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We investigate the dust growth in oxygen-rich stellar outflows for a set ofnine well-observed massive supergiants with optically thin dust shells. Modelsof the infrared emission from their circumstellar dust shells are compared totheir observed infrared spectra so as to derive the essential parameters thatgovern dust formation in the extended envelope of these stars. The resultsobtained from the comparative study are also compared with the predictions of amodel for silicate dust condensation solely based on laboratory data and basicstellar properties. The infrared emission in the wavelength range between 6 and25 mu can be reproduced rather well by a mixture of non-stoichiometriciron-bearing silicates, alumina, and metallic iron dust particles for all nineobjects. The observed spectra obtained from three objects, mu Cep, RW Cyg, andRS Per, can be reproduced by a stationary and (essentially) sphericallysymmetric outflow which enables a direct comparison with predictions from atheoretical dust growth model. The temperature at the onset of massive silicatedust growth is of the order of 920 K and the corresponding outflow velocity ofthe order of the sound velocity for these objects. The condensation temperaturesuggests that the silicate dust grows on the corundum dust grains that areformed well in the interior of the silicate dust shell at a much highertemperature. Our results propose that regarding the two major problems of dustformation in stellar outflows: (i) formation of seed nuclei; (ii) their growthto macroscopic dust grains, we are gradually coming close to a quantitativeunderstanding of the second item.

Non-stoichiometric amorphous magnesium-iron silicates in circumstellar dust shells. Dust growth in outflows from supergiants