DISCOVERY OF CO2 EMISSION IN AGB STARS WITH THE 13 MU-M DUST FEATURE

We have detected strong emission lines at 13.48, 13.87, 14.97, 15.40 a nd 16.28 mu m in the ISO/SWS spectra of O-rich AGE stars. These lines are only found in the spectra of Miras and semi-regular variables when they also show the 13 mu m dust emission feature. The lines appear ju st resolved in the high-resolution (AOT06, resolution similar to 1500) SWS spectra that we recently obtained. Here we report the identificat ion of these emission lines as the Q-branches of re-vibrational transi tions in CO2 molecules. This identification is corroborated by our SWS Fabry-Perot observation of the 13.87 mu m line in W Hya where individ ual Q-branch components of the 10(0)0 Sigma(g)(+) - 01(1)0 Pi(u) trans ition of CO2 have been detected. The 15.40 mu m line is probably due t o (CO2)-C-13. We speculate that the simultaneous occurence of the 13 m u m dust feature and the CO2 emission lines indicates the existence of a warm (similar to 650-1250 K) gas layer close to the star where both the 13 mu m dust and the CO2 emission lines are formed.