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

在10 ~ 20k的温度下，需要在ISM中高效的粉尘形成过程，以保持粉尘破坏和形成过程之间的平衡。这一过程被认为发生在分子云中，气态难熔物质聚集在残存的尘埃颗粒上，并最终在这些颗粒上形成固体层。在项目的第一个阶段，硅质和碳质固体的冷冷凝在没有额外能量的情况下已经被实验证明。压缩层的结构和组成类似于在演化恒星的星周壳中形成的尘埃物质。在第二个项目阶段，我们将重点关注冷表面上碳质和硅质物质的同时增加，以及原子和分子前体最终形成固体材料。由于星际尘埃的观测清楚地证实了分离的硅酸盐和碳质颗粒的形成，即使硅质和碳质气态物质可能会反应形成碳化物或硅有机化合物，我们将研究可能阻止碳化物或具有C-Si键的化合物形成的可能情况。VUV光子的作用可能引发特定分子种类的解吸过程或引发选择性缩合过程的化学反应，必须进行实验研究。为了全面了解星际尘埃的冷冷凝过程，我们将研究这些冷凝物的光谱特性、不同比例的碳质和硅质物质在颗粒表面聚集时碳和硅酸盐的形成效率以及界面上的化学键对光谱特性的影响。

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.