Experimental studies on the low-temperature condensation of cosmic dust in the interstellar medium

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An efficient dust formation process in the ISM at temperatures of about 10 to 20 K is required to keep the balance between dust destruction and formation processes. This process is supposed to occur in molecular clouds where gaseous refractory species accrete on surviving dust grains and finally form a solid layer on those grains. In the first project phase, the cold condensation of siliceous and carbonaceous solids without additional energy has been experimentally proven. The structure and composition of the condensed layers is similar to dust materials that are formed in circumstellar shells of evolved stars. In the second project phase, we focus our activities on the simultaneous accretion of carbonaceous and siliceous species on cold surfaces and the final formation of solid materials from atomic and molecular precursors. Since observations of interstellar dust clearly verify the formation of separated silicates and carbonaceous grains even though siliceous and carbonaceous gaseous species may react to form carbides or silicon organic compounds, we will study possible scenarios that may prevent the formation of carbides or compounds having C-Si bonds. The role of VUV photons that may trigger desorption processes of specific molecular species or chemical reactions provoking selective condensation processes has to be studied experimentally. The spectral properties of such condensates, efficiencies of carbon and silicate formation for different ratios of carbonaceous and siliceous species that accrete on the surfaces of grains, and the chemical bonds at interfaces with regard to the spectral properties will be studied to completely understand the cold condensation process of interstellar dust.