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Armagh Astronomer Solves Star Mystery

An international group of astronomers, including Dr Simon Jeffery of the Armagh Observatory, has used the Hubble Space Telescope to verify the origin of a very unusual and rare type of star. The group's studies confirm that the so-called "extreme helium stars" are formed by the merger of two white dwarf stars. The work has been published recently in The Astrophysical Journal.

The Hubble Space Telescope has revealed chemical evidence for the idea that a rare type of hydrogen-depleted supergiant star is created through the merger of two white dwarfs. Scientists have located 21 so-called "extreme helium stars" in our Galaxy. The unusually luminous stars are much larger and hotter than the Sun despite being less massive. Their name reflects their abundance of surface helium and peculiar lack of hydrogen.

Since the discovery of the first extreme helium star in 1942, two theories have come to dominate the discussion of their origin. One, dubbed the final-flash (FF) model, suggests an outer helium layer of a cooling white dwarf ignites, producing an extra thermal pulse that balloons the star into a giant. Under such circumstances, an extreme helium star might result as the giant then contracts.

The alternative theory, known as the double-degenerate (DD) model, begins with a binary star system involving a helium-rich white dwarf and a more massive carbon-oxygen-rich white dwarf orbiting one another for billions of years. If the two eventually got too close, the smaller star would rapidly be broken into a disc, and the larger star would consume it, becoming a supergiant star enriched in surface helium.

In 2002, Simon Jeffery and Hideyuki Saio of Tohoku University in Japan, showed how the DD model worked with a detailed computation. Now Jeffery and colleagues from the Indian Institute of Astrophysics and the University of Texas, US, have published chemical abundances for 17 extreme helium stars and found that they largely match the DD model's predictions.

The group made detailed studies of the ultraviolet light coming from seven extreme helium stars with Hubble's Space Telescope Imaging spectrograph and of the optical light from the telescopes in Texas and India. This data provided them with the specified amount of at least two dozen different chemical elements present in each star they studied, including heavy elements yttrium and zirconium.

The Hubble results match up well with predicted compositions from models of the composition of a star formed through the merger of two white dwarf stars in which the helium-core white dwarf is torn apart, and forms a thick disk around the carbon-oxygen white dwarf. Then, in a process taking only a few minutes, the disk is gravitationally pulled into the carbon-oxygen white dwarf.

What happens next depends of the mass of the newly formed star. If it is above a certain mass, called the Chandrasekhar limit, it will explode as a Type Ia supernova. However, if the mass is below this limit, the new merged star will balloon up into a supergiant, eventually becoming an extreme helium star.

"The new compositions go a lot further than the previous ones," Jeffery says. "We see them in about the right quantities to match what we would expect from the DD model." There is one exception, though - oxygen. The team expected most of the oxygen to have been destroyed and turned into nitrogen during the stars' hydrogen-burning phases. But in eight out of 10 cases they measured oxygen at levels similar to the assumed initial abundances.

"We don't really understand it," Jeffery admits. But he adds that if a star is shredded in 3 or 4 minutes, "there will be lots of things that we probably don't understand".

FOR FURTHER INFORMATION PLEASE CONTACT: Simon Jeffery at the Armagh Observatory, College Hill, Armagh, BT61 9DG. Tel.: 028-3752-2928; FAX: 028-3752-7174; csjarm.ac.uk; Website: star.arm.ac.uk.

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Last Revised: 2006 March 13th
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