CO2 emission in EP Aqr: Probing the extended atmosphere

We present an analysis of the ISO/SWS full resolution scan between 12.5 and 16.5 mu m of the O-rich AGE star EP Aqr, exhibiting a number of strong CO2 emission bands. We have developed a simple LTE model to calculate theoreti cal CO2 spectra assuming a single-layer slab geometry and compared the SWS observations to this model in order to infer the physical properties of the extended atmosphere. The single layer slab model is able to reproduce the individual band profiles quite well with optically thick bands (column dens ities typically 10(18.5) cm(-2)). The derived excitation temperatures for t he different bands are in the range T similar to 350 - 700 K in a region wh ich extends from similar to 4 - 9R*. The fundamental CO2 bending mode at 14 .98 mu m furthermore shows evidence for an optically thin component arising from a much cooler (T similar to 100 K) and more extended (R-em similar to 100R*) layer. The strong spectral signature of (CO2)-C-13 in the spectrum allows an (uncertain) determination of the C-12/C-13 ratio similar to 10. T he parameters derived for the CO2 bands allow us to estimate the local temp erature and density structure of the extended atmosphere. We find that the derived local gas temperatures are somewhat lower than predicted by hydrody namical model calculations whereas the local gas densities are in good agre ement with these models when using CO2 abundances derived from chemical net work calculations. The CO2 layer extends from close to the stellar photosph ere to the inner part of the dust forming region which makes it a unique ne w probe of the whole extended atmosphere and the region where dust formatio n takes place.