FIRST MEASURE OF URANIUM ABUNDANCE IN A STAR

FIRST MEASURE OF URANIUM ABUNDANCE IN A STAR It is a great discovery, obtained thanks to the high resolution spectrograph UVES, assembled on one of the 8m-diameter telescopes of the ESO-VLT in Paranal (Chile). For the first time, an international team (led by Roger Cayrel, from Paris Observatory), could measure one uranium line in absorption in a star. Roger Cayrel and his collaborators (among them P. Francois, V. Hill, F. Spite and M. Spite, of Paris Observatory) observe the stars the most deficient in heavy elements, produced by the stellar nucleosynthesis (mainly ejecta from supernovae of type II). Their goal is to study the first stars to have been formed in the Universe, because formed from gas not yet polluted by other stars, and then go back to the formation of the Galaxy. The detailed analysis of element abundances in these very primitive objects (certainly more primitive than the clouds of the Lyman-alpha forest observed at the redshift of 4) must allow to better understand the origin of the significant variations of chemical composition from object to object, observed at these very weak metallicities. Until now only thorium ( 232 Th), has been used to this end, but its half-life time is 14.0 billion year. Its decline is thus at maximum of only a factor two, and the uncertainty on the age determination, taking into account all other uncertainties, is about 4 to 5 billion year, that is to say a third of the age of the Universe. With uranium, which declines by a factor 8 in the same time, one hopes to have three times less error. However, before being able to announce an age with this precision, it will be necessary to reduce the uncertainty which still exists, in atomic physics, on the one hand on the oscillator strength of the line (a team from the CEA is mobilized on this determination) and on the other hand on the production ratio between uranium with respect to thorium and the stable elements the most nearby in mass number (from Os to Bi). For the moment the preliminary estimate of the age of the uranium formation, practically the age of the Galaxy,which had then formed only less than one thousandth of the current number of stars, is of 12.5± 3 Gyr. The error will be soon reduced by half.

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Reference : (article in Nature, Thursday 8 Feb 01, astro-ph/0104357) Contact : Roger Cayrel (Département DASGAL, Observatoire de Paris) Acknowledgements : The authors are grateful to Guy Simon and to Lydia Tchang-Brillet, respectively to have provided results of the infra-red survey DENIS before publication, and to have advised and directed them towards physicists specialists in actinides. No correct partition function of Uranium existed at the time of the observation reported here (our thanks to the colleague of IMCCE who helped for the numerisation of atomic tables.

Dernière modification le 4 mars 2013