ESO ASTROPHYSICS SYMPOSIA, Munchen, Germany, 18 - 21 October 2003, vol.1, no.1, pp.398-400
Evolution of AGB stars is still under heavy investigations. In this study, we have combined theoretical studies and observations. Comparing theoretical models and observations of oxygen rich AGB stars we have obtained a clear evidance that overall optical depth (T,) shows a strong correlation with ratio of 9.8 mu m and 18 gm band strenghts [9.8/18]. Correlation between the ratio [9.8/18] and (tau(v)) is not effected by stellar temperature and chemical composition of the shell, but inner boundry (sublimation) temperature of the shell plays a critical role on range of optical depth and stage of evolution of dust shell.