We have developed a simple LTE model for calculating sample spectra for the case of single-layer slab models. We quantitatively compared the model spectra with the SWS observations of EP Aqr for the different CO2 bands in order to infer the physical properties of the extended atmosphere. The single layer model is able to reproduce on an individual basis the observed band profiles very well even though in some cases the effects of a temperature stratification is discernible in the band profiles. All the bands do show significant optical depth effects.
The parameters derived for the CO2 bands allow us to estimate the local temperature and density structure of the extended atmosphere. When comparing these results with recent hydrodynamical model atmosphere calculations, it is found that the temperatures are slightly lower while the densities are most likely higher than predicted by these models. Both from the temperature and density structure we conclude that our LTE assumption is justified.
The CO2 bands are a good probe to study the outer
extended atmosphere of AGB stars and the region where dust formation
is supposed to take place. Further thermal modeling of other O-rich
AGB stars that exhibit these bands is in progress and will be
presented in a forthcoming paper.
© European Southern Observatory (ESO) 2000
Online publication: August 17, 2000