Spectropolarimetry of the massive post-red supergiants IRC +10420 and HD 179821
Spectropolarimetry of the massive post-red supergiants IRC +10420 and HD 179821
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大质量后红超巨星 IRC 10420 和 HD 179821 的分光偏振测量
DOI：
10.1111/j.1365-2966.2008.12889.x
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发表时间：
2008
影响因子：
4.8
通讯作者：
Patel M
中科院分区：
文献类型：
--
作者：
Patel M
我们给出了两个大质量红后超巨星IRC+10420和HD 179 821的中分辨率光谱偏振和长期光偏振测量。这些数据提供了关于它们的星周物质以及它们的演化的新信息。在IrC+10420中，H-α谱线的极化与连续谱不同，说明电子散射区不是球对称的。在偏振测量中观察到的长期变化可能与沿着扩展反射星云的短轴的轴对称结构有关。长期测光显示，直到90年代中期，这颗恒星的温度一直在上升，此后，光学系统中的光球通量趋于平稳。由于光度变化大多是在红色探测到的，它们不会敏感地追踪到较高的恒星温度。因此，这颗恒星是否已经停止了它的温度上升并不明显。对于HD 179 821，我们没有发现任何吸收或发射线上的偏振效应，但在15年内观测到了非常大的偏振变化，约为5%。在同一时期，光学光度法在0.2毫克的水平上显示出适度的变化。这是意想不到的，因为较大的偏振变化通常伴随着强烈的光度学变化。对于这一令人费解的事实，我们讨论了几种解释。由于没有证据表明在恒星附近存在热的尘埃物质，大多数实验似乎都失败了，这涉及到恒星周围的不对称物质。需要注意的是，稀少可用的近红外测光技术可能错过了强烈偏振活动的周期。这种变化可以用一个非径向脉动的光球的存在来解释。光度学的变化暗示，在过去的10年里，通过两个光谱亚类的变化导致了温度的升高。
We present medium resolution spectropolarimetry and long-term photo-polarimetry of two massive post-red supergiants, IRC +10 420 and HD 179 821. The data provide new information on their circumstellar material as well as their evolution. In IRC +10 420, the polarization of the Hα 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 axisymmetric 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. So, it is not obvious whether the star has halted its increase in temperature or not. For HD 179 821, we find no polarization effects across any absorption or emission lines, but observe very large polarization changes of the order of 5 per cent over 15 yr. During the same period, the optical photometry displayed modest variability at the 0.2 mag level. This is unexpected, because large polarization changes are generally accompanied by strong photometric changes. 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. A caveat is that the sparsely available near-infrared photometry could have missed periods of strong polarization activity. The variations may 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 10 yr.