Sunday, June 5, 2016

How dusty are photoevaporative winds?

How dusty are photoevaporative winds?


Hutchinson et al


Not very. We perform dusty smoothed particle hydrodrodynamic (SPH) calculations of photoevaporation in protoplanetary discs. We use unequal-mass particles to resolve more than five orders of magnitude in disc/outflow density and a one-fluid formulation to efficiently simulate an equivalent magnitude range in drag stopping time. We find that only micron sized dust grains and smaller can be entrained in photoevaporative winds. This makes it difficult to explain the dust holes seen in transition discs using photoevaporation and implies that only small grains can be transported to the outer disc by this mechanism. A pileup of micron sized dust grains can occur in the upper atmosphere at critical radii in the disc as grains decouple from the low-density wind. Entrainment is a strong function of location in the disc, resulting in a size sorting of grains in the outflow---the largest grain being carried out between 10--20AU. The peak dust density for each grain occurs at the inner edge of its entrainment region.

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