High-precision abundances of elements in solar twin stars - Trends with stellar age and elemental condensation temperature
High-precision abundances of elements in solar twin stars - Trends with stellar age and elemental condensation temperature
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太阳双星中元素的高精度丰度——恒星年龄和元素凝结温度的趋势
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发表时间：
2015
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通讯作者：
P. Nissen
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作者：
P. Nissen
上下文对太阳和太阳双星大气中元素丰度的高精度测定表明，太阳中难熔元素和挥发性元素的比例非常低。这导致了这样的建议，丰度比[X/Fe]和元素凝聚温度TC之间的关系，可以被用作星星周围存在类地行星的标志。目标。在太阳附近的21颗太阳双星和太阳（通过小行星的反射光观测）的S/N为600的HARPS光谱被用来确定非常精确的（σ = 0.01 dex）元素丰度差，以了解[X/Fe]与TC和其他参数（如恒星年龄）的相关性。方法. C，O，Na，Mg，Al，Si，S，Ca，Ti，Cr，Fe，Ni，Zn和Y的丰度是使用MARCS模型大气的弱和中强光谱线的等效宽度得出的，参数由Fe线的激发和电离平衡确定。考虑了非LTE效应，并将其考虑到某些元素。此外，精密度（σ 0. 8G yr）恒星年龄是通过在log g-Teff图中的Yonsei-Yale等时线之间插值得到的。结果太阳中难熔元素和挥发性元素的比值比大多数太阳孪星中的低（只有一颗星星具有与太阳相同的[X/Fe]-TC分布），但对于许多恒星，[X/Fe]和TC之间的关系并不明确。对于几种元素，[X/Fe]和恒星年龄之间有非常紧密的相关性，在8 Gyr的年龄间隔内，振幅高达10.20 dex，而[Fe/H]和年龄之间缺乏相关性。虽然[Mg/Fe]随着年龄的增长而增加，但s过程元素钇表现出相反的行为，这意味着[Y/Mg]可以用作银河系演化的灵敏计时器。Na/Fe和Ni/Fe比值与恒星年龄没有很好的相关性，但定义了一个紧密的Ni-Na关系，类似于以前在更贫金属的恒星中发现的关系，尽管幅度较小。C/O比随时间变化很小，[C/Fe]和[O/Fe]变化幅度约为0.15 dex。结论. [X/Fe]对恒星年龄的依赖性和[Ni/Fe]-[Na/Fe]的变化使[X/Fe]-TC关系作为恒星周围存在类地行星的可能标志的使用变得复杂。各种丰度比的年龄趋势提供了新的限制超新星产量和银河系的化学演化，和太阳金属丰度恒星的C/O的缓慢演变是感兴趣的讨论系外行星的组成。
Context. High-precision determinations of abundances of elements in the atmospheres of the Sun and solar twin stars indicate that the Sun has an unusually low ratio between refractory and volatile elements. This has led to the suggestion that the relation between abundance ratios, [X/Fe], and elemental condensation temperature, TC, can be used as a signature of the existence of terrestrial planets around a star. Aims. HARPS spectra with S /N 600 for 21 solar twin stars in the solar neighborhood and the Sun (observed via reflected light from asteroids) are used to determine very precise (σ ∼ 0.01 dex) differential abundances of elements in order to see how well [X/Fe] is correlated with TC and other parameters such as stellar age. Methods. Abundances of C, O, Na, Mg, Al, Si, S, Ca, Ti, Cr, Fe, Ni, Zn, and Y are derived from equivalent widths of weak and medium-strong spectral lines using MARCS model atmospheres with parameters determined from the excitation and ionization balance of Fe lines. Non-LTE effects are considered and taken into account for some of the elements. In addition, precise (σ 0. 8G yr) stellar ages are obtained by interpolating between Yonsei-Yale isochrones in the log g ‐ Teff diagram. Results. It is confirmed that the ratio between refractory and volatile elements is lower in the Sun than in most of the solar twins (only one star has the same [X/Fe]-TC distribution as the Sun), but for many stars, the relation between [X/Fe] and TC is not well defined. For several elements there is an astonishingly tight correlation between [X/Fe] and stellar age with amplitudes up to ∼0.20 dex over an age interval of eight Gyr in contrast to the lack of correlation between [Fe/H] and age. While [Mg/Fe] increases with age, the s-process element yttrium shows the opposite behavior meaning that [Y/Mg] can be used as a sensitive chronometer for Galactic evolution. The Na/Fe and Ni/Fe ratios are not well correlated with stellar age, but define a tight Ni‐Na relation similar to that previously found for more metal-poor stars albeit with a smaller amplitude. Furthermore, the C/O ratio evolves very little with time, although [C/Fe] and [O/Fe] change by ∼0.15 dex. Conclusions. The dependence of [X/Fe] on stellar age and the [Ni/Fe]‐ [Na/Fe] variations complicate the use of the [X/Fe]-TC relation as a possible signature for the existence of terrestrial planets around stars. The age trends for the various abundance ratios provide new constraints on supernovae yields and Galactic chemical evolution, and the slow evolution of C/O for solar metallicity stars is of interest for discussions of the composition of exoplanets.