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Asymptotic Giant Branch (AGB) stars are typified by strong dust-driven,molecular outflows. For long, it was believed that the molecular setup of thecircumstellar envelope of AGB stars is primarily determined by the atmosphericC/O ratio. However, recent observations of molecules such as HCN, SiO, and SOreveal gas-phase abundances higher than predicted by thermodynamic equilibrium(TE) models. UV-photon initiated dissociation in the outer envelope ornon-equilibrium formation by the effect of shocks in the inner envelope may bethe origin of the anomolous abundances. We aim at detecting (i.) a group of`parent' molecules (CO, SiO, HCN, CS), predicted by the non-equilibrium studyof Cherchneff (2006) to form with almost constant abundances independent of theC/O ratio and the stellar evolutionary stage on the Asymptotic Giant Branch(AGB), and (ii.) few molecules, such as SiS and SO,which are sensitive to theO- or C-rich nature of the star. Several low and high excitation rotationaltransitions of key molecules are observed at mm and sub-mm wavelengths withJCMT and APEX in four AGB stars: the oxygen-rich Mira WX Psc, the S star W Aql,and the two carbon stars V Cyg and II Lup. A critical density analysis isperformed to determine the formation region of the high-excitation molecularlines. We detect the four `parent' molecules in all four objects, implyingthat, indeed, these chemical species form whatever the stage of evolution onthe AGB. High-excitation lines of SiS are also detected in three stars withAPEX, whereas SO is only detected in the oxygen-rich star WX Psc. This is thefirst multi-molecular observational proof that periodically shocked layersabove the photosphere of AGB stars show some chemical homogeneity, whatever thephotospheric C/O ratio and stage of evolution of the star.

Detection of `parent' molecules from the inner wind of AGB stars as tracers of non-equilibrium chemistry