Planetesimal Formation in the Dead Zone of Protoplanetary Disks

Details Planetesimal Formation in the Dead Zone of Protoplanetary Disks Planetesimal Formation in the Dead Zone of Protoplanetary Disks

Oct 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010dps....42.2403b&link_type=abstract

American Astronomical Society, DPS meeting #42, #24.03; Bulletin of the American Astronomical Society, Vol. 42, p.958

Astronomy and Astrophysics

Astronomy

Scientific paper

Planetesimals are building blocks of both terrestrial and giant planets, yet their formation remains a mystery. The main difficulty comes from the meter-size barrier: meter-sized bodies suffer from rapid radial drift towards the central star due to the gas drag. Recently, it has been found that the inclusion of back-reaction from solids to gas leads to a powerful drag instability: the streaming instability (SI). SI generates turbulence and is able to concentrate centimeter to meter sized bodies into dense clumps, triggering gravitational collapse to form planetesimals directly, bypassing the meter barrier. We conduct local 2D and 3D hybrid simulations of particles and gas in the midplane of protoplanetary disks (PPDs) using the Athena code. Particles and gas are coupled aerodynamically, characterized by the dimensionless stopping time κ=Ωts. Magnetorotational turbulence is neglected as appropriate for the dead zone of PPDs. Self-gravity is ignored since we focus on the precursor of planetesimal formation: particle clumping. We have systematically explored the parameter space relevant for planetesimal formation, including: 1) A wide distribution of particle sizes (κ) 2) The height-integrated solid to gas mass ratio (Z); and 3) The strength of the disk radial pressure gradient (Π). The saturated state of our simulations is characterized by particle settling balanced by turbulent diffusion due to the SI, and the particle drift velocities are well described by a multi-species NSH equilibrium, which generalizes the original Nakagawa-Sekiya-Hayashi solution to include multiple particle sizes. We find that favorable conditions for planetesimal formation via SI include: large particles (κ>0.01), large solid abundance (Z>0.01), and small pressure gradient (Π<0.05). Moreover, there exists two positive feedback loops with respect to the enrichment of local disk solid abundance and grain growth. These results suggest that planetesimal formation may be less difficult than previously thought.

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