Understanding B-type supergiants in the low metallicity environment of the SMC
Understanding B-type supergiants in the low metallicity environment of the SMC
复制标题
了解 SMC 低金属丰度环境中的 B 型超巨星
DOI：
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发表时间：
2003
期刊：
影响因子：
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通讯作者：
H Germany
中科院分区：
文献类型：
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作者：
C. Trundle;D. Lennon;J. Puls;Peter Schneider Isaac Newton Group of Telescopes;La Pama;S. T. Q. U. Belfast;Belfast;N. I. U. Munchen;H Germany
本文利用在vlt上获得的高分辨紫外光学数据，对7颗SMC B型超巨星和1颗巨星进行了光谱分析。fastwind是一种非LTE的球形线覆盖模式大气代码，用于推导这些恒星的大气和风参数及其绝对丰度。质量损失率，来自H α配置文件，在与金属丰度依赖的理论预测不一致。事实上，SMC恒星的风动量似乎与银河系B型恒星的风动量光度关系（WLR）非常一致，这是一个令人困惑的结果，因为线驱动风理论预测了金属丰度的依赖性。然而，银河系的恒星是使用未覆盖的模型大气进行分析的，这可能会掩盖任何对金属丰度的依赖。在超巨星中观察到平均氮增加了14倍，而在巨星AV 216中只存在4倍的富集。在相同质量范围内的O型矮星和超巨星中也观察到类似的过量氮，这表明额外的氮是在恒星仍然在主序星上时产生的。这些氮的富集可以重现目前的恒星演化模型，其中包括旋转诱导的混合，只有当大的初始旋转速度为300公里s-1的调用。如此大的自转速度似乎与观测到的O型恒星和B型超巨星的v sin i分布不一致。因此，建议目前可用的恒星演化模型需要更有效的混合较低的旋转速度。
Spectroscopic analyses of 7 SMC B-type supergiants and 1 giant have been undertaken using high resolution optical data obtained on the vlt with uves. fastwind, a non-LTE, spherical, line-blanketed model atmosphere code was used to derive atmospheric and wind parameters of these stars as well as their absolute abundances. Mass-loss rates, derived from H α profiles, are in poor agreement with metallicity dependent theoretical predictions. Indeed the wind-momenta of the SMC stars appear to be in good agreement with the wind-momentum luminosity relationship (WLR) of Galactic B-type stars, a puzzling result given that line-driven wind theory predicts a metallicity dependence. However the galactic stars were analysed using unblanketed model atmospheres which may mask any dependence on metallicity. A mean nitrogen enhancement of a factor of 14 is observed in the supergiants whilst only an enrichment of a factor of 4 is present in the giant, AV216. Similar excesses in nitrogen are observed in O-type dwarfs and supergiants in the same mass range, suggesting that the additional nitrogen is produced while the stars are still on the main-sequence. These nitrogen enrichments can be reproduced by current stellar evolution models, which include rotationally induced mixing, only if large initial rotational velocities of 300 km s -1 are invoked. Such large rotational velocities appear to be inconsistent with observed $v sin i$ distributions for O-type stars and B-type supergiants. Hence it is suggested that the currently available stellar evolution models require more efficient mixing for lower rotational velocities.