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Kinematic Substructure of Halo Stars from HIPPARCOS Observations

With highly accurate parallaxes and proper motions observed by HIPPARCOS, we discuss the distributions of velocities and metallicities for a sample of stars selected from the Lowell Proper Motion Catalog. Cluster analysis is used to search for natural groups in the sample and display them graphically. Data presented in this Letter support the picture that the halo is not dynamically mixed. We also demonstrate that the large-scale kinematic substructure in the halo population is responsible for differences in the asymmetric drift and the imbalance in W velocity derived from different surveys. Our results suggest that the accretion of stellar aggregates (e.g., globular clusters, satellite galaxies, or Searle & Zinn "fragments") plays an important part in the formation of the Galaxy.