在LMC中的双峰式转折集群NGC 1806中没有化学异常的证据
No evidence of chemical anomalies in the bimodal turnoff cluster NGC 1806 in the LMC
摘要：
我们研究了NGC 1806的化学组成，这是一个巨大的、中等年龄的球状星团，显示出双主序的转折。我们分析了该星团中8颗巨星的高分辨率光谱样本（用甚大望远镜的FLAMES进行了加密），发现平均铁含量为[Fe/H]=--0.60 +- 0.01 dex，没有证据表明轻元素（Na，O，Mg，Al）的丰度存在内在的星与星之间的变化。结合光学测光和近紫外哈勃空间望远镜测光得到的（m_（F814 W）; m_（F336 W）-m_（F814 W））色星等图显示了一个狭窄的红巨星分支，从而排除了星团中C和N丰度的内在变化。这些发现表明，NGC 1806并没有拥有化学上不同的子种群，与在古老的球状星团中发现的不同。反过来，这表明双主序列关闭现象不能用通常用来解释在旧球状星团中观察到的化学异常的自富集过程来解释。其他解决方案（即，恒星旋转、星团之间的合并或与巨大分子云的碰撞）应该被设想来解释这类球状星团。
英文摘要：
We have studied the chemical composition of NGC 1806, a massive, intermediate-age globular cluster that shows a double main sequence turnoff. We analyzed a sample of high-resolution spectra (secured with FLAMES at the Very Large Telescope) for 8 giant stars, members of the cluster, finding an average iron content of [Fe/H]=--0.60 +- 0.01 dex and no evidence of intrinsic star-to-star variations in the abundances of light elements (Na, O, Mg, Al). Also, the (m_(F814W); m_(F336W)-m_(F814W)) color-magnitude diagram obtained by combining optical and near-UV Hubble Space Telescope photometry exhibits a narrow red giant branch, thus ruling out intrinsic variations of C and N abundances in the cluster. These findings demonstrate that NGC 1806 does not harbor chemically distinct sub-populations, at variance with what was found in old globular clusters. In turn, this indicates that the double main sequence turnoff phenomenon cannot be explained in the context of the self-enrichment processes usually invoked to explain the chemical anomalies observed in old globulars. Other solutions (i.e., stellar rotation, merging between clusters or collisions with giant molecular clouds) should be envisaged to explain this class of globulars.
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