Early evolution of the Galactic halo revealed from Hipparcos observations of metal-poor stars

Abstract

The kinematics of 122 red giants and 124 RR Lyrae variables in the solar neighborhood is studied using accurate measurements of their proper motions by the Hipparcos astrometry satellite, combined with the published photometric distances, metal abundances and radial velocities. A majority of these sample stars have metal abundances with [Fe/H]<-1 and thus represent the old stellar populations in the Galaxy. The halo component with [Fe/H]<-1.6 is characterized by no systemic rotation and a radially elongated velocity ellipsoid. About 16% of such metal-poor stars have low orbital eccentricities e<0.4, and we see no evidence for the correlation between [Fe/H] and e. Based on the model for the e distribution of orbits, we show that this fraction of low e stars for [Fe/H]<-1.6 is explained from the halo component alone, without introducing the extra disk component claimed by recent workers. This is also supported by no significant change of the e distribution with the height from the Galactic plane. This metal-weak thick disk component appears to comprise only about 10% for -1.6< [Fe/H]<-1 and 20% for -1.4<[Fe/H]<-1. It is also verified that the disk has the mean rotation of about 195 km/s and the vertical extent of 1 kpc, which is consistent with the thick disk dominating at [Fe/H]=-0.6 to -1. The implications of these results for the early evolution of the Galaxy are also presented.