The field BHB stars, along with the RR Lyrae variables and the carbon stars, are excellent tracers of the galactic halo stellar population. A detailed knowledge of their chemical and dynamical characteristics, therefore, is essential to understand whether the Galaxy formed by a monolithic collapse or in a hierarchical accretion/merging process.
A first problem our group was interested in, in collaboration with Kinman (NOAO) and Castelli (OATs), was the definition of the local halo characteristics using BHB stars in the solar neighborhood brighter than V = 11, i.e. nearer than about 1.2 kpc. Various types of observational data were collected and analyzed in order to select a pure sample of true BHB stars and eliminate the contamination from A-type stars belonging to the disk population: particularly important were high-resolution spectra to derive accurate elemental abundances, as well as radial and rotation velocities. We were thus able to isolate about 30 BHB stars (Kinman et al. 2000). Further dynamical studies will compare their galactic orbits with those of other nearby halo stars.
The questions of whether the high galactic halo is in retrograde rotation and how the velocity dispersion and flattening of the halo vary with height above the galactic plane are still controversial. They could be settled by studying halo stars nearer than about 10 kpc. In collaboration with Kinman, Buzzoni (TNG) and Chavez (Mexico) we are studying a sample of about 150 BHB and RR Lyrae stars near the North Galactic Pole, in the magnitude range from V=11 to 16, by means of radial velocities and GSC2 proper motions. Spectroscopic observations at the TNG are planned for spring 2001.