Planetesimal Formation Through Streaming and Gravitational Instabilities |
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Authors: | Anders Johansen Hubert Klahr |
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Institution: | 1.Leiden Observatory,Leiden University,Leiden,The Netherlands;2.Max-Planck-Institut für Astronomie,Heidelberg,Germany |
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Abstract: | Planets form in circumstellar discs as dust grains collide together and form ever larger bodies. However a major bottleneck
occurs for bodies with sizes around a few centimetres or larger. These rocks and boulders have very poor sticking properties
and spiral into the star in a few hundred years due to friction with the slower rotating gas. A possible way to overcome this
“meter-barrier” is to have local concentrations of rocks and boulders that become gravitationally unstable and contract to
form planetesimals of several kilometers in size. We present the results of numerical simulations of the coupled motion of
gas and rocks in protoplanetary disc mid-planes. The gas rotates slightly sub-Keplerian due to the global, radial pressure
gradient of the disc, causing a net velocity difference between the gas and the solids. This relative motion is in turn unstable
to streaming instabilities, and the saturated state of the turbulence is characterised by dense particle clumps that are fed
by the radial drift of isolated rocks. For realistic protoplanetary disc parameters the clumps are gravitationally unstable
and contract on the time-scale of a few orbits to form bodies of several hundred kilometers in size. Magnetic fields may further
augment the particle concentrations due to relatively long-lived high pressure bumps that form in magnetorotational turbulence.
However, magnetic fields are not crucial to the gravoturbulent formation of planetesimals, rather magnetised turbulence allows
collapse to occur for flows that are on the average less affected by the particles, i.e. for lower particle column densities. |
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