Lithium abundance in the globular cluster M4: from the turn‐off to the red giant branch bump
Lithium abundance in the globular cluster M4: from the turn‐off to the red giant branch bump
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DOI：
10.1111/j.1365-2966.2010.17884.x
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发表时间：
2010-10
影响因子：
4.8
通讯作者：
A. Mucciarelli;M. Salaris;L. Lovisi;F. Ferraro;L. Lanzoni;S. Lucatello;R. W. A. Department;Bologna University;I. R. R. Institute-I.-R.-R.-Institute-1864572367;Liverpool John Moores University;UK.;I. Padova;Italy.
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作者：
A. Mucciarelli;M. Salaris;L. Lovisi;F. Ferraro;L. Lanzoni;S. Lucatello;R. W. A. Department;Bologna University;I. R. R. Institute-I.-R.-R.-Institute-1864572367;Liverpool John Moores University;UK.;I. Padova;Italy.
我们给出了球状星团M4中87颗恒星的Li和Fe丰度，这是利用甚大望远镜上长颈鹿收集的高分辨率光谱获得的。目标范围从岔道到红色巨型树枝凸起。关闭星的Li丰度是均匀的，平均值为A(Li)=2.3±0.02Dex(σ=0.10Dex)，与在其他球状星团和晕场星中测得的Li丰度上限一致，也证实了M4与Wilkinson微波各向异性探测器+大爆炸核合成(WMAP+BBNS)所预测的原始Li丰度的差异。A(Li)的整体行为作为有效温度的函数，使我们能够识别Li演化中的两个主要液滴，这两个液滴是由于第一次疏浚和红巨星分支碰撞后的额外混合事件。M4的铁含量测量结果为[Fe/H]=−1.10±0.01Dex(σ=0.07Dex)，没有出现系统的矮星和巨星之间的偏移量。Li和Fe丰度在整个演化路径上的行为与包括纯原子扩散在内的理论模型不相容，指出需要考虑对流区以下的额外湍流混合，从而能够抑制原子扩散。A(Li)的测量值及其在关闭星中的均匀性允许我们对M4关闭星内部的Li轮廓的形状施加强烈的约束。A(Li)在有效温度下的整体行为可以用不同的原始Li丰度重现，这取决于所采用的湍流混合的类型。人们不能复制从WMAP+BBNS A(Li)开始并采用Richard，Michaud&Rich所描述的湍流混合的全球趋势，其效率与Korn等人所使用的相同。对NGC6397中锂的含量进行解释。事实上，这样的解并不能很好地同时再现在旋转星和红巨星上观测到的Li丰度。然而，WMAP+BBNS A(Li)可以被复制，假设有更有效的湍流混合能够到达更深的恒星区域，在那里Li被燃烧。我们的结论是，宇宙学上的Li差异不容易解决，目前对恒星内部的湍流缺乏了解，未来需要努力很好地理解这种非正则过程的真实性质。
We present Li and Fe abundances for 87 stars in the globular cluster M4, obtained by using high-resolution spectra collected with GIRAFFE at the Very Large Telescope. The targets range from the turn-off up to the red giant branch bump. The Li abundance in the turn-off stars is uniform, with an average value equal to A(Li)= 2.30 ± 0.02 dex (σ= 0.10 dex), consistent with the upper envelope of Li content measured in other globular clusters and in the halo field stars, confirming also for M4 the discrepancy with the primordial Li abundance predicted by Wilkinson Microwave Anisotropy Probe+ big bang nucleosynthesis (WMAP+BBNS). The global behaviour of A(Li) as a function of the effective temperature allows us to identify the two main drops in the Li evolution due to the first dredge-up and to the extra-mixing episode after the red giant branch bump. The measured iron content of M4 results to [Fe/H]=−1.10 ± 0.01 dex (σ= 0.07 dex), with no systematic offsets between dwarf and giant stars. The behaviour of the Li and Fe abundances along the entire evolutionary path is incompatible with theoretical models including pure atomic diffusion, pointing out that an additional turbulent mixing below the convective region needs to be taken into account, able to inhibit the atomic diffusion. The measured value of A(Li) and its homogeneity in the turn-off stars allow us to put strong constraints on the shape of the Li profile inside the M4 turn-off stars. The global behaviour of A(Li) with the effective temperature can be reproduced with different pristine Li abundances, depending on the kind of adopted turbulent mixing. One cannot reproduce the global trend that starts from the WMAP+BBNS A(Li) and adopts the turbulent mixing described by Richard, Michaud & Richer with the same efficiency as that used by Korn et al. to explain the Li content in NGC 6397. In fact, such a solution is not able to well reproduce simultaneously the Li abundance observed in turn-off and red giant branch stars. However, the WMAP+BBNS A(Li) can be reproduced assuming a more efficient turbulent mixing able to reach deeper stellar regions where the Li is burned. We conclude that the cosmological Li discrepancy cannot be easily solved with the present, poor understanding of the turbulence in the stellar interiors, and a future effort to well understand the true nature of this non-canonical process is needed.