Cosmic dust formation at cryogenic temperatures

Within a project aimed at studying the formation of interstellar silicates in dense molecular clouds, we have carried out experiments on the accretion of SiO molecules at cryogenic temperatures using SiO-doped superfluid He nanodroplets and solid Ne matrices. Mass spectrometry revealed the formation of Si_xO_x oligomers in the doped droplets, i.e., at a temperature of 0.37 K. Therefore the reactions that produce the oligomers have no energy barrier at their entrance channel. Reaction energies were experimentally determined and the results of theoretical calculations were found to be consistent with the measurements. Absorption spectroscopy at UV and mid-IR wavelengths was performed at various stages of the annealing of SiO-doped Ne matrices and after their complete evaporation. It showed that the matrices were actually doped with Si_xO_x (x = 1-3) species which disappeared during annealing to be replaced with a condensate characterized by a broad IR spectrum peaking near 9.5 μm. High-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy investigations showed that the condensate had on the whole a homogeneous, amorphous structure and the formula SiO. Still, the study of the IR spectra indicated some degree of disproportionation which increased during warming from 8 to 294 K. We have concluded that SiO molecules can accrete at temperatures as low as 13 K into a solid compound. These preliminary experiments have demonstrated the relevance of the techniques to the study of accretion at low temperature. More particularly their results allow us to envisage the formation of silicates in the interstellar medium.