The Quintuplet cluster - III. Hertzsprung-Russell diagram and cluster age
The Quintuplet cluster - III. Hertzsprung-Russell diagram and cluster age
复制标题
五联体星团 - III。
DOI：
10.1051/0004-6361/201117534
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发表时间：
2012
影响因子：
6.5
通讯作者：
L. Oskinova
中科院分区：
文献类型：
--
作者：
A. Liermann;W. Hamann;L. Oskinova
五连星是银河中心 (GC) 的三个大质量星团之一，位于人马座 A* 投影中约 30 度。我们的目标是构建星团的赫罗图（HRD），以研究其演化并限制其恒星形成历史。为此，我们使用最完整的五重星光谱目录。基于 K 波段光谱，我们在假设星团均匀变红的情况下确定了目录中所有恒星的恒星温度和光度。我们在由此产生的 HRD 中发现了两组：早期型 OB 星和晚期型 KM 星，彼此分开。通过与Geneva恒星演化模型进行比较，我们得出OB星的初始质量超过8M圆点。在 HRD 中，这些恒星沿着等时线分布，对应的年龄约为 4 Myr。这证实了之前的考虑，其中类似的年龄估计是基于星团中演化的沃尔夫-拉叶星的存在。我们推导出各种光谱亚型组（例如 N-WR/N-O、N-WC/N-WN）的数量比率，并将其与种群合成模型的预测进行比较。我们发现，大约 3.3 到 3.6 Myr 之前的恒星形成瞬时爆发是形成五联星团的最可能情况。此外，我们应用质量-光度关系来构造星团的初始质量函数（IMF）。我们发现国际货币基金组织有点头重脚轻的迹象。根据光谱特征，LHO 目录中的晚型恒星是红巨星分支 (RGB) 恒星或红超巨星 (RSG)。假设它们位于银心附近，我们可以推导出它们的光度。与恒星演化模型的比较表明，这些恒星的初始质量低于 15 M 圆点，这意味着它们需要大约 15 Myr (RSG) 甚至超过 30 Myr (RGB) 才能演化到现阶段。人们可能会怀疑这些晚型恒星在物理上并不属于五重星团。事实上，它们中的大多数都没有资格成为星团成员，因为它们的径向速度与星团平均速度相差太大。尽管如此，LHO 星表中最亮的 RGB/RSG 恒星中的五颗均具有五连星的平均径向速度，因此仍然高度怀疑是受引力束缚的成员。如果是这样，这将对集群的形成和演化场景提出挑战。
The Quintuplet, one of three massive stellar clusters in the Galactic center (GC), is located about 30 pc in projection from Sagittarius A*. We aim at the construction of the Hertzsprung-Russell diagram (HRD) of the cluster to study its evolution and to constrain its star-formation history. For this purpose we use the most complete spectral catalog of the Quintuplet stars. Based on the K-band spectra we determine stellar temperatures and luminosities for all stars in the catalog under the assumption of a uniform reddening towards the cluster. We find two groups in the resulting HRD: early-type OB stars and late-type KM stars, well separated from each other. By comparison with Geneva stellar evolution models we derive initial masses exceeding 8 M-circle dot for the OB stars. In the HRD these stars are located along an isochrone corresponding to an age of about 4 Myr. This confirms previous considerations, where a similar age estimate was based on the presence of evolved Wolf-Rayet stars in the cluster. We derive number ratios for the various spectral subtype groups (e.g. N-WR/N-O, N-WC/N-WN) and compare them with predictions of population synthesis models. We find that an instantaneous burst of star formation at about 3.3 to 3.6 Myr ago is the most likely scenario to form the Quintuplet cluster. Furthermore, we apply a mass-luminosity relation to construct the initial mass function (IMF) of the cluster. We find indications for a slightly top-heavy IMF. The late-type stars in the LHO catalog are red giant branch (RGB) stars or red supergiants (RSGs) according to their spectral signatures. Under the assumption that they are located at about the distance of the Galactic center we can derive their luminosities. The comparison with stellar evolution models reveals that the initial masses of these stars are lower than 15 M-circle dot implying that they needed about 15 Myr (RSG) or even more than 30 Myr (RGB) to evolve into their present stage. It might be suspected that these late-type stars do not physically belong to the Quintuplet cluster. Indeed, most of them disqualify as cluster members because their radial velocities differ too much from the cluster average. Nevertheless, five of the brightest RGB/RSG stars from the LHO catalog share the mean radial velocity of the Quintuplet, and thus remain highly suspect for being gravitationally bound members. If so, this would challenge the cluster formation and evolution scenario.