大质量后红超巨星IRC +10420和HD 179821的光谱偏振研究
Spectropolarimetry of the Massive Post-Red Supergiants IRC +10420 and HD 179821
摘要：
我们提出了两个大质量的后红超巨星IRC +10420和HD 179821的中分辨率光谱偏振测量和长期光偏振测量。这些数据提供了关于它们的拱星物质及其演化的新信息。在IRC +10420中，H α线的偏振与连续谱的偏振不同，这表明电子散射区不是球对称的。观测到的长期变化的偏振可以与轴对称的结构，沿扩展反射星云的短轴。长期测光显示，该恒星的温度一直上升到90年代中期，之后光球的光学通量趋于稳定。由于光度变化主要是在红色探测，它们不能灵敏地追踪恒星的高温。因此，这颗恒星是否停止了温度的上升并不明显。对于HD 179821，我们没有发现任何吸收或发射线的偏振效应，但在15年内观察到非常大的偏振变化，约为5%。出乎意料的是，在同一时期，光学测光在0.2等水平上显示出适度的变化。几种解释这一令人困惑的事实进行了讨论。其中大部分涉及到星周物质的不对称性，似乎都失败了，因为没有证据表明恒星附近存在热的尘埃物质。或者，这种变化可以用非径向脉动光球的存在来解释。光度学的变化暗示了温度的增加，这与过去十年中两个光谱子类的变化相对应。
英文摘要：
We present medium resolution spectropolarimetry and long term photo-polarimetry of two massive post-red supergiants, IRC +10420 and HD 179821. The data provide new information on their circumstellar material as well as their evolution. In IRC +10420, the polarization of the Halpha line is different to that of the continuum, which indicates that the electron-scattering region is not spherically symmetric. The observed long term changes in the polarimetry can be associated with an axi-symmetric structure, along the short axis of the extended reflection nebulosity. Long term photometry reveals that the star increased in temperature until the mid-nineties, after which the photospheric flux in the optical levelled off. As the photometric changes are mostly probed in the red, they do not trace high stellar temperatures sensitively. And so, it is not obvious whether the star has halted its increase in temperature or not. For HD 179821 we find no polarization effects across any absorption or emission lines, but observe very large polarization changes of order 5% over 15 years. Unexpectedly, during the same period, the optical photometry displayed modest variability at the 0.2 magnitude level. Several explanations for this puzzling fact are discussed. Most of which, involving asymmetries in the circumstellar material, seem to fail as there is no evidence for the presence of hot, dusty material close to the star. Alternatively, the variations can be explained by the presence of a non-radially pulsating photosphere. Changes in the photometry hint at an increase in temperature corresponding to a change through two spectral subclasses over the past ten years.
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