DUST COAGULATION IN DENSE MOLECULAR CLOUDS - THE FORMATION OF FLUFFY AGGREGATES

Dust extinction observations and the calculation of gas-dust dynamics indicate that in dense clumps of molecular clouds dust grains coagulat e efficiently. We set up a detailed model for the dust coagulation pro cess in dense cores of molecular clouds without beginning star formati on. We took into account the effects of thermal, turbulent, gravitatio nal motion, motion from incidential particle asymmetries, grain rotati on, charges, and the accretion of molecules onto the particles. For mo st effects, we developed new formalisms. For the first time, we explic itly considered the irregularity and changing fluffiness of the cluste rs produced in the coagulation process. The basis for this treatment w as an independent numerical simulation of the structure of such aggreg ates. Here, we considered especially the behaviour of aggregates small er than the fractal limit and composed of subgrains with a spectrum of sizes. We fitted the structure parameters by analytic functions which were used in the final model for the coagulation of the interstellar particles. With this model we carried out numerous simulations of the evolution of dust grain distributions in dense cores. The particles we re characterized by two parameters, the particle mass and a quantity r elated to the internal density of the particles. Different gas densiti es, clump models, accretion rates, and initial grain size distribution s were investigated. It was found that the main force driving the aggr egation of dust particles in dense clumps is turbulence at gas densiti es below 10(8) H-atoms per cm3 and Brownian motion at higher densities . The coagulation velocity is considerably influenced by electric char ges on the grains. Both dust coagulation and ice accretion lead to a r apid growth of the smallest particles whereas the upper grain size lim it is only slightly shifted. The resulting size and density distributi on will be narrow on the grain mass scale but broad in the internal de nsity parameters of the coagulates. The total opacity of the resulting distributions of fluffy dust agglomerates was calculated using effect ive-medium theories combined with a core-mantle model for the aggregat e particles. The far infrared absorptivity is enhanced by the factor 3 (at 200 mum) in the first steps of the coagulation process and hardly influenced by the further coagulation. For gas densities between 10(6 ) and 10(9) CM-3 and timescales below 10(5) yrs, the coagulation proce ss is efficient in changing the optical properties of the dust particl es but not in the production of large heavy particles.