

LONDON: The discovery of a rare ancient star with an extremely primitive chemical composition could force scientists to rethink star formation models.

One of four ‘primitive’ stars ever discovered, the rare 13-billion-year-old star, called SDSS J102915+172927, was found hovering about 3,500 light-years above the disc of the Milky Way. Scientists led by Elisabetta Caffau of the University of Heidelberg in Germany used data from the European Southern Observatory’s Very Large Telescope (ESO VLT) in Chile to deduce the star’s age through its chemical composition.

Apart from its rareness, the star has a number of intriguing features. The first is its size, just four-fifths of the Sun’s mass. “It was believed that stars with masses lower than even the Sun’s mass actually can’t form from matter that contains so little carbon and oxygen,” said co-author Hans Ludwig, also from Heidelberg, of the study published in Nature today.

This is because carbon and oxygen atoms are 12 to 16 times heavier than hydrogen, and in a smaller star, their higher density goes a long way toward providing the pressure needed to ignite the gas at the core of the nascent star. Since this diminutive star managed to form without their help, some star formation theories need a fresh look.

Paradoxical star

After cooling for a few hundred thousand years following the Big Bang, the matter in the universe condensed into atoms. This gas, almost exclusively made up of hydrogen and helium with a small amount of lithium, accumulated into stars.

As the stars created heavier elements like carbon and iron through fusion reactions, and then scattered these elements into the cosmos with supernovae explosions, the array of elements that gathered into new stars became more varied. But the signatures of heavier elements didn’t show up in the glow of the ancient star, marking it out as having formed when the universe was young.

How did it happen?

The star has the lowest amount of elements heavier than helium of all stars yet studied. Particularly noteworthy is just how low the amount of lithium and carbon contained is, according to John Norris of Australia National University in Canberra, who wrote the accompanying ‘News & Views’ article in Nature. “These stars are very difficult to find,” he added.

The star is at least 40 times shorter on lithium than expected, given the amount that should have been present in the early universe. Only one of the other three primitive stars is suitable for estimating how much lithium was about when it formed, and it too contains only about one-hundredth of the expected lithium.

Caffau and Ludwig both trust the theoretical calculations for the amount of lithium in the early universe, so the question is – how did the lithium disappear? Caffau admitted, “We have no idea why and when this happened.”

The odd one out

When it comes to carbon content, the newly discovered primitive star is definitely the odd one out. All three of the previously known stars contain a lot of carbon, at least compared to the amount of iron in them. Theorists took this then-consistent information and set about deducing how the gas in the early universe could have collected a disproportionate amount of carbon, Norris said.

Both the iron and carbon would have come from the few supernovae that preceded even these archaic stars. Modern supernovae would have created a ratio of carbon to iron similar to that in the new primitive star, but to account for three carbon-heavy stars, theorists suggested a different type of supernova – just strong enough to blow out the carbon while retaining most of the heavier iron.

But since the new star’s carbon is proportional to its iron, the theorists will have to think again about how to reconcile these observations. “That’s where we’ll go in the future,” said Norris. And he’s optimistic that astronomers will soon have more than four primitive stars to help build the picture of the early universe. The researchers have suggested that between five and 50 others are waiting to be discovered in data already gathered in the Sloan Digital Sky Survey, made at the Apache Point Observatory in New Mexico.

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