INSTABILITIES OF STELLAR DISKS

The stability criteria are obtained for arbitrary localized perturbati ons in the plane of a stellar disk. The complete stabilization require s that Toomre's stability parameter Q be greater than the critical par ameter Q(c) calculated as a function of the parameter alpha(2) = 2 Ohm /r\Ohm'\, which characterizes the extent to which the disk rotation is differential. In particular, for the most interesting case of a flat rotation curve (alpha(2) = 2), the quantity Q(c) turns out to be a uni versal constant (Q(c) approximate to 3.15). The criteria for disk stab ility relative to bending of its plane are also briefly discussed.