ACCRETION IN PROTOPLANETARY DISKS BY COLLISIONAL FUSION

The formation of a solar system such as ours is believed to have followed a multi-stage process around a protostar and its associated accretion disk. Whipple first noted that planetesimal growth by particle agglomeration is strongly influenced by gas drag, and Cuzzi and colleagues have shown that when midplane particle mass densities approach or exceed those of the gas, solid-solid interactions dominate the drag effect. The size dependence of the drag creates a 'bottleneck' at the meter scale with such bodies rapidly spiraling into the central star, whereas much smaller or larger particles do not. Independent of whether the origin of the drag is angular momentum exchange with gas or solids in the disk, successful planetary accretion requires rapid planetesimal growth to kilometer scales. A commonly accepted picture is that for collisional velocities Vc above a certain threshold value, V th∼ 0.1-10 cm s-1, particle agglomeration is not possible; elastic rebound overcomes attractive surface and intermolecular forces. However, if perfect sticking is assumed for all ranges of interparticle collision speeds the bottleneck can be overcome by rapid planetesimal growth. While previous work has dealt with the influences of collisional pressures and the possibility of particle fracture or penetration, the basic role of the phase behavior of matter-phase diagrams, amorphs, and polymorphs-has been neglected. Here, it is demonstrated for compact bodies that novel aspects of surface phase transitions provide a physical basis for efficient sticking through collisional melting/amorphization/polymorphization and subsequent fusion/annealing to extend the collisional velocity range of primary accretion to ΔVc ∼ 1-100 m s-1 >> V th, which encompasses both typical turbulent rms speeds and the velocity differences between boulder-sized and small grains ∼1-50 m s-1. Therefore, as inspiraling meter-sized bodies collide with smaller particles in this high velocity collisional fusion regime they grow sufficiently rapidly to ∼0.1-1 km scale and settle into stable Keplerian orbits in ∼105 years before photoevaporative wind clears the disk of source material. The basic theory applies to low and high melting temperature materials and thus to the inner and outer regions of a nebula.