Studies on physical and chemical processes in the outer atmospheres of red supergiants by means of high precision spectroscopy

1. 利用高分辨率CCD进行AGB恒星的高色散光谱研究：利用冈山天体物理天文台的双星光谱仪引入的高分辨率CCD，可以观测到暗至10等的特殊恒星的高色散光谱。通过这个新工具，我们已经发现，在某些情况下，CH恒星的^<12>C/^<13>C比率非常大，并且在不同的恒星之间表现出很大的变化。此外，我们确定了j型恒星的^<12>C/^<13>C比率，包括在IRAS LRS中显示硅酸盐发射的特殊碳星。发现j型星的^<12>C/^<13>C比值分布在2 ~ 10之间，具有硅酸盐发射的特殊碳星的比值出现在该范围的上限。这为我们理解特殊碳星的演化提供了一个重要的观测约束，需要进一步的观测来扩展我们的结果。利用FTS对高分辨率红外光谱进行定量分析：利用FTS实现的高精度、高分辨光谱，我们首次发现CO一阶泛音波段的低激发谱线存在光球吸收无法解释的强度异常。结果表明，过量吸收产生于冷亮恒星外层大气中一个额外的分子形成区。这一事实可以看作是对分子在外层大气中由于分子冷却而产生热不稳定性而形成分子理论的观测证实。其次，我们发现冷巨星的光谱线的微分位移和不对称性往往比其他恒星更大。这一事实意味着不依赖于深度和各向同性的高斯模型不能被证明是正确的。由于这个原因，只有通过能够独立于微湍流分析的弱谱线，才能以可接受的精度测定丰度。从CO第二泛音的微弱谱线中，我们发现早期M巨星的碳丰度与G-K巨星几乎相同，而在温度较低的发光M巨星中，碳丰度又进一步下降了0.3个指数。然而，这样的结果很难被包括第二次和第三次打捞在内的标准恒星演化理论所理解，因此，应该存在一个AGB演化过程中碳逐渐减少的演化序列。这与M ->MS ->S ->SC ->C ->PN（富碳）序列形成鲜明对比，后者显示出由于第三次疏通而逐渐增加的碳。这种分歧的原因及其对疏浚的影响仍有待澄清。少

1. High dispersion spectroscopy of AGB stars by high resolution CCD : By the use of high resolution CCD introduced to the coude spectrograph of Okayama Astrophysical Observatory, high dispersion spectra of peculiar stars as dark as 10 mag. can be observed . By this new tool, we already found that ^<12>C/^<13>C ratios in CH stars are extremely large in some cases and show large variation from star to star. Also, we determined ^<12>C/^<13>C ratios in J-type stars, including peculiar carbon stars that show silicate emission in IRAS LRS. It is found that ^<12>C/^<13>C ratios in J-type stars distribute between 2 and 10, and those of peculiar carbon stars with silicate emission appear at the upper limit of this range. This offers an important observational constrain in our understanding on the evolution of the peculiar carbon stars and further observations should be carried out to extend our result.2. Quantitaive analysis of the high resolution infrared spectra by FTS : By means of high prec … More ision spectroscopy realized by FTS, we first found that low excitation lines of the CO first overtone bands show intensity anomalies that cannot be explained by photospheric absorption alone. It is shown that the excess absorption originates in an extra molecular formation zone in the outer atmosphere of cool luminous stars. This fact can be regarded as an observational confirmation of the theory of molecular formation by the thermal instability due to molecular cooling in the outer atmosphere. Second, it is found that the differential shifts and asymmetries of spectral lines originating from stellar photosphere tend to be larger in cool giants than in other stars. This fact implies that the depth-independent and isotropic Gaussian model for micro-turbulence cannot be justified. For this reason, abundance determination with acceptable accuracy can be possible only by weak lines that can be analyzed independently of micro-turbulence. From weak lines of the CO second overtone, we found that the carbon abundances in early M giant stars are almost the same as in G-K giants, while they show further decrease by another 0.3dex in cooler luminous M giants. Such a result, however, is difficult to understand by the standard theory of stellar evolution including the second and third dredge-ups, thus, there should be an evolutionary sequence that shows gradual decrease of carbon during AGB evolution. This shows marked contrast to a sequence of M ->MS ->S ->SC ->C ->PN (C-rich) that shows gradual increase of carbon due to the third dredge-up. The reason of such a bifurcation and its implication for dredge-up remain to be clarified. Less