Early dust evolution in protostellar accretion disks

We investigate dust dynamics and evolution during the formation of a protostellar accretion disk around intermediate-mass stars via two-dimensional numerical simulations. Using three different detailed dust models, compact spherical particles, fractal ballistic particle cluster agglomeration (BPCA) grains, and ballistic cluster cluster agglomeration (BCCA) grains, we find that even during the early collapse and the first similar to 10(4) yr of dynamical disk evolution, the initial dust size distribution is strongly modified. Close to the disk's midplane coagulation produces dust particles of sizes of several times 10 mum (for compact spherical grains) up to several millimeters (for fluffy BCCA grains), whereas in the vicinity of the accretion shock front (located several density scale heights above the disk), large velocity differences inhibit coagulation. Dust particles larger than about 1 mum segregate from the smaller grains behind the accretion shock. Because of the combined effects of coagulation and grain segregation, the infrared dust emission is modified. Throughout the accretion disk a Mathis, Rumpl, & Nordsieck dust distribution provides a poor description of the general dust properties. Estimates of the consequences of the freezing out of molecules in protostellar disks should consider strongly modified grains. Physical model parameters such as the limiting sticking strength and the grains' resistivity against shattering are crucial factors determining the degree of coagulation reached. In dense regions (e.g., in the midplane of the disk) a steady state is quickly attained; for the parameters used here the coagulation timescale for 0.1 mum dust particles is similar to1 yr (10(-12) g cm(-3)/rho). High above the equatorial plane coagulation equilibrium is not reached as a result of the much lower densities. Here the dust size distribution is affected primarily by differential advection, rather than coagulation. The influence of grain evolution and grain dynamics on the disk's near-infrared continuum appearance during the disk's formation phase is only slight because the most strongly coagulated grains are embedded deep within the accretion disk.