NLTE determination of the sodium abundance in a homogeneous sample of extremely metal-poor stars
NLTE determination of the sodium abundance in a homogeneous sample of extremely metal-poor stars
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DOI：
10.1051/0004-6361:20066232
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发表时间：
2007-01
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影响因子：
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通讯作者：
S. Andrievsky;M. Spite;S. Korotin;F. Spite;P. Bonifácio;R. Cayrel;V. Hill;P. F. Gepi;U. Cnr
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文献类型：
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作者：
S. Andrievsky;M. Spite;S. Korotin;F. Spite;P. Bonifácio;R. Cayrel;V. Hill;P. F. Gepi;U. Cnr
极贫金属（EMP）恒星的像差比很好地指示了银河系演化最早阶段气体的化学成分。LTE分析发现，在金属丰度较低的情况下，与大多数其他元素相比，[Na/Fe]与[Fe/H]的离散度惊人地大，在巨星中，[Na/Fe]随着金属丰度的降低而降低。众所周知，钠线的形成对非LTE效应非常敏感，为了牢固地建立早期银河系中钠丰度的行为，我们使用了在VLT用UVES获得的EMP星样本的高质量观测，并且我们考虑了钠的非LTE线形成。用改进的MULTI程序计算了54颗EMP巨星和关闭星（其中33颗[Fe/H]< -3.0）的两条钠D共振线（在EMP星的光谱中只有这些钠线是可探测的）的轮廓，并与观测光谱进行了比较。在-4 <[Fe/H]<-2.5范围内的这些新测定中，[Na/Fe]和[Na/Mg]几乎是恒定的，具有低散射。在关闭恒星和“未混合”巨星（位于低RGB）中：[Na/Fe] = -0.21 +/- 0.13或[Na/Mg] = -0.45 +/- 0.16。这些值与最近在附近贫金属星中测定的[Na/Fe]和[Na/Mg]值非常一致。此外，我们确认所有的富钠星都是“混合”星（即，巨人位于碰撞后，经历了额外的混合）。没有一颗关闭的恒星富含钠。因此，在某些混合巨星中观察到的钠增强很可能是深度混合的结果。
Abundance ratios in extremely metal-poor (EMP) stars are a good indication of the chemical composition of the gas in the earliest phases of the Galaxy evolution. It had been found from an LTE analysis that at low metallicity, and in contrast with most of the other elements, the scatter of [Na/Fe] versus [Fe/H] was surprisingly large and that, in giants, [Na/Fe] decreased with metallicity. Since it is well known that the formation of sodium lines is very sensitive to non-LTE effects, to firmly establish the behaviour of the sodium abundance in the early Galaxy, we have used high quality observations of a sample of EMP stars obtained with UVES at the VLT, and we have taken into account the non-LTE line formation of sodium. The profiles of the two resonant sodium D lines (only these sodium lines are detectable in the spectra of EMP stars) have been computed in a sample of 54 EMP giants and turn-off stars (33 of them with [Fe/H]< -3.0) with a modified version of the code MULTI, and compared to the observed spectra. With these new determinations in the range -4 <[Fe/H]< -2.5, both [Na/Fe] and [Na/Mg] are almost constant with a low scatter. In the turn-off stars and "unmixed" giants (located in the low RGB): [Na/Fe] = -0.21 +/- 0.13 or [Na/Mg] = -0.45 +/- 0.16. These values are in good agreement with the recent determinations of [Na/Fe] and [Na/Mg] in nearby metal-poor stars. Moreover we confirm that all the sodium-rich stars are "mixed" stars (i.e., giants located after the bump, which have undergone an extra mixing). None of the turn-off stars is sodium-rich. As a consequence it is probable that the sodium enhancement observed in some mixed giants is the result of a deep mixing.