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arXiv 1106.0508astro-ph.SR
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早期磁性B型星:X射线发射和风性质

Early magnetic B-type stars: X-ray emission and wind properties

L. M. Oskinova, H. Todt, R. Ignace, J. C. Brown, J. P. Cassinelli, W. -R. Hamann

摘要:

本文对大质量磁性早期B型星的X射线发射和风性质进行了全面的研究。专门的XMM-牛顿观测获得了三颗恒星xi 1 CMa,V2052 Oph和zeta Cas。我们报告的X射线发射的V2052 Oph和zeta Cas的第一次检测。观测结果表明,我们的程序星的X射线谱是相当软的。我们编制了完整的早期B型恒星的样本,检测到的磁场和现有的X射线测量,以研究是否X射线发射可以作为一个一般代理恒星磁性。我们发现硬而强的X射线发射并不一定与磁场的存在相关。利用非LTE铁覆盖模型大气,分析了5颗具有磁场的非超巨星B星的紫外光谱。后者是用PoWR代码计算的,该代码处理光球和风,也考虑了X射线。我们的模型准确地拟合恒星光球光谱的光学和紫外。模型中包含了与观测结果相一致的X射线发射参数、温度和通量。我们证实了早先的发现,即X射线发射材料的填充因子非常高。我们的分析表明,这里研究的磁性早B型星具有弱风。质量损失率显着低于流体动力学一致的模型预测。我们发现,虽然X射线强烈影响风的电离结构,这种效果是不足以减少总辐射加速度。当X射线在所观察到的强度和温度下被考虑时,仍然有足够的辐射加速来驱动比我们从UV谱线经验推断的更强的质量损失。(节略)

英文摘要:

We present a comprehensive study of X-ray emission and wind properties of massive magnetic early B-type stars. Dedicated XMM-Newton observations were obtained for three stars xi1 CMa, V2052 Oph, and zeta Cas. We report the first detection of X-ray emission from V2052 Oph and zeta Cas. The observations show that the X-ray spectra of our program stars are quite soft. We compile the complete sample of early B-type stars with detected magnetic fields to date and existing X-ray measurements, in order to study whether the X-ray emission can be used as a general proxy for stellar magnetism. We find that hard and strong X-ray emission does not necessarily correlate with the presence of a magnetic field. We analyze the UV spectra of five non-supergiant B stars with magnetic fields by means of non-LTE iron-blanketed model atmospheres. The latter are calculated with the PoWR code, which treats the photosphere as well as the the wind, and also accounts for X-rays. Our models accurately fit the stellar photospheric spectra in the optical and the UV. The parameters of X-ray emission, temperature and flux are included in the model in accordance with observations. We confirm the earlier findings that the filling factors of X-ray emitting material are very high. Our analysis reveals that the magnetic early type B stars studied here have weak winds. The mass-loss rates are significantly lower than predicted by hydrodynamically consistent models. We find that, although the X-rays strongly affect the ionization structure of the wind, this effect is not sufficient in reducing the total radiative acceleration. When the X-rays are accounted for at the intensity and temperatures observed, there is still sufficient radiative acceleration to drive stronger mass-loss than we empirically infer from the UV spectral lines. (abridged)

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