THE SPACE DENSITY AND KINEMATICS OF BLUE METAL-POOR MAIN-SEQUENCE STARS NEAR THE SOLAR CIRCLE

From analysis of a photometrically selected sample of 175 metal-poor field stars with main sequence gravities (hereafter BMPs) and UBV colors blueward of the most metal-poor globular cluster main-sequence turnoffs, 0.15<(B - V)0<0.35, and properties of the candidate lists of the HK objective prism survey, we calculate the space density and a suitably defined specific frequency of BMPs within approximately 2 kpc of the Sun. We consider two cases. If we adopt the luminosities and color distribution of globular cluster blue stragglers (hereafter BS) for BMPs, the BMP space density is approximately 350 kpc-3, from which we obtain a specific frequency S(BMP)-8, a value 9 times larger than that of BSs in globular clusters. From this result, we conclude that cluster-type BSs are but a minor component of the field BMPs and that the remainder must be of a different nature. If we adopt luminosities of metal-poor models halfway between the Zero Age Main Sequence and the phase of core hydrogen exhaustion, we obtain a space density of 450 (+300,-150) kpc-3 and a specific frequency S(BMP) = 10(+5,-3). From a subsample of 107 BMPs with available radial velocities we derive a galactic rotation of 128 km s-1 and an isotropic velocity dispersion of sigma(rphitheta) approximately 90 km s-1 values intermediate between those of halo and thick-disk populations. From analysis of a larger sample of stars on 0. 15<(B - V)0<0.35 binned by a crude line-blanketing parameter, we find that our results are insensitive to adopted BMP selection criteria: none of these subsamples of A- and early F-type stars above the galactic plane possess disk kinematics. The region of the UBV two-color diagram occupied by BMPs could be populated by metal-deficient, main-sequence gravity stars with ages substantially younger than those of the metal-poor halo. Because we cannot imagine how or where the observed local population of BMPs could have been produced within our galaxy during the past 3 to 10 Gyr, we suggest that BMPs are the bluest members of metal-poor intermediate-age main sequences accreted, probably, from dwarf spheroidal satellites of the Milky Way during the past 10 Gyr. We discuss observational consequences of this suggestion.