疏散星团Stephenson 2周围的红超巨星
Red supergiants around the obscured open cluster Stephenson 2
摘要：
在银河系的一个小区域，靠近盾牌座臂的底部和长棒的顶端，发现了几个红超巨星星团。星族合成模型表明，它们必须非常巨大，才能容纳如此多的超巨星。其中，史蒂芬森2，拥有26个RSG的核心分组，是银河系中最大的星团的有力候选者。我们探讨了星系团的实际大小及其与超密度的可能联系。我们已经在DENIS，USNO-B1和2 MASS之间进行了交叉匹配，以识别Ste 2周围的候选遮蔽明亮的红色恒星，并在附近的控制区域中找到>600个候选者。其中超过400颗恒星的亮度足以让4米级的望远镜进行观测。我们用位于La Palma的WHT望远镜上的AF 2观测了一个约250颗恒星的子样本，获得了7500- 9000 A范围内的中分辨率光谱。我们推导出所有这些对象的光谱类型和光度类，并测量它们的径向速度。我们的目标是G和K超巨星，晚期（>=M4）M巨星，M型亮巨星（光度II类）和超巨星。我们发现~35个RSG的径向速度与Ste 2成员相似，分布在两个调查区域。此外，我们发现~40个径向速度与物理协会原则上不相容的RSG。我们的研究结果表明，恒星2不是一个孤立的星团，而是一个巨大结构的一部分，可能包含数百个RSG，径向速度与银河经度（距离约6 kpc）的终极速度一致。此外，我们还发现了沿着这条视线不同距离处的几个大质量恒星群的证据[简略]。
英文摘要：
Several clusters of red supergiants have been discovered in a small region of the Milky Way close to the base of the Scutum-Crux Arm and the tip of the Long Bar. Population synthesis models indicate that they must be very massive to harbour so many supergiants. Among them, Stephenson 2, with a core grouping of 26 RSGs, is a strong candidate to be the most massive cluster in the Galaxy. It is located close to a region where a strong over-density of RSGs had been found. We explore the actual cluster size and its possible connection to this over-density. We have performed a cross-match between DENIS, USNO-B1 and 2MASS to identify candidate obscured luminous red stars around Ste 2, and in a control nearby region, finding >600 candidates. More than 400 are sufficiently bright in I to allow observation with a 4-m class telescope. We have observed a subsample of ~250 stars, using AF2 on the WHT telescope in La Palma, obtaining intermediate-resolution spectroscopy in the 7500--9000A range. We derive spectral types and luminosity classes for all these objects and measure their radial velocities. Our targets are G and K supergiants, late (>=M4) M giants, and M-type bright giants (luminosity class II) and supergiants. We find ~35 RSGs with radial velocities similar to Ste 2 members, spread over the two areas surveyed. In addition, we find ~40 RSGs with radial velocities incompatible in principle with a physical association. Our results show that Ste 2 is not an isolated cluster, but part of a huge structure likely containing hundreds of RSGs, with radial velocities compatible with the terminal velocity at this Galactic longitude (and a distance ~6kpc). In addition, we find evidence of several populations of massive stars at different distances along this line of sight [ABRIDGED].
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