Iso Mid-Infrared Spectroscopy Of Galactic Bulge Agb Stars

Aims. To study the nature of Bulge AGB stars and in particular their circumstellar dust, we have analysed mid-infrared spectra obtained with the ISOCAM CVF spectrometer in three Bulge fields.Methods. The ISOCAM 5-16.5 mu m CVF spectra were obtained as part of the ISOGAL infrared survey of the inner Galaxy. A classification of the shape of the 10 mu m dust feature was made for each case. The spectra of the individual sources were modelled using a radiative transfer model. Different combinations of amorphous silicates and aluminium-oxide dust were used in the modelling.Results. Spectra were obtained for 29 sources of which 26 are likely to be Bulge AGB stars. Our modelling shows that the stars suffer mass loss rates in the range of 10(-8)-5 x 10(-7) M-circle dot/yr, which is at the low end of the mass-loss rates experienced on the thermally pulsing AGB. The luminosities range from 1700 to 7700 L-circle dot as expected for a population of AGB stars with M-init of 1.5-2 M-circle dot. In agreement with the condensation sequence scenario, we find that the dust content is dominated by Al2O3 grains in this sample of low mass-loss rate stars.