Experimental study of the formation and destruction of carbonates in circumstellar and interstellar environments and their impact on the oxygen depletion in the interstellar medium

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Carbonates are found in various astrophysical environments including circumstellar envelopes around evolved stars, protoplanetary disks, solar and extrasolar planets, and comets. Their formation and destruction in astrophysical environments is a complex interplay between different refractory and nonrefractory solids, gaseous components, and irradiation with energetic photons. Carbonates can be formed from silicates and carbonaceous material. A considerable amount of carbon and in particular of oxygen can be sequestered in such inorganic and organic carbonates, which provides a possible solution to the oxygen depletion problem in the interstellar medium. In the course of the project, we are going to simulate the formation and destruction of carbonates in the laboratory at conditions prevailing in various astrophysical environments. Inorganic and organic carbonates will be produced from carbonaceous and silicate dust analogues produced in our laboratory. The carbonation of silicates will be realized by gas phase condensation in CO2/H2O-containing atmospheres, gas-surface reactions between CO, CO2, and H2O molecules and cosmic silicate analogues, reactions of cold silicates or silicate/carbon mixtures with molecular ice composed of CO, CO2, and H2O. In addition, the effect of simultaneous ultraviolet irradiation on the carbonate formation produced from silicates either in the gas phase or at the interface grains/ice will be investigated. The exposure of carbon grains to a bombardment of oxygen atoms and a flow of CO and CO2 molecules will be used to synthesize organic carbonates. Their formation is expected to occur in two reaction steps including the formation of epoxide functional groups and their reaction into cyclic carbonates. IR spectroscopy will be used to monitor the formation of inorganic and organic carbonates in situ. The formation rates will be determined as a function of temperature covering a range between 10 K and ~400 K. IR spectroscopy, electron microscopy, and organic analyses will be used to explain the chemical and structural modifications of the silicate and carbon grains. Besides the formation rate of carbonates, their photon-induced destruction by VUV irradiation plays an important role and has to be analyzed. The results of our studies will provide us significant information on the spectral properties of cosmic carbonates and their potential role in astrochemical and astrophysical processes such as the depletion of oxygen and the formation of complex organic molecules in the interstellar medium, as well as the formation of planets.