Study of the evolution of the structure of circumstellar dust envelopes based on the observations of infrared spectra

该项目的科学目标是通过研究演化阶段与星周尘埃包层结构之间的关系，澄清渐近巨星分支（AGB）的演化。在本项目开始之初，我们就意识到从观测上追踪CDE结构的演化变化是非常重要的。因此，我们观察到各种AGB星的CDE在近红外波段（NIR）。根据观测结果，我们发现AGB星中的碳星的空间分布可能与富氧AGB星的空间分布不同，这一结果表明AGB星的演化过程可能与其在银河系中的位置有关，而位置的不同可能是由于物理条件的不同而引起的。因此，我们在近红外光谱中新观测到银河系中约450颗碳星，以澄清碳星与富O AGB星空间分布的差异。我们的观测结果表明，碳星的空间分布不同于富氧AGB星，AGB星的演化过程取决于在银河系中的位置。

The scientific objective of this project is to clarify the evolution of asymptotic giant branch (AGB) stars by investigating the relation between evolutionary stages and the structure of circumstellar dust envelopes (CDE). At the beginning of this project, we consicered that it is very important to trace the evolutionary change of the structure of CDE observationally. Thus we observed variety of AGB stars with CDE in the near-infrared (NIR) wavelength region. As this observational results, we found that the space distribution of carbon stars (one kind of AGB stars) is likely to be different from that of O-rich AGB stars with thickCDE.This result indicate that the evolutionary process of AGB stars may depend on the location in the Galaxy caused by some difference in the physical condition which depends on the location. Thus we have newly observed about 450 carbon stars in the Galaxy in the NIR to clarify the difference in the space distribution of carbon stars from that of O-rich AGB stars. Our observed results suggest that the space distribution of carbon stars is different from that of O-rich AGB stars and that the evolutionary process of AGB stars depend on the location in the Galaxy.