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arXiv 0906.4330astro-ph.GA
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疏散星团NGC 6475（M 7）T = 4500-10000 K恒星化学丰度的光谱研究

A spectroscopic study of the Open Cluster NGC 6475 (M 7) Chemical Abundances from stars in the range T = 4500-10000 K

Sandro Villanova, Giovanni Carraro, Ivo Saviane

备注：
8 pages, 5 eps figures, in press in Astronomy and Astrophysics, abstract shortened to fit in
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摘要：

星团是研究银河系及其邻居的动力学和化学演化的关键对象。特别是化学成分是从不同种类的恒星（热主序星，冷主序星，水平分支星，RGB星）使用不同的方法。我们的目标是将这些方法应用于疏散星团NGC 6475的恒星。通过对最重要的元素进行普查，我们将能够测试它们的一致性。我们从ESO数据库中选取了疏散星团NGC 6475的7颗恒星的高信噪比高分辨率光谱，它们的T $_{\rm eff}$范围为4500-10000 K，光度等级为V（矮星）和III（巨星）。我们测定了几种元素的化学丰度。对于热星（T $_{\rm eff}$ $>$ 9000 K），我们采用巴耳末线拟合方法得到大气参数。对于冷星（T $_{\rm eff}$ $<$ 6500 K），我们使用FeI/II丰度平衡方法。对于两组恒星，必须使用不同的线列表。使用LTE近似。对于受NLTE偏差影响的元素（C、N、O、Na、Mg），进行了校正。通过测量谱线的等效宽度，得到了许多元素的丰度。对于那些只有混合线可用的元素（O，He），使用了真实光谱与合成光谱的比较。V和Ba考虑了超精细结构。

英文摘要：

Clusters of stars are key objects for the study of the dynamical and chemical evolution of the Galaxy and its neighbors. In particular chemical composition is obtained from different kinds of stars (hot main-sequence stars, cool main-sequence stars, horizontal-branch stars, RGB stars) using different methodologies. Our aim is to apply these methodologies to the stars of the Open Cluster NGC 6475. Obtaining a census of the most important elements we will be able to test their consistence. We finally want to establish more robust fundamental parameters for this cluster.We selected high S/N high resolution spectra of 7 stars of the Open Cluster NGC 6475 from the ESO database covering the T$_{\rm eff}$ range 4500-10000 K and of luminosity class V (dwarf) and III (giants). We determined the chemical abundances of several elements. For hot stars (T$_{\rm eff}$$>$9000 K) we applied the Balmer Lines fitting method to obtained atmospheric parameters. For cool stars (T$_{\rm eff}$$<$6500 K) we used the FeI/II abundance equilibrium method. For the two groups of stars the use of different line-lists was mandatory. LTE approximation was used. For elements affected by NLTE deviation (C,N,O,Na,Mg) corrections were applied. The abundance of many elements were obtained from the measurement of the equivalent width of spectral lines. For those elements for which only blended lines were available (O, He) comparison of real spectrum with synthetic ones was used. Hyperfine structure was taken in account for V and Ba.

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