Scenarios of planetesimal formation have been plagued by a difficult puzzle: micron-sized dust grains that accompany the gas around a new star at extraordinarily low density ( 10-10 g/cm3) must somehow clump together to form kilometer-sized objects and eventually planets. Direct interaction between grains at such low densities would be so rare as to be negligible, and at such low concentration the kinetic energy of grains will always be too large to allow gravitational collapse to occur. Although gravity pull dust grains toward the mid-plane of the protoplanetary disk, our numerical simulations show that shear instabilities disrupt the dust layer before a Goldreich-Ward gravitational instability could occur. However, 3D, long-lived vortices provide just the kind of environment required to concentrate dust enough for agglomeration or gravitation to create planetesimals. Our numerical simulations have shown that 3D vortices exist as stable, long-lived solutions to the equations of motion of the gas in the disk around a star. These simulations indicate that 3D vortices form from breaking internal gravity waves that are generated by turbulent motion of the gas or by the interaction of existing 3D vortices. The simulations also show that 3D vortices are stable attractors of dust grains. Notably, the simulations show that dust grains can be stabilized even in vortices off the mid-plane of the disk, where the "downward” (toward the mid-plane of the disk) pull of gravity must be counterbalanced by "upward” (away from the mid-plane) gas drag, in much the same way that hailstones are lofted in thunderstorms.
AAS/Division for Planetary Sciences Meeting Abstracts #39
- Pub Date:
- October 2007