冷标准星外层的结构
Structure of the outer layers of cool standard stars
摘要：
内容：在晚型红巨星中，当我们移动到赫兹-罗素图中较晚的光谱类型时，外层大气层的结构会发生有趣的变化：色球层总是存在，但日冕辐射减少，并且有一股冷的大质量风介入。目的：大多数研究都集中在短波长观测，本文探讨了色球和风对长波长测光测量的影响。研究方法：观测的光谱能量分布进行了比较的理论预测的MARCS大气模型的样本9 K-和M-巨星。发现的差异解释使用基本模型的通量发射来自色球层或电离风。结果：对于9个样本星中的7个，在（亚）毫米和/或厘米波长处检测到明显的通量过剩。过量的精确开始取决于所考虑的恒星。波长短于约1毫米的通量很可能由光学厚的色球层主导，其中光学厚的电离风是较长波长的主要通量贡献者。结论：虽然所研究的K-和M-巨星的光学到中红外光谱可以很好地用辐射平衡大气模型来表示，但在较高海拔处的色球层和/或电离恒星风的存在在（亚）毫米和厘米波长范围内占主导地位。通量过剩的存在也影响了这些恒星在（亚）毫米和厘米波长范围内作为基准分光光度校准器的作用。
英文摘要：
Context: Among late-type red giants, an interesting change occurs in the structure of the outer atmospheric layers as one moves to later spectral types in the Hertzsprung-Russell diagram: a chromosphere is always present, but the coronal emission diminishes and a cool massive wind steps in. Aims: Where most studies have focussed on short-wavelength observations, this article explores the influence of the chromosphere and the wind on long-wavelength photometric measurements. Methods: The observational spectral energy distributions are compared with the theoretical predictions of the MARCS atmosphere models for a sample of 9 K- and M-giants. The discrepancies found are explained using basic models for flux emission originating from a chromosphere or an ionized wind. Results: For 7 out of 9 sample stars, a clear flux excess is detected at (sub)millimeter and/or centimeter wavelengths. The precise start of the excess depends upon the star under consideration. The flux at wavelengths shorter than about 1 mm is most likely dominated by an optically thick chromosphere, where an optically thick ionized wind is the main flux contributor at longer wavelengths. Conclusions: Although the optical to mid-infrared spectrum of the studied K- and M-giants is well represented by a radiative equilibrium atmospheric model, the presence of a chromosphere and/or ionized stellar wind at higher altitudes dominates the spectrum in the (sub)millimeter and centimeter wavelength ranges. The presence of a flux excess also has implications on the role of these stars as fiducial spectrophotometric calibrators in the (sub)millimeter and centimeter wavelength range.
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