Rossby wave instability of Keplerian accretion disks

We find a linear instability of nonaxisymmetric Rossby waves in a thin nonm agnetized Keplerian disk when there is a local maximum in the radial profil e of a key function L(r) drop F(r)S-2/Gamma(r), where F-1 = (z) over cap . (V x v)/Sigma is the potential vorticity, S = P/Sigma(Gamma) is the entropy , Sigma is the surface mass density, P is the vertically integrated pressur e, and r is the adiabatic index. We consider in detail the special case whe re there is a local maximum in the disk entropy profile S(r). This maximum acts to trap the waves in its vicinity if its height-to-width ratio max(S)/ Delta r is larger than a threshold value. The pressure gradient derived fro m this entropy variation provides the restoring force for the wave growth. We show that the trapped waves act to transport angular momentum outward. A plausible way to produce an entropy variation is when an accretion disk is starting from negligible mass and temperature, therefore, negligible entro py. As mass accumulates by either tidal torquing, magnetic torquing, or Roc he-lobe overflow, confinement of heat will lead to an entropy maximum at th e outer boundary of the disk. Possible nonlinear developments from this ins tability include the formation of Rossby vortices and the formation of spir al shocks. What remains to be determined from hydrodynamic simulations is w hether or not Rossby wave packets (or vortices) "hold together" as they pro pagate radially inward.