Convective cooling of protoplanetary disks and rapid giant planet formation

The rapid formation of self-gravitating clumps of gas and dust in a marginally gravitationally unstable disk requires a reasonably efficient means of cooling the disk gas. Clumps form on the dynamical timescale of a few orbital periods in the disk instability scenario. Radiative transfer is not able to cool the midplanes of optically thick protoplanetary disks fast enough to permit the disk instability mechanism to form dense clumps. However, vertically oriented convective cells, driven by the temperature gradient between the disk's midplane and surface, appear to be capable of cooling the disk midplanes on the desired timescale. We demonstrate this possibility by analyzing in detail the vertical convective energy fluxes in the first three-dimensional radiative hydrodynamics model of clump formation by disk instability, which we presented in 2001. Similarly robust convective fluxes occur in the models we presented in 2002. Transient convective cells can be seen in all of these simulations, with vertical velocities ( similar to0.1 km s(-1)) and energy fluxes large enough to cool the outer disk at the desired rate.