ar X iv : a st ro - p h / 01 04 35 7 v 1 2 2 A pr 2 00 1 Measurement of stellar age from ura - nium decay

The ages of the oldest stars in the Galaxy indicate when star formation began, and provide a minimum age for the Universe. Radioactive dating of mete-oritic material 1 and stars 2 relies on comparing the present abundance ratios of radioactive and stable nuclear species to the theoretically predicted ratios of their production. The radioisotope 232 Th (half-life 14 Gyr) has been used to date Galactic stars 2−4 , but it decays by only a factor of two over the lifetime of the Universe. 238 U (half-life 4.5 Gyr) is in principle a more precise age indicator, but even its strongest spectral line, from singly ionized uranium at a wavelength of 385.957 nm, has previously not been detected in stars 4−7. Here we report a measurement of this line in the very metal-poor star CS31082-0018, a star which is strongly overabundant in its heavy elements. The derived uranium abundance, log(U/H) = −13.7 ± 0.14 ± 0.12 yields an age of 12.5 ± 3 Gyr, though this is still model dependent. The observation of this cosmochronometer gives the most direct age determination of the Galaxy. Also, with improved theoretical and laboratory data, it will provide a highly precise lower limit to the age of the Universe. We are currently undertaking a large programme of high-resolution spectroscopy-at the ESO Very Large Telescope and UVES 9 spectrograph-of extremely metal-poor stars selected primarily from the Ca II HK survey of ref.8. The star CS31082-001 (V-band magnitude V = 11.7) has been identified as very metal-poor (T.C.B. et al., manuscript in preparation). One of us (the observer, V.H.) has noted that this star is similar to another well studied very metal-poor star, CS22892-052 6,10,11 , in that it exhibits a large enhancement (relative to iron) of elements formed by the neutron-capture r-process. High-resolution spectroscopic observations of CS22892-052 had allowed a precise measurement of 232 Th from the 401.9nm line, but based on the abundance of this slowly decaying species, the age of this star remained fairly uncertain: 15.6 ± 4.6 Gyr (ref.4). Moreover, the newly discovered star CS31082-001 exhibits considerably less contamination of the atomic line spectrum by molecular bands of CH and CN than does CS22892-052. The Th II line at 401.9 nm is also unusually strong and clear of contaminants; in fact, a total of 14 Th II lines are detected , 11 of which were selected for measurement. Only the 401.9nm line has …