Circumstellar Dust Envelopes of M Giant Stars

IRAS data of M giant stars are studied using a spherical dust envelope model. The infrared colors and the strengths of the 10 µm silicate emission band of the IRAS M stars which show the emission band in LRS spectra are well explained by the model with dirty silicate grains. However, some additional grains which have no feature at 10 µm are required in order to explain the spectra of the M stars which do show no features or very weak band emission at 10 µm.