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SUMMARY:On the validity of the super-particle approximation of planetesima
 ls in simulations of gravitational collapse - Rein\, H (Cambridge)
DTSTART:20090929T110000Z
DTEND:20090929T113000Z
UID:TALK20340@talks.cam.ac.uk
CONTACT:Mustapha Amrani
DESCRIPTION:The formation mechanism of planetesimals in protoplanetary dis
 cs is hotly debated. Currently\, the favoured model involves the accumulat
 ion of meter-sized objects within a turbulent disc\, followed by a phase o
 f gravitational instability. At best one can simulate a few million partic
 les numerically as opposed to the several trillion meter-sized particles e
 xpected in a real protoplanetary disc. Therefore\, single particles are of
 ten used as super-particles to represent a distribution of many smaller pa
 rticles. However\, the super- particle approximation is not always valid w
 hen applied to planetesimal formation because the system can be marginally
  collisional (of order one collision per particle per orbit). The super-pa
 rticle approximation is valid only when the system is collisionless. In ma
 ny recent numerical simulations this is not the case and the approach lead
 s to spurious results and enhanched clumping. We present new results from 
 numerical simulations of planetesimal formation through gravitational inst
 ability. A scaled system is studied that does not require the use of super
 -particles. We find that the scaled particles can indeed be used to model 
 the initial phases of clumping if the porperties of the scaled particles a
 re chosen such that all important timescale in the system are equivalent t
 o what is expected in a real protoplanetary disc. This method is explained
  in detail in this paper and we give constraints on the number of particle
 s that one has to use in order to achieve numerical convergence.\nIn order
  to illustrate this we simplify the system: the evolution of particles is 
 studied in a local shearing box\; the particle- particle interactions such
  as gravity\, physical collisions\, and gas drag are solved directly but a
  constant background shear flow without any feedback from the particles is
  assumed. We compare this new method to the standard super-particle approa
 ch and find significant discrepancies in both the require- ment for gravit
 ational collapse and the resulting clump statistics. Our study shows that 
 the formation of planetesimals in a trubulent disk is much harder than pre
 viously reported.
LOCATION:Seminar Room 1\, Newton Institute
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