SIMBAD references

We present an analysis of the ISO/SWS full resolution scan between 12.5 and 16.5 µm of the O-rich AGB star EP Aqr, exhibiting a number of strong CO₂ emission bands. We have developed a simple LTE model to calculate theoretical 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 densities typically 10^18.5cm^–2). The derived excitation temperatures for the different bands are in the range T∼350-700K in a region which extends from ∼4-9R_*. The fundamental CO₂ bending mode at 14.98µm furthermore shows evidence for an optically thin component arising from a much cooler (T∼100K) and more extended (R_em∼100R_*) layer. The strong spectral signature of ¹³CO₂ in the spectrum allows an (uncertain) determination of the ¹²C/¹³C ratio ∼10. The parameters derived for the CO₂ bands allow us to estimate the local temperature and density structure of the extended atmosphere. We find that the derived local gas temperatures are somewhat lower than predicted by hydrodynamical model calculations whereas the local gas densities are in good agreement with these models when using CO₂ abundances derived from chemical network calculations. The CO₂ layer extends from close to the stellar photosphere to the inner part of the dust forming region which makes it a unique new probe of the whole extended atmosphere and the region where dust formation takes place.