Cayrel R, Hill V, Beers T C, Barbuy B, Spite M, Spite F, Plez B, Andersen J, Bonifacio P, François P, Molaro P, Nordström B, Primas F
Observatoire de Paris-Meudon, DASGAL, France.
Nature. 2001 Feb 8;409(6821):691-2. doi: 10.1038/35055507.
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 meteoritic material and stars relies on comparing the present abundance ratios of radioactive and stable nuclear species to the theoretically predicted ratios of their production. The radioisotope 232Th (half-life 14 Gyr) has been used to date Galactic stars, but it decays by only a factor of two over the lifetime of the Universe. 238U (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. Here we report a measurement of this line in the very metal-poor star CS31082-001, 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.
银河系中最古老恒星的年龄表明了恒星形成开始的时间，并为宇宙提供了一个最小年龄。对陨石物质和恒星进行放射性测年，依赖于将放射性和稳定核素的当前丰度比与理论预测的生成比进行比较。放射性同位素钍232（半衰期140亿年）已被用于测定银河系恒星的年龄，但在宇宙的寿命期间它仅衰减两倍。铀238（半衰期45亿年）原则上是一个更精确的年龄指标，但即使是其最强的光谱线，来自波长为385.957纳米的单电离铀，此前在恒星中也未被探测到。在这里，我们报告了在极贫金属恒星CS31082 - 001中对这条谱线的测量，这颗恒星的重元素严重过剩。推导得出的铀丰度，log（U/H） = - 13.7 +/- 0.14 +/- 0.12，得出年龄为125亿年 +/- 30亿年，不过这仍依赖于模型。对这种宇宙计时器的观测给出了对银河系最直接的年龄测定。此外，随着理论和实验室数据的改进，它将为宇宙年龄提供一个高精度的下限。
