Pulsation Phase-Dependent Dust Shell Models for Oxygen-rich Asymptotic Giant Branch Stars

Abstract

We investigate the spectral energy distributions (SEDs) of oxygen-rich asymptotic giant branch (AGB) stars at different pulsation phases using infrared observational data, including data from the Infrared Space Observatory. Comparing the results of detailed radiative transfer model calculations with observations, we explore the changes of the relevant parameters of the dust shells and central stars depending on the pulsation phase. We find that the schemes of the SED changes for low mass-loss rate O-rich AGB (LMOA) stars are quite different from those for high mass-loss rate O-rich AGB (HMOA) stars. For LMOA stars, we find that the dust formation temperature is much lower than 1000 K, the stronger stellar winds produce more dust grains, and the dust shell optical depth increases at the maximum phase. For HMOA stars, the deep silicate absorption features show significant variations depending on the pulsation phase, mainly due to changes in the properties of the dust shells. Considering the dust formation and evaporation processes in HMOA stars, we propose three possible dust models to explain the SED changes. Contrary to previous investigations, we find that the models that do not require dust evaporation at the maximum phase produce SEDs similar to the observations.

Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands, and the United Kingdom) and with the participation of ISAS and NASA.