The structure of the galactic halo outside the solar circle as traced by the blue horizontal branch stars

A complete sample of blue horizontal branch (BHB) stars in the magnitude range 13.0 less than or equal to V less than or equal to 16.5 is isolated in two Galactic fields that have previously been searched for RR Lyrae variables: SA 57 in the Northern Polar Cap and the Lick Astrograph field RR 7 in the Anticenter (l = 183 deg, b = +37 deg). These BHB stars are a subset of the AF stars found in the Case Low-Dispersion Northern Survey; lists of these AF stars were made available by the late Nick Sanduleak. In the color range 0.00 less than or equal to (B - V)0 less than or equal to +0.20, we can distinguish the BHB stars among these AF stars by comparing them both with well known local field horizontal branch (FHB) stars and also the BHB members of the halo globular clusters M3 and M92. The criteria for this comparison include (1) a (u - B)K color index (derived from photoelectric observations using the Stroemgren u filter and the Johnson B and V filters) that measures the size of the Balmer jump, (2) a spectrophotometric index lambda that measures the steepness of the Balmer jump, and (3) a parameter D0.2 that is the mean width of the H-delta and H-gamma Balmer lines measured at 20 percent of the continuum level. These criteria give consistent results in separating BHB stars from higher gravity main sequence AF stars in the color range 0.00 less than or equal to (B - V)0 less than or equal to +0.20. All three photometric and spectrophotometric criteria were measured for 35 stars in the SA 57 field and 37 stars in the RR 7 field that are in the color range (B - V)0 less than or equal to +0.23 and in the magnitude range 13.0 less than or equal to V less than or equal to 16.5. For a small number of additional stars only (u - B)K was obtained. Among the AF stars that are fainter than B = 13 and bluer than (B - V)0 = +0.23, about half of those in the SA 57 field and about one third of those in the lower latitude RR 7 field are BHB stars. Isoabundance contours were located empirically in plots of the pseudoequivalent width versus (B - V)0 for the lines of Mg II lambda-4481 A, Ca II lambda-3933 A, and Fe I lambda-4272 A. Solar abundances were defined by the data from main sequence stars in the Pleiades and Coma open clusters. Data from the BHB stars in M3 and M92 defined the (Fe/H) = -1.5 and -2.2 isoabundance contours, respectively. Metallicities of all stars were estimated by interpolating the measured pseudoequivalent widths in these diagrams at the observed (B - V)0. The distribution of (Fe/H) found for the BHB stars in this way is very similar to that which we found for the RR Lyrae stars in the same fields using the Preston Delta-S method. The space densities of these BHB stars were analyzed both separately and together with earlier observations of field BHB stars. This analysis supports a two-component model for the halo of our Galaxy that is similar in many respects to that proposed by Hartwick although our discussion refers only to the region outside the solar circle.