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Project Progress

It has been suggested that km-sized objects can form through the fragmentation of a gravitationally unstable layer of solid materials in a nebula. However, the difference between the rotational velocities of gas and dust particles in a gaseous disk produces a shear between these layers, which in turn results in turbulence. This turbulence and prevents solid objects from accumulating and increasing their local density to the necessary value for starting gravitational instability. To overcome this problem, models of gravitational instability consider objects with initial sizes larger than 50 cm and do not account for the growth of mm-sized particles (which are formed though hitting and sticking of micron-sized objects) to several cm in size. In this project, we have shown that the key in resolving this issue and filling the gap between mm-sized particles and 50 cm-sized objects is the interactions of dust particles with gas in the vicinity of pressure-enhanced regions. The combined effect of gas-drag and pressure-gradients causes micron-sized dust particles to migrate towards pressure-enhanced regions, while growing to a few millimeter in size (Figure 1).

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These objects accumulate in the location of the maximum gas pressure, where gas and dust rotate with similar Keplreian velocity and no shear-induced turbulence exists. Settling dust particles in the location of maximum gas pressure form clumps of several centimeter in size (Figure 2).

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The appearing and disappearing of pressure enhancements attracts these clumps and forms larger accumulations. These large accumulations, in turn form km-sized bodies by coalescing with one another and/or undergoing gravitational instability.