The nature of the circumstellar CO2 emission from M giants

The 13-16 mu m region observed by the Infrared Space Observatory (ISO) of s everal ABG stars are discussed. We present and analyse spectra of seven M g iants which show carbon dioxide features. To explain the features of the ba nds, we suggest they originate from two different circumstellar layers, one being a warm and high density layer close to the star, possibly making the 15 mu m band optically thick, and the other being a large, cold and optica lly thin layer extending far out in the wind. This could explain the differ ence in temperatures of the different bands found in the analysis of the sp ectra and the number of molecules needed for the emission. It is demonstrated that in spite of the bands probably not being formed in vibrational LTE, the temperatures can be estimated from the widths of the b ands.