Protostars and Planets VI, Heidelberg, July 15-20, 2013
Formation of planetesimals in collapsing particle clouds
Wahlberg Jansson, Karl (Lund Observatory)
Johansen, Anders (Lund Observatory)
In protoplanetary disks around young stars planets form from initially μm-sized dust particles. In the formation process the size of the particles increases with many orders of magnitude: μm dust to km-sized planetesimals to planets with sizes up to >10,000 km. Formation of planets is done in different stages with different governing forces. From planetesimals to planets gravity is the dominant force and planetesimals can be accumulated to form terrestrial planets and giant planet cores. The earliest stages can also be explained by coagulation of dust particles. When small dust particles collide they can stick together via surface forces, van der Waals forces. This process works up to mm-cm-sized particles when particles tend to bounce or fragment rather than stick when they collide. I work on one way to overcome this barrier.
I investigate a scenario where I have a self-gravitating cloud of particles of a certain size that is in virial equilibrium. I place the cloud at some distance from the Sun and assume that the initial size of the cloud is equal to the Hill sphere of the mass of the cloud. In this cloud, as the particles move around, collisions between particles will occur. Next I assume that these collisions are not elastic so some energy is dissipated in each collision. Since the cloud is in virial equilibrium it will therefore get more and more bound and contract for each collision. This is a runaway process and results in the collapse of the cloud into a solid body. If you do an analytic approximation of this collapse time you find that a Pluto mass cloud of cm-sized particles at a Pluto distance from the Sun and assume perfectly inelastic collisions it collapses on a time scale of order years only. However collisions might not be perfectly inelastic and during the collapse the particles start to move faster and faster and at some point the collisions will result in fragmentation instead of bouncing. Nonetheless this is an interesting result which I plan to investigate in computer simulations where I can add an initial size distribution, coagulation and fragmentation to the calculations.
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