Dust coagulation and fragmentation in molecular clouds. I. How collisions between dust aggregates alter the dust size distribution

In dense molecular clouds collisions between dust grains alter the ISM-dust size distribution. We study this process by inserting the results from detailed numerical simulations of two colliding dust aggregates into a coagulation model that computes the dust size distribution with time. All collisional outcomes -- sticking, fragmentation (shattering, breakage, and erosion) -- are included and the effects on the internal structure of the aggregates are also tabulated. The dust aggregate evolution model is applied to an homogeneous and static cloud of temperature 10 K and gas densities between 10^3 and 10^7 cm^-3. The coagulation is followed locally on timescales of ~10^7 yr. We find that the growth can be divided into two stages: a growth dominated phase and a fragmentation dominated phase. Initially, the mass distribution is relatively narrow and shifts to larger sizes with time. At a certain point, dependent on the material properties of the grains as well as on the gas density, collision velocities will become sufficiently energetic to fragment particles, halting the growth and replenishing particles of lower mass. Eventually, a steady state is reached, where the mass distribution is characterized by a mass spectrum of approximately equal amount of mass per logarithmic size bin. The amount of growth that is achieved depends on the cloud's lifetime. If clouds exist on free-fall timescales the effects of coagulation on the dust size distribution are very minor. On the other hand, if clouds have long-term support mechanisms, the impact of coagulation is important, resulting in a significant decrease of the opacity on timescales longer than the initial collision timescale between big grains.