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

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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