疏散星团作为银河系化学演化的关键示踪物。III。Berkeley 20、Berkeley 29、Collinder 261和Melotte 66的元素丰度
Open clusters as key tracers of Galactic chemical evolution. III. Element abundances in Berkeley 20, Berkeley 29, Collinder 261, and Melotte 66
摘要：
银河系疏散星团长期以来被认为是研究铁和其他金属径向分布的最佳工具之一。我们利用VLT的FLAMES收集了大样本疏散星团中明亮巨星的UVES光谱，这些恒星跨越了银河系中心距离、年龄和金属丰度的广泛范围。我们在这里提出的结果，为四个集群：伯克利20和伯克利29，两个最遥远的集群在样本中;科林斯261，最古老的和最小的银河系中心距离之一; Melotte 66。使用光谱代码MOOG和Kurucz模型大气进行了等效宽度分析，以获得Fe，Al，Mg，Si，Ca，Ti，Cr，Ni，Ba的丰度;通过直接线轮廓拟合获得非LTE Na丰度。我们获得了两个反中心星系团Be 20的太阳下金属丰度（[Fe/H]=-0.30，rms = 0.02）和Be 29（[Fe/H]=-0.31，rms = 0.03），对于Mel 66（[Fe/H]=-0.33，rms = 0.03），位于银河系第三象限，而Cr 261位于银河系中心，具有较高的金属丰度（[Fe/H]=+0.13，rms = 0.05 dex）。α元素Si、Ca和Ti，以及Fe峰元素Cr和Ni一般接近太阳能; s过程元素Ba增强。Na丰度的非LTE计算表明太阳标度值，这表明先前在疏散星团的巨星中确定的Na增强可能是由于被忽视的非LTE效应。我们的结果支持Fe径向梯度在距离银河系中心约10 - 11 kpc处存在陡峭的负斜率，而在外盘中，[Fe/H]分布似乎是平坦的。所有测量的元素比是在非常好的协议与那些发现类似的金属盘星和银心距离似乎没有趋势。
英文摘要：
Galactic open clusters are since long recognized as one of the best tools for investigating the radial distribution of iron and other metals. We employed FLAMES at VLT to collect UVES spectra of bright giant stars in a large sample of open clusters, spanning a wide range of Galactocentric distances, ages, and metallicities. We present here the results for four clusters: Berkeley 20 and Berkeley 29, the two most distant clusters in the sample; Collinder 261, the oldest and the one with the minimum Galactocentric distance; Melotte 66. Equivalent width analysis was carried out using the spectral code MOOG and Kurucz model atmospheres to derive abundances of Fe, Al, Mg, Si, Ca, Ti, Cr, Ni, Ba; non-LTE Na abundances were derived by direct line-profile fitting. We obtain subsolar metallicities for the two anticenter clusters Be 20 ([Fe/H]=-0.30, rms=0.02) and Be 29 ([Fe/H]=-0.31, rms=0.03), and for Mel 66 ([Fe/H]=-0.33, rms=0.03), located in the third Galactic quadrant, while Cr 261, located toward the Galactic center, has higher metallicity ([Fe/H]=+0.13, rms=0.05 dex). The alpha-elements Si, Ca and Ti, and the Fe-peak elements Cr and Ni are in general close to solar; the s-process element Ba is enhanced. Non-LTE computations of Na abundances indicate solar scaled values, suggesting that the enhancement in Na previously determined in giants in open clusters could be due to neglected non-LTE effects. Our results support the presence of a steep negative slope of the Fe radial gradient up to about 10-11 kpc from the Galactic center, while in the outer disk the [Fe/H] distribution seems flat. All the elemental ratios measured are in very good agreement with those found for disk stars of similar metallicity and no trend with Galactocentric distance seems to be present.
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