Velocity evolution of disk stars due to gravitational scattering by giant molecular clouds

Abstract

We investigate the evolution of velocity dispersion of disk stars due to gravitational scattering by giant molecular clouds (GMCs) through numerical integration of orbits in a differentially rotating disk. We find that the evolution of the velocity dispersion can be divided into two phases. In the first phase where the velocity dispersion is small and the relative velocity to the GMC is determined by the shear velocity of the galactic disk, the radial and tangential components of the velocity dispersion increase with time as σR,σθ∝t1/2, while the vertical component as σz∝exp(t). In the later phase where the velocity dispersion becomes large enough to govern the relative velocity, all the components increase as σR,σθ,σz∝t1/4 and ratios among them converge to σR:σθ:σz≂1:0.7:0.6, independent of the initial conditions of velocity dispersion. These behaviors agree well with the observed velocity dispersion of disk stars with various ages.