82 research outputs found
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 10,000 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 10 micons (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 micron 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 steady-state is
quickly attained. 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.Comment: 24 pages, 16 figures, to appear in ApJ 550 (2001
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