The Possible Influence of Interstellar Clouds on Stellar Velocities. II.

Abstract

The increase with time of random stellar velocities, as a result of gravitational encounters with interstellar cloud complexes, has been recomputed, taking into account the presence of differential galactic rotation. As a result of such nonuniform rotation, the clouds will have velocities relative to one another even if their random velocities are zero; the gravitational potential of these clouds will he a fluctuating function of time; and the stars will gain kinetic energy from the clouds. To explain the increase of velocity dispersion with advancing spectral type along the main sequence, the mass of a typical cloud complex must be in the neighborhood of 106m0, the value found previously; but the random velocity of a cloud complex, as a whole, is irrelevant and may be vanishingly small. Since inhomogeneities of density with the required scale of some 300 parsecs or more seem indicated by the extinction observations, it seems not unlikely that star-cloud encounters are, in fact, responsible for the greater velocity dispersion of the later-type, older stars of population type I.