Kinematic compression and expansion of the velocity distributions of particles in gas flows.

Abstract

Velocity distribution functions showing kinematic compression and expansion are derived in the context of gravitational forces. Whereas kinematic compression is well known in laser ion-beam spectroscopy, kinematic expansion is a new concept. In our analysis, collisionless and weak-collision cases are considered. A brief discussion shows that the weak-collision case may be appropriate in certain regions of the interstellar medium and that kinematic compression and expansion could have interesting observational consequences. In particular, as we show elsewhere, kinematic expansion may be important in the formation of broad and asymmetric quasistellar object emission lines.