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

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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.