三个古老的晕次巨星：精确的视差、成分、年龄和对球状星团的影响
Three Ancient Halo Subgiants: Precise Parallaxes, Compositions, Ages, and Implications for Globular Clusters
摘要：
最古老恒星的最精确年龄是那些附近的晕次巨星，因为它们几乎完全依赖于测量的弛豫和绝对氧丰度。在这项研究中，我们使用哈勃太空望远镜上的精细制导传感器来确定星族II亚巨星HD 84937、HD 132475和HD 140283的三角视差，精度为2.1%或更高。高质量的光谱已被用来获得它们的表面丰度的O，Fe，Mg，Si，和Ca，这是假定为0.1 - 0.15德克斯小于其初始丰度由于扩散的影响。将等时线与$(\log\,T_{\rm eff}, M_V)$图上的三颗次巨星进行比较，依次得出HD 84937、HD 132475和HD 140283的年龄分别为$12.08 \pm 0.14, 12.56 \pm 0.46$和$14.27 \pm 0.38$ Gyr，其中每个误差条只包括视差不确定性。总的不确定度估计为$\sim\pm 0.8$ Gyr（在接近关闭的恒星HD 84937的情况下更大）。虽然HD 140283的年龄比宇宙微波背景辐射$\sim$ 0.4 - 0.5 Gyr的年龄大，但这种差异是在$< 1\,σ$的水平上。然而，第一颗星族II恒星显然是在大爆炸后不久形成的。（忽略扩散过程的恒星模型似乎被排除在外，因为它们预测HD 140283是比宇宙更古老的$\sim 1.5$ Gyr。晕场亚巨星似乎比金属丰度相似的球状星团更老：如果假设距离接近天琴座RR标准烛光所暗示的距离，M92和M5分别比HD 140283和HD 132475年轻$\sim 1.5$和$\sim 1.0$ Gyr。
英文摘要：
The most accurate ages for the oldest stars are those obtained for nearby halo subgiants, because they depend almost entirely on just the measured parallaxes and absolute oxygen abundances. In this study, we have used the Fine Guidance Sensors on the Hubble Space Telescope to determine trigonometric parallaxes, with precisions of 2.1% or better, for the Population II subgiants HD 84937, HD 132475, and HD 140283. High quality spectra have been used to derive their surface abundances of O, Fe, Mg, Si, and Ca, which are assumed to be 0.1-0.15 dex less than their initial abundances due to the effects of diffusion. Comparisons of isochrones with the three subgiants on the $(\log\,T_{\rm eff}, M_V)$-diagram yielded ages of $12.08 \pm 0.14, 12.56 \pm 0.46$, and $14.27 \pm 0.38$ Gyr for HD 84937, HD 132475, and HD 140283, in turn, where each error bar includes only the parallax uncertainty. The total uncertainty is estimated to be $\sim\pm 0.8$ Gyr (larger in the case of the near-turnoff star HD 84937). Although the age of HD 140283 is greater than the age of the universe as inferred from the cosmic microwave background by $\sim$ 0.4-0.5 Gyr, this discrepancy is at a level of $< 1\,σ$. Nevertheless, the first Population II stars apparently formed very soon after the Big Bang. (Stellar models that neglect diffusive processes seem to be ruled out as they would predict that HD 140283 is $\sim 1.5$ Gyr older than the universe.) The halo field subgiants appear to be older than globular clusters of similar metallicities: if distances close to those implied by the RR Lyrae standard candle are assumed, M92 and M5 are younger than HD 140283 and HD 132475 by $\sim 1.5$ and $\sim 1.0$ Gyr, respectively.
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