Non-stoichiometric amorphous magnesium-iron silicates in circumstellar
  dust shells. Dust growth in outflows from supergiants
  We investigate the dust growth in oxygen-rich stellar outflows for a set of
nine well-observed massive supergiants with optically thin dust shells. Models
of the infrared emission from their circumstellar dust shells are compared to
their observed infrared spectra so as to derive the essential parameters that
govern dust formation in the extended envelope of these stars. The results
obtained from the comparative study are also compared with the predictions of a
model for silicate dust condensation solely based on laboratory data and basic
stellar properties. The infrared emission in the wavelength range between 6 and
25 mu can be reproduced rather well by a mixture of non-stoichiometric
iron-bearing silicates, alumina, and metallic iron dust particles for all nine
objects. The observed spectra obtained from three objects, mu Cep, RW Cyg, and
RS Per, can be reproduced by a stationary and (essentially) spherically
symmetric outflow which enables a direct comparison with predictions from a
theoretical dust growth model. The temperature at the onset of massive silicate
dust growth is of the order of 920 K and the corresponding outflow velocity of
the order of the sound velocity for these objects. The condensation temperature
suggests that the silicate dust grows on the corundum dust grains that are
formed well in the interior of the silicate dust shell at a much higher
temperature. Our results propose that regarding the two major problems of dust
formation in stellar outflows: (i) formation of seed nuclei; (ii) their growth
to macroscopic dust grains, we are gradually coming close to a quantitative
understanding of the second item.
