The kinematics of Population II stars

Abstract

Simple dynamical models are made to study the implications of recent kinematic data for Population II stars in the Galactic halo. The models assume a scale-free distribution of stars orbiting within a spherical potential with a flat rotation curve. This potential is consistent with the observed properties of the globular cluster system. A highly flattened star distribution can reproduce the velocity dispersion behavior reported by Ratnatunga and Freeman in 1985 in a variety of high-latitude fields. In particular it can reproduce the small dispersion they observed out to 25 kpc toward the Galactic pole. These data appear to require a spatial distribution which differs substantially both from that of globular clusters and from those assumed by standard models for star count data. The dynamical models require the halo giants to be on near-circular orbits, in conflict with the traditional collapse picture for the formation of Population II.