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 2D numerical simulations. Using three different detailed dust models, compact spherical particles, fractal BPCA grains, and BCCA grains, we find that even during the early collapse and the first ∼ 10 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 10μm (for compact spherical grains) up to several mm (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 μm segregate from the smaller grains behind the accretion shock. Due to the combined effects of coagulation and grain segregation the infrared dust emission is modified. Throughout the accretion disk a MRN 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 mid-plane of the disk) a steadystate is quickly attained; for the parameters used here the coagulation time scale for 0.1 μm dust particles is ∼ 1 yr (10−12 g cm−3/%). High above the equatorial plane coagulation equilibrium is not reached due to 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. Subject headings: accretion, accretion disks — hydrodynamics — radiative transfer — solar system: formation — stars: formation