The 10 µm Feature of M-Type Stars in the Large Magellanic Cloud and the Dust Condensation Sequence

Abstract

We present 7-14 μm 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 μm region, which is seen in either emission or (self-) absorption. The shape of the feature changes with increasing mass-loss rate, M˙, 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 M˙ 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.

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 UK) and with the participation of ISAS and NASA.