天鹅座OB2中氧和硅的丰度：OB慢旋转体样品中的化学均匀性
Oxygen and silicon abundances in Cygnus OB2: Chemical homogeneity in a sample of OB slow rotators
摘要：
天鹅座OB 2是银河系中一个富含OB的星协，在过去的2000 - 2500万年里经历了激烈的恒星形成。它的恒星星族显示出年龄和银河经度之间的相关性。探索其恒星成分的化学组成，我们将能够检查出生分子云的均匀程度和自富集过程的可能影响。我们的目标是确定天鹅座OB 2中8个早期型慢旋转星样本的硅和氧丰度，以检查整个协会中可能存在的不均匀性，以及是否存在化学成分与银河经度的相关性。我们已经进行了光谱分析的样本晚O和早期B星的低转速，这已被选定，以便覆盖整个协会的地区。我们进行了分析的基础上的等效宽度的金属线，翅膀的H巴耳末线和FASTWIND恒星大气模型，以确定其恒星的基本参数以及硅和氧的表面丰度。我们推导出整个区域的硅和氧丰度的相当均匀的分布，平均值为12+log（Si/H）=7.53 $\pm$ 0.08 dex和12+log（O/H）=8.65 $\pm$ 0.12 dex。我们在天鹅座OB 2中发现了均匀的化学成分，没有明显的化学自富集证据，尽管该地区历史上有强烈的恒星风和可能的超新星迹象。与不同的恒星风和超新星的化学富集方案相比，恒星形成效率不明显高于10%。我们发现的均匀性程度与基于HII区域观察到的银河系氧梯度一致。我们还发现，天鹅座OB 2内的氧散射至少是相同的顺序比HII地区在类似的银河系中心的距离。
英文摘要：
Cygnus OB2 is a rich OB association in the Galaxy which has experienced intense star formation in the last 20-25 Myr. Its stellar population shows a correlation between age and Galactic longitude. Exploring the chemical composition of its stellar content we will be able to check the degree of homogeneity of the natal molecular cloud and possible effects of self-enrichment processes. Our aim is to determine silicon and oxygen abundances for a sample of eight early-type slow rotators in Cygnus OB2 in order to check possible inhomogeneities across the whole association and whether there exists a correlation of chemical composition with Galactic longitude. We have performed a spectroscopic analysis of a sample of late O and early B stars with low rotational velocity, which have been chosen so as to cover the whole association area. We have carried out an analysis based on equivalent widths of metal lines, the wings of the H Balmer lines and FASTWIND stellar atmosphere models to determine their stellar fundamental parameters as well as the silicon and oxygen surface abundances. We derive a rather homogeneous distribution of silicon and oxygen abundances across the region, with average values of 12+log(Si/H)=7.53$\pm$0.08 dex and 12+log(O/H)=8.65$\pm$0.12 dex. We find a homogeneous chemical composition in Cygnus OB2 with no clear evidence for significant chemical self-enrichment, despite indications of strong stellar winds and possible supernovae during the history of the region. Comparison with different scenarios of chemical enrichment by stellar winds and supernovae point to star forming efficiencies not significantly above 10%. The degree of homogeneity that we find is consistent with the observed Milky Way oxygen gradient based on HII regions. We also find that the oxygen scatter within Cygnus OB2 is at least of the same order than among HII regions at similar Galactocentric distance.
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