Early dust evolution in protostellar accretion disks

We investigate dust dynamics and evolution during the formation of a protos tellar accretion disk around intermediate-mass stars via two-dimensional nu merical simulations. Using three different detailed dust models, compact sp herical 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 dy namical disk evolution, the initial dust size distribution is strongly modi fied. Close to the disk's midplane coagulation produces dust particles of s izes of several times 10 mum (for compact spherical grains) up to several m illimeters (for fluffy BCCA grains), whereas in the vicinity of the accreti on shock front (located several density scale heights above the disk), larg e velocity differences inhibit coagulation. Dust particles larger than abou t 1 mum segregate from the smaller grains behind the accretion shock. Becau se of the combined effects of coagulation and grain segregation, the infrar ed 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 mol ecules in protostellar disks should consider strongly modified grains. Phys ical 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 dis k) a steady state is quickly attained; for the parameters used here the coa gulation 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 distri bution is affected primarily by differential advection, rather than coagula tion. The influence of grain evolution and grain dynamics on the disk's nea r-infrared continuum appearance during the disk's formation phase is only s light because the most strongly coagulated grains are embedded deep within the accretion disk.