Stability of a Collisional Galactic Disk

Abstract

Until recently, only collisionless models have been investigated in the kinetic treatment of stellar disk stability (e.g., Fridman and Polyachenko [1984]). This is due to the fact that the frequency of ordinary binary stellar gravitational (elastic) encounters in the Galaxy is much smaller than the variation of the gravitational field for the process being studied. However, in the pioneering paper Spitzer and Schwarzschild (1951) proposed a different kind of encounter: interaction of stars with gas clouds of the interstellar medium having a mass of rougly 10⁶ M _⊙. In recent years this hypothesis was partially confirmed by observations: it was discovered in the Galaxy a few thousand giant molecular clouds of mass M _c ≥ 10⁵ M _⊙. Other evidence of dynamical relaxation of the star-cloud disk in the solar neighborhood was found by Grivnev and Fridman (1990); the time of relaxation was estimated equal to τ = (2 − 4) × 10⁹ years. Hence the study of collisional star-cloud system is not only of academic interest — on the time span t ≥ 10⁹ years an actual galaxy may be a collisional ensemble of stars and clouds.