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RSAA News of the Month: November 2002 (Further Detail)

A Stellar Relic from the Early Milky Way

 

It was November 2001 and ANU astronomer Prof Mike Bessell could hardly believe his eyes. A spectrum that he had just taken on the ANU 2.3m telescope of a star, HE0107-5240, one of many thousands that had been selected by his collaborator Dr Norbert Christlieb from the Hamburg/ESO Survey, had the weakest calcium lines that he had ever seen. After 25 years it seemed likely that they had found a star more metal-deficient than CD -38 245 the previously most metal deficient star found by Bessell in 1977. In the intervening period an immense effort by many astronomers had gone into searching for more such stars but nothing had been found with lower abundances. Many astronomers considered that the limit had perhaps been reached. Was the new star the breakthrough?

Dr Christlieb quickly wrote out a proposal to the European Southern Observatory requesting special Director's discretionary time to check out this remarkable star using the most powerful high dispersion spectrograph. Approval was quickly granted and within a month a VLT spectrum had been taken and was sent to Hamburg. "WOOOWWWW!!!" emailed Christlieb to their collaborators as soon as he looked at the high resolution spectra of the two stars. "We have not just beaten CD -38 245 we have thrashed it. The new star could be more than 10 times lower in abundance."

Over the next 6 months Christlieb and his collaborators studied the spectra in great detail and in the October 31 edition of Nature their early results are finally published.

"This is, in a way, the closest we have come to the conditions directly after the Big Bang by studying stars," say Dr. Christlieb and Prof. Bengt Gustafsson, who in collaboration lead the chemical analysis. "But obviously a lot must have happened between the Big Bang and the formation of this star. In spite of its extreme metal-poorness it evidently carries some metals, and they were most probably formed in a still more early massive star that exploded as a supernova. Moreover, the star has an abnormally large content of carbon and nitrogen. Those elements may possibly have been formed by nuclear reactions with helium and hydrogen inside the star. After that, material was mixed to the stellar surface. It is also possible that a neigbouring star polluted our star by transferring mass towards the end of its life. However, we do not trace any more direct evidence that shows that this has happened."

The discovery of HE 0107-5240 is important because it demonstrates that stars with masses slightly below that of the Sun can form from very metal-poor gas. Most of the present theoretical calculations show that it is very difficult to form low-mass stars shortly after the Big Bang, because metals are needed to efficiently cool gas clouds as they contract to form stars. However, the existence of HE 0107-5240, a star with about 0.8 solar masses, reveals that Nature has found a way to achieve enough cooling. It therefore appears that many of the theoretical models need to be improved.

If a star of about 0.8 solar masses and 1/200,000 of the metal content of the Sun did indeed form in the early Universe, then it should also have been possible for low-mass Population III stars -consisting only of material formed in the Big-Bang- to form. If so, they would have survived until today, giving us the chance to find them with large, systematic searches like the Hamburg/ESO Survey. Because only 1/4 of the 8000 candidates identified in that survey have been vetted as of today, it is well possible that such a star will eventually be found in these efforts.

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