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Project Progress

The formation of planetesimals is one of the main unresolved issues of planetary science. Traditionally it is assumed that km-sized objects are formed through collision and growth of cm-sized bodies. However, numerical simulations have shown that the relative velocities of cm-sized objects are so large that their collisions may result in fragmentation. To overcome this difficulty, an alternative mechanism has recently been proposed by Youdin and Shu (2002) in which planetesimals are formed through fragmentation of a gravitationally unstable disk of cm-sized objects at the midplane of the nebula. While small particles settle on the midplane, they form a rich layer of particulate material, which subsequently collapses and breaks into km-sized bodies.

The main issue with this scenario is that because of the differences between the rotational velocities of gas and solid objects, a layer of cm-sized particles will undergo shear-induced turbulence prior to reaching the critical density required for its gravitational instability. In his research, Haghighipour has shown that in a dynamically evolving disk, regions may appear where the density of the gas is locally enhanced, and shear-induced turbulence is non-existence. The presence of pressure gradients on both sides of these structures causes solid objects to undergo inward and outward migrations, and accumulate at the location of maximum gas density (Figure 1). At these locations, pressure gradient vanishes and particles and gas rotate at the same Keplerian velocity. As a result, no shear-induced turbulence is created, and solid objects can settles on the midplane and increase its density to reach the critical value (Figure 2). Haghighipour has shown that this mechanism will facilitate planetesimals formation through gravitational instability, and can help km-sized objects to form in a short time. A paper on the results of this project is in preparation for submission to the Astrophysical Journal.

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