Velocity dispersion of stars and giant molecular clouds

An attempt is made to reproduce the age-dependence of the velocity dispersion of stars on the basis of the stochastic gravitational interaction between stars and the giant molecular clouds (GMCs). The theory of random walk in the field of harmonic oscillation is employed, and the velocity dispersion is formulated under the dynamical friction of stars in their orbital motion, which was ignored by Fujimoto (1980). Firstly, if the dynamical friction is not effective, the GMCs are very responsible for the age-dependence of the velocity dispersion of stars: the observed velocity dispersion of 60-80 km/s is reproduced in t = 10 to the 10th yr for reasonable parameters of the GMCs. Secondly, if the dynamical friction cannot be disregarded, the GMCs are not responsible, and other mechanisms, such as gravitational scattering by massive black holes in the galactic halo and by local matter condensation due to the disk instability, should be considered. The formal application of dynamical friction is also considered.