1998A&A...330L..17J

We have detected strong emission lines at 13.48, 13.87, 14.97, 15.40 and 16.28µm in the ISO/SWS spectra of O-rich AGB stars. These lines are only found in the spectra of Miras and semi-regular variables when they also show the 13µm dust emission feature. The lines appear just resolved in the high-resolution (AOT06, resolution ∼1500) SWS spectra that we recently obtained. Here we report the identification of these emission lines as the Q-branches of ro-vibrational transitions in CO₂ molecules. This identification is corroborated by our SWS Fabry-Perot observation of the 13.87µm line in W Hya where individual Q-branch components of the 10⁰0{SIGMA}⁺_g - 01¹0{PI}_u transition of CO₂ have been detected. The 15.40µm line is probably due to ¹³CO₂. We speculate that the simultaneous occurence of the 13µm dust feature and the CO₂ emission lines indicates the existence of a warm (∼650-1250K) gas layer close to the star where both the 13µm dust and the CO₂ emission lines are formed.