追踪恒星中的混合：疏散星团NGC 2516、毕宿星团和M67的铍新观测
Tracing mixing in stars: new beryllium observations of the open clusters NGC 2516, Hyades, and M67
摘要：
恒星中铍和锂丰度的测定为研究恒星内部迄今为止知之甚少的额外混合过程提供了一个关键工具。我们测量了三个疏散星团中的Be，补充现有的Be调查，并旨在收集一个更完整的经验方案的演化作为恒星年龄和温度的函数。具体来说，我们分析了NGC 2516，毕宿星团和M 67的成员的VLT/UVES光谱，以确定其Be和Li丰度。在前两个星团中，我们关注的是温度低于5400 K的恒星，而M 67的样本包括温度高于6150 K的恒星，以及两颗亚巨星和两颗蓝离散星。我们还计算了标准的演化模型的基础上的0.9 Mo星的Be的演化。我们发现不同温度区间和年龄的恒星有不同的经验行为。温度高于6150 K的恒星表现出Be贫化，并遵循Be与Li的相关性，而在~6150-5600 K范围内的恒星中Be是未贫化的。温度低于5400 K的NGC 2516成员没有耗尽任何Be，但类似温度的老毕星团确实显示出一些耗尽。Be在次巨星和蓝离散星中严重耗尽。温暖恒星的结果与以前的研究一致，支持了在这种温度下混合是由旋转驱动的假设。这同样适用于从“李间隙”演化而来的两个次巨星。这种机制并不是太阳型恒星的主要机制。我们发现，冷毕宿星的亏损不能简单地解释的对流区的深度增加的效果。最后，两个蓝色离散星的不同Be含量表明它们是由两个不同的过程形成的（即，冲突与二进制合并）。
英文摘要：
Determinations of beryllium abundance in stars, together with lithium, provide a key tool to investigate the so far poorly understood extra-mixing processes at work in stellar interiors. We measured Be in three open clusters,complementing existing Be surveys, and aiming at gathering a more complete empirical scenario of the evolution of Be as a function of stellar age and temperature. Specifically, we analyzed VLT/UVES spectra of members of NGC 2516, the Hyades, and M 67 to determine their Be and Li abundances. In the first two clusters we focused on stars cooler than 5400 K, while the M 67 sample includes stars warmer than 6150 K, as well as two subgiants and two blue stragglers. We also computed the evolution of Be for a 0.9 Mo star based on standard evolutionary models. We find different emprical behaviours for stars in different temperature bins and ages. Stars warmer than 6150 K show Be depletion and follow a Be vs. Li correlation while Be is undepleted in stars in the ~6150-5600 K range. NGC 2516 members cooler than 5400 K have not depleted any Be, but older Hyades of similar temperature do show some depletion. Be is severely depleted in the subgiants and blue stragglers. The results for warm stars are in agreement with previous studies, supporting the hypothesis that mixing in this temperature regime is driven by rotation. The same holds for the two subgiants that have evolved from the "Li gap". This mechanism is instead not the dominant one for solar-type stars. We show that Be depletion of cool Hyades cannot simply be explained by the effect of increasing depth of the convective zone. Finally, the different Be content of the two blue stragglers suggests that they have formed by two different processes (i.e., collisions vs. binary merging).
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