The 10 μm feature of M-type stars in the Large Magellanic Cloud and the dust condensation sequence

We present 7 - 14 mu m Infrared Space Observatory (ISO) spectroscopy of 12 M-type evolved stars in the Large Magellanic Cloud (LMC), in order to study the dust mineralogy and condensation process around these stars. The sample stars show a broad dust feature in the 7 - 14 mu m region, which is seen in either emission or (self-) absorption. The shape of the feature changes with increasing mass-loss rate,, suggesting a change in dust. mineralogy as the central star evolves. At low mass-loss rates amorphous alumina and amorphous silicates are observed, while at high mass-loss rates only amorphous silicates are seen, in agreement with the classical condensation sequence expected for these materials. We find a clear correlation between and the peak wavelength. position of the broad dust feature. Our data suggest a strong dependence of the dust mineralogy on the temperature at the dust condensation radius.