Discovery of CO_2 emission in AGB stars with the 13mu M dust feature

Abstract

We have detected strong emission lines at 13.48, 13.87, 14.97, 15.40 and 16.28mu m 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 13mu m 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 13.87mu m line in W Hya where individual Q-branch components of the 10(0}0Sigma ({+}_{g)) - 01(1}0Pi_ {u) transition of CO₂ have been detected. The 15.40mu m line is probably due to (13) CO₂. We speculate that the simultaneous occurence of the 13mu m dust feature and the CO₂ emission lines indicates the existence of a warm ( ~ 650-1250K) gas layer close to the star where both the 13mu m dust and the CO₂ emission lines are formed. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA. The SWS is a joint project of SRON and MPE.