沿M30的两个BSS序列的FLAMES和XSHOOTER光谱
FLAMES and XSHOOTER spectroscopy along the two BSS sequences of M30
摘要：
本文利用甚大望远镜的FLAMES和XSHOOTER对球状星团（GC）M30中的15颗蓝离散星（BSS）进行了光谱观测。已经选择了目标来对发现的两个BSS序列进行采样，沿着蓝色序列有7个BSS，沿着红色序列有8个BSS。特别是，几乎所有观测到的BSS的自转速度都小于~30 km/s，只有一个（蓝色）快速旋转的BSS（>90 km/s）被确认为WUMa双星。这一旋转速度分布类似于在47 Tuccott和NGC 6397中获得的结果，而M4仍然是迄今为止研究的唯一一个拥有大部分快速旋转BSS的GC。所有温度超过~7800 K的恒星（无论母恒星的BSS序列如何）都显示出比正常星团恒星更大的铁丰度，并且随着有效温度的变化趋势明显。这种行为表明，由辐射悬浮驱动的粒子传输机制发生在这些恒星的光球层中，就像已经在NGC 6397中观察到的BSS一样。最后，4个属于红色序列的BSS（不受辐射悬浮的影响）显示出强烈的[O/Fe]耗尽，相对于红巨星分支和水平分支恒星的丰度。这种O-耗尽是相容的化学签名预期在BSS中形成的二元系统中的传质过程，在协议中提出的机制形成BSS中的红色序列。
英文摘要：
We present spectroscopic observations acquired with FLAMES and XSHOOTER at the Very Large Telescope for a sample of 15 Blue Straggler Stars (BSSs) in the globular cluster (GC) M30. The targets have been selected to sample the two BSS sequences discovered, with 7 BSSs along the blue sequence and 8 along the red one. No difference in the kinematical properties of the two groups of BSSs has been found. In particular, almost all the observed BSSs have projected rotational velocity lower than ~30 km/s, with only one (blue) fast rotating BSS (>90 km/s), identified as a W UMa binary. This rotational velocity distribution is similar to those obtained in 47 Tucanae and NGC 6397, while M4 remains the only GC studied so far harboring a large fraction of fast rotating BSSs. All stars hotter than ~7800 K (regardless of the parent BSS sequence) show iron abundances larger than those measured from normal cluster stars, with a clearcut trend with the effective temperature. This behaviour suggests that particle trasport mechanisms driven by radiative levitation occur in the photosphere of these stars, as already observed for the BSSs in NGC 6397. Finally, 4 BSSs belonging to the red sequence (not affected by radiative levitation) show a strong depletion of [O/Fe], with respect to the abundance measured in Red Giant Branch and Horizontal Branch stars. This O-depletion is compatible with the chemical signature expected in BSSs formed by mass transfer processes in binary systems, in agreement with the mechanism proposed for the formation of BSSs in the red sequence.
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