Cold condensation of dust in the ISM.

The condensation of complex silicates with pyroxene and olivine composition under conditions prevailing in molecular clouds has been experimentally studied. For this purpose, molecular species comprising refractory elements were forced to accrete on cold substrates representing the cold surfaces of surviving dust grains in the interstellar medium. The efficient formation of amorphous and homogeneous magnesium iron silicates at temperatures of about 12 K has been monitored by IR spectroscopy. The gaseous precursors of such condensation processes in the interstellar medium are formed by erosion of dust grains in supernova shock waves. In the laboratory, we have evaporated glassy silicate dust analogs and embedded the released species in neon ice matrices that have been studied spectroscopically to identify the molecular precursors of the condensing solid silicates. A sound coincidence between the 10 microm band of the interstellar silicates and the 10 microm band of the low-temperature siliceous condensates can be noted.