Abundances from High-Resolution Spectra of Kinematically Interesting Halo Stars
Abundances from High-Resolution Spectra of Kinematically Interesting Halo Stars
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DOI：
10.1086/338898
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发表时间：
2002-03
期刊：
影响因子：
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通讯作者：
A. Stephens;A. Boesgaard
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作者：
A. Stephens;A. Boesgaard
银河系光环中有些恒星的轨道特性与大多数光环群体明显不同。解释这些运动学上奇特的恒星的一种建议是，它们通过附近矮星系的分裂而被银河系引力所吞没。这一假设的一个检验在于确定异常晕星相对于正常（“原生”）晕场恒星的化学成分。由于附近低质量球状或不规则星系中恒星形成速度较慢，预计可能吸积的恒星具有不同的化学富集度。我们已经获得了具有不寻常轨道特性的贫金属晕矮星的阶梯光谱：来自外晕、“高”晕和逆行轨道。我们的光谱具有高光谱分辨率 (35,000–48,000) 和高信噪比（中值 140），主要来自具有 HIRES 的 Keck I 10 m 望远镜（53 颗星），但也来自 KPNO Mayall 4 m 望远镜的中阶梯光栅光谱仪（三颗星）。光谱覆盖很大的光谱范围：∼4000–6800 Å。测量了大约 9000 条线的强度，以确定恒星参数（Teff、log g、[Fe/H] 和 xi）和 10 种元素的丰度，包括 Na、α 聚变产物（Mg、Si、Ca、Ti）、铁峰元素（Cr、Fe、Ni）和中子俘获元素（Y、Ba）。计算了每颗恒星的大气模型，用于确定 LTE 假设下的成分。丰度与运动学性质的比较没有显示出特殊的趋势，除了α聚变元素平均值与Fe的比率与远河恒星的相关性较弱，因为最外层恒星的[α/Fe]比率低于远河恒星的比率。吸积晕恒星的特征被认为是相对于 Fe 而言缺乏 α 元素（例如 O、Mg、Si、Ca、Ti）（与正常恒星相比），这是矮星系内恒星形成的零星爆发的结果。然而，我们还没有发现具有如此[α/Fe]的亚太阳比的新恒星。对于 BD +80°245，我们得出 [α/Fe] = -0.22 与之前的结果一致。对于我们的所有四种 α 元素比率，[Mg/Fe]、[Si/Fe]、[Ca/Fe] 和 [Ti/Fe]，我们发现在低金属丰度下具有高值，而在高金属丰度下则降低到接近太阳值。平均 [α/Fe] 与 [Fe/H] 的斜率约为 -0.15。在给定的金属丰度下，外晕恒星的 [α/Fe] 低于内晕恒星。在高 [Fe/H] 和低 [Fe/H] 条件下，[α/Fe] 的分布超出了单独的测量误差。看来[Na/Fe]和[Mg/Fe]的比率一起增加。 [Ni/Fe]的比例相对于太阳能持续轻度不足。 [Ba/Fe]的比率随着[Fe/H]的增加而增加。我们样本中运动学上奇特的恒星必定起源于远离银河系中心的局部恒星形成区域。它们不带有累积种群的化学特征。很明显，我们样本中的化学富集主要来自 II 型超新星，只有一小部分来自 Ia 型超新星。在遥远的晕中恒星形成事件开始后，我们的恒星最多会形成~1 Gyr。从这些数据中得出的一般结论是，新生银河系的形成并不是由矮星系（如目前围绕银河系运行的星系）的后期吸积所主导。然而，银河系演化早期碎片的同化是结构形成分层模型的自然副产品，可以解释晕族的许多特性。
There are stars in the halo of the Galaxy whose orbital properties are distinctly different from the majority of the halo population. One suggestion to account for these kinematically peculiar stars is that they have been gravitationally subsumed by the Milky Way through the breakup of nearby dwarf galaxies. One test of this hypothesis lies in determining the chemical composition of the deviant halo stars relative to that of normal ("native") halo field stars. The possibly accreted stars are predicted to have different chemical enrichments due to a slower rate of star formation in nearby low-mass spheroidal or irregular galaxies. We have obtained echelle spectra of metal-poor halo dwarfs with unusual orbital properties: from the outer halo, from the "high" halo, and on retrograde orbits. Our spectra are at high spectral resolution (35,000–48,000) and high signal-to-noise ratios (median value 140), primarily from the Keck I 10 m telescope with HIRES (53 stars), but also from the echelle spectrometer of the KPNO Mayall 4 m Telescope (three stars). The spectra cover a large spectral range: ∼4000–6800 Å. The strengths of approximately 9000 lines have been measured to determine the stellar parameters (Teff, log g, [Fe/H], and ξ) and the abundances of 10 elements, including Na, α-fusion products (Mg, Si, Ca, Ti), iron peak elements (Cr, Fe, Ni), and neutron capture elements (Y, Ba). A model atmosphere was computed for each star, which was used to determine the composition under the assumption of LTE. The comparison of the abundances with the kinematic properties revealed no special trends, except that the ratio of the mean of the α-fusion elements to Fe is weakly correlated with stellar apogalactica in the sense that the outermost stars have lower ratios of [α/Fe] than those with smaller apogalactica. The hallmark of an accreted halo star is presumed to be a deficiency (compared with normal stars) of the α-elements (e.g., O, Mg, Si, Ca, Ti) with respect to Fe, a consequence of sporadic bursts of star formation within the dwarf galaxies. However, we have found no new stars with such subsolar ratios of [α/Fe]. For BD +80°245 we derive [α/Fe] = -0.22 in agreement with earlier results. For all four of our α-element ratios, [Mg/Fe], [Si/Fe], [Ca/Fe], and [Ti/Fe], we find high values at low metallicities that decrease to near-solar values at high metallicities. The slope of the mean [α/Fe] with [Fe/H] is approximately -0.15. At a given metallicity, the outer halo stars have lower [α/Fe] than the inner halo stars. There is a spread in [α/Fe] beyond the individual measurement errors at both high and low [Fe/H]. It appears that the ratios of [Na/Fe] and [Mg/Fe] increase together. The ratio of [Ni/Fe] is persistently mildly deficient relative to solar. The ratio of [Ba/Fe] increases with [Fe/H]. The kinematically peculiar stars in our sample must have had their origins in localized star-forming regions far removed from the Galactic center. They do not carry the chemical signature of an accreted population. It seems clear that the chemical enrichment in our sample comes primarily from Type II supernovae with only a small component from Type Ia supernovae. Our stars would have formed at most ∼1 Gyr after the beginning of a star formation episode in the remote halo. The general conclusion extracted from these data is that the formation of the nascent Milky Way was not dominated by the late accretion of dwarf galaxies like the ones that currently orbit the Galaxy. However, the assimilation of fragments early in the evolution of the Galaxy is a natural by-product of hierarchical models of structure formation and can explain many properties of the halo population.