The effects of retrograde stars on the stability of flat stellar disks are examined. The models used are the isochrones with various fractions of retrograde stars. The most unstable global modes are obtained by numerically integrating the linearized collisionless Boltzmann equation. The results show that the growth rate of the one-armed mode becomes higher as the number of retrograde stars increases. On the contrary, the growth rates of the two- and three-armed modes are lowered by retrograde stars. Thus, the one-armed mode grows faster than the two- and three-armed modes when the disk contains a large number of retrograde stars. The behavior of the one-armed mode can be explained in terms of the inner Lindblad resonance of the wave with retrograde stars.

Key words: Global instabilities; Linear modal calculations; Retrograde stars; Stellar dynamics.