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arXiv 0710.0579astro-ph
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主序星上A型星演化的晚期阶段：Praesepe星团8 Am星观测到的化学丰度和扩散模型之间的比较

Late stages of the evolution of A-type stars on the main sequence: comparison between observed chemical abundances and diffusion models for 8 Am stars of the Praesepe cluster

L. Fossati, S. Bagnulo, R. Monier, S. A. Khan, O. Kochukhov, J. Landstreet, G. Wade, W. Weiss

备注：
Accepted for publication on A&A
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摘要：

目标。我们的目标是提供观测约束扩散模型，预测特殊的化学丰度在大气中的Am星。我们还打算检查是否可以用弱磁场的存在来解释化学特性和缓慢旋转。方法.我们已经获得了高分辨率，高信噪比光谱的八个先前分类的Am星，两个正常的A-型星和一个蓝离散星，被认为是Praesepe集群的成员。对于所有这些恒星，我们已经确定了大量化学元素的基本参数和光球丰度，其精度比以往任何时候都要高。对于其中7颗恒星，我们还获得了圆偏振光谱，并应用LSD技术来约束纵向磁场。结果在所有分析过的恒星中都没有发现磁场。HD 73666是一颗蓝离散星，之前被认为是Ap（Si）星，现在被证明拥有正常A型星的丰度。未确认HD 72942的AM分类。对于HD 73709，我们还计算了合成的Delta-a光度，与观测结果吻合良好。有一个普遍良好的协议之间的丰度预测的扩散模型和值，我们已经获得了剩余的Am星。然而，观测到的Na和S丰度分别偏离预测0.6 dex和>0.25 dex。在我们样本中的三颗恒星中，Li似乎过多。

英文摘要：

Aims. We aim to provide observational constraints on diffusion models that predict peculiar chemical abundances in the atmospheres of Am stars. We also intend to check if chemical peculiarities and slow rotation can be explained by the presence of a weak magnetic field. Methods. We have obtained high resolution, high signal-to-noise ratio spectra of eight previously-classified Am stars, two normal A-type stars and one Blue Straggler, considered to be members of the Praesepe cluster. For all of these stars we have determined fundamental parameters and photospheric abundances for a large number of chemical elements, with a higher precision than was ever obtained before for this cluster. For seven of these stars we also obtained spectra in circular polarization and applied the LSD technique to constrain the longitudinal magnetic field. Results. No magnetic field was detected in any of the analysed stars. HD 73666, a Blue Straggler previously considered as an Ap (Si) star, turns out to have the abundances of a normal A-type star. Am classification is not confirmed for HD 72942. For HD 73709 we have also calculated synthetic Delta-a photometry that is in good agreement with the observations. There is a generally good agreement between abundance predictions of diffusion models and values that we have obtained for the remaining Am stars. However, the observed Na and S abundances deviate from the predictions by 0.6 dex and >0.25 dex respectively. Li appears to be overabundant in three stars of our sample.

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