The Dust Sequence along the AGB

With Spitzer--IRS we observed a homogeneous sample of AGB stars in the Galactic Bulge. The Spitzer--IRS wavelength range 5.2 - 38 mum contains the major spectral features of aluminium oxides and magnesium--iron oxides as well as those of amorphous and various crystalline silicates. Previous studies have shown these components to be abundant in AGB environments, albeit with highly varying relative abundances. The stars in our sample all originate from about 1.5 solar mass stars, and differ only in their ages along the AGB. Moreover, this sample fully covers the range in AGB mass-loss rates, from the onset of mass loss on the AGB up to the superwind phase. We can therefore link for the first time the spectral differences in dust composition directly to the age on the AGB. Our observations provide a unique observational baseline for studies of dust evolution in AGB stars in our own and other galaxies.