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G.R. Stewart (University of Colorado)
Published models of the radial temperature profile of protoplanetary disks show a steep temperature drop between 2AU and 6AU from the central star. The midplane temperature profiles exhibit plateaus and jumps that are caused by the condensation of dust grains and ice grains at different temperatures. Downward steps in the temperature profile imply a local decrease in the radial pressure support such that the angular velocity of the gas will exhibit local minima. This kind of rotation curve produces local minima and maxima in the vorticity of the gas flow and is therefore subject to a Rossby wave instability (Lovelace et al. 1999). Numerical simulations suggest that this kind of instability can produce large vortices and enhanced angular momentum transport in the disk (Li et al. 2001).
A reduced model of the Rossby wave instability can be derived by taking advantage of the small ratio between the radial and azimuthal length scales of the instability. In particular, it is easy to show that the vorticity is much greater than the divergence of the perturbed gas flow. The azimuthal gas flow is determined by a balance between the coriolis force and the radial pressure gradient and is therefore "geostrophic." However, the radial velocity is strongly non-geostrophic. This situation is very similar to what happens during frontalgenesis in the Earth's atmosphere. The reduced model takes a very simple form if the radial length scale is substantially greater than the disk scale height. An outstanding question is: Do these Rossby vortices live long enough to enhance the formation of planetesimals in protoplanetary disks?
This work was supported by NASA's Origins of Solar Systems Program.
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Bulletin of the American Astronomical Society, 34, #3
© 2002. The American Astronomical Society.