T. Birnstiel, H. Klahr, B. Ercolano
Context: The global size and spatial distribution of dust is an important
ingredient in the structure and evolution of protoplanetary disks and in the
formation of larger bodies, such as planetesimals. Aims: We aim to derive
simple equations that explain the global evolution of the dust surface density
profile and the upper limit of the grain size distribution and which can
readily be used for further modeling or for interpreting of observational data.
Methods: We have developed a simple model that follows the upper end of the
dust size distribution and the evolution of the dust surface density profile.
This model is calibrated with state-of-the-art simulations of dust evolution,
which treat dust growth, fragmentation, and transport in viscously evolving gas
disks. Results: We find very good agreement between the full dust-evolution
code and the toy model presented in this paper. We derive analytical profiles
that describe the dust-to-gas ratios and the dust surface density profiles well
in protoplanetary disks, as well as the radial flux by solid material "rain
out", which is crucial for triggering any gravity assisted formation of
planetesimals. We show that fragmentation is the dominating effect in the inner
regions of the disk leading to a dust surface density exponent of -1.5, while
the outer regions at later times can become drift-dominated, yielding a dust
surface density exponent of -0.75. Our results show that radial drift is not
efficient in fragmenting dust grains. This supports the theory that small dust
grains are resupplied by fragmentation due to the turbulent state of the disk.
View original:
http://arxiv.org/abs/1201.5781
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