INSTABILITIES OF COUNTERROTATING STELLAR DISKS

Stellar disks having two equal populations of stars rotating in opposi te directions are subject to several distinct instabilities. Here we f ocus on the six lowest-order modes: three within the disk plane and th ree bending modes. The form and vigor of the principal instabilities i n any individual model vary with the balance between radial and azimut hal pressure and with disk thickness: thin radially hot models have di sruptive axisymmetric bending instabilities (bell modes), while an in- plane lopsided instability is the most disruptive for cool models. All instabilities are weakened, but most rather slowly, by increasing dis k thickness. The nonlinear evolution of the more unstable models takes an exotic variety of forms including lopsided and warped disks, perma nent asymmetries about the initial plane, and pairs of counterrotating bars. Some of the radially hotter models acquire a thick subcomponent in the disk center resembling a bulge in appearance. Remarkably, howe ver, the instabilities in a model having intermediate radial pressure cause rather mild changes and lead to an apparently stable, moderately thin, and almost axisymmetric disk. This model indicates that the SO galaxy NGC 4550, which has recently been discovered to possess two cou nterrotating disks, does not require large quantities of unseen mass t o stablize it. This final state is also an unusual counterexample to t he claim that instabilities should always thicken any nonrotating stel lar system flatter than E7.