Two populations among the metal-poor field RR Lyrae stars

Abstract

We compute the spatial velocity components and the galactic orbital elements for 209 metal-poor RRLyrae (ab) variable stars in the solar neighborhood using proper motions, radial velocities, and photometric distances available in the literature. The computed orbital elements and published heavy element abundances are used to study relationships between the chemical, spatial, and kinematical characteristics of nearby field RR Lyrae variables. We observe abrupt changes in the stellar spatial and kinematical characteristics when the peculiar velocities relative to the local standard of rest cross the threshold value, km s-1. This provides evidence that the general population of metal-poor RRLyrae stars is not uniform, and includes two spherical subsystems occupying different volumes in the Galaxy. Based on the agreement between typical parameters of corresponding subsystems of field RRLyrae stars and of the globular clusters, studied by us earlier, we conclude that metal-poor stars and globular clusters can be subdivided into two populations, but using different criteria for stars and clusters. We suppose that field stars with velocities below the threshold value and clusters with extremely blue horizontal branches form the spherical, slowly rotating subsystem of the proto-disk halo (related by its origin to the Galactic thick disk). It has a negligible, but non-zero, vertical metallicity gradient. Field stars with fast motion and clusters with redder horizontal branches constitute the spheroidal subsystem of the accreted outer halo, which is approximately two times larger in size than the first subsystem. It has absolutely no metallicity gradients, most of its stars have eccentric orbits, many stars display retrograde motion in the Galaxy, and their ages are comparatively low, supporting the hypothesis that the objects in this subsystem have an extragalactic origin.