Molecular Features in ISO-SWS Spectra of O-Rich Long-Period Variables

Abstract

The combination of ISO-SWS spectra from projects fkersch:orichsrv/ orichsrl and jhron:varlpv/varlpv2 allows us to cover a wide range of pulsational characteristics among AGB variables of type Mira, SRa, SRb and Lb and to follow selected objects through their pulsational cycle in order to probe for the interrelation of pulsation, stellar structure and mass loss. As expected we found features of CO, SiO, H2O, OH and probably CO2 in the ISO-SWS spectra of AGB variables. All bandheads predicted by spectra from CO and SiO linelists can be clearly seen. The features of CO, SiO and OH are very prominent in the spectra of relatively hot SR variables, whereas they are only weak or disappear completely in cool Mira stars. The spectra of cool Mira variables are dominated by absorption of H2O and maybe CO2. Synthetic CO and SiO spectra based on hydrostatic atmospheres predict very strong bands for the coolest stars, which is absolutely not in agreement with the observations. Also for hotter SR variables some of the features come out too strong (Figs. 1 and 2). However, we do not expect hydrostatic models to be able to reproduce the atmospheres of the coolest and most extended AGB variables (Miras). Exploratory spectra based on dynamical models predict a strong variability of spectral features, which has already been observed. Any interpretation of ISO-SWS spectra of cool giants has to take into account the molecular absorption caused by water.