Growing hydrodynamic modes in Keplerian accretion disks during secondary perturbations: Elliptical vortex effects

The origin of hydrodynamic turbulence, and in particular of an anomalously enhanced angular momentum transport, in accretion disks is still an unsolved problem. This is especially important for cold disk systems that are practically neutral in charge, since the turbulence in those systems cannot be of magnetohydrodynamic origin. While the flow must exhibit some instability and then turbulence in support of the transfer of mass inward and angular momentum outward, according to the linear perturbation theory, in the absence of magnetohydrodynamic effects, it should always be stable. We demonstrate that the three-dimensional secondary disturbance to the primarily perturbed disk, consisting of elliptical vortices, gives significantly large hydrodynamic growth in such a system and hence may suggest a transition to an ultimately turbulent state. This result is essentially applicable to accretion disks around quiescent cataclysmic variables, in protoplanetary and star-forming disks, and the outer region of disks in active galactic nuclei, where the gas is significantly colder, and thus the magnetic Reynolds number is smaller than 10(4).