The structure of the galactic halo outside the solar circle as traced by the blue horizontal branch stars
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A complete sample of blue horizontal branch (BHB) stars in the magnitude range 13.0V16.5 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, b=+37). 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. The completeness of the sample was confirmed by reference to the photometric survey of SA 57 by Stobie & Ishida [AJ, 93, 624 (1987)] that is complete to fainter than V=18. In the color range 0.00(B-V)0+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 Strmgren u filter and the Johnson B and V filters) that measures the size of the Balmer jump, (2) a spectrophotometric index that measures the steepness of the Balmer jump, and (3) a parameter D0.2 that is the mean width of the H and H 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(B-V)0+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+0.23 and in the magnitude range 13.0V16.5. 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 4481 , Ca II 3933 and Fe I 4272 . 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 S method. The space densities of these BHB stars were analyzed both separately and together with earlier observations of field BHB stars given by Arnold & Gilmore [MNRAS, 257, 225 (1992)], Sommer-Larsen & Christensen [MNRAS, 219, 537 (1986)], and Preston et al. [ApJ, 375, 121 (1991a)]. This analysis supports a two-component model for the halo of our Galaxy that is similar in many respects to that proposed by Hartwick [The Galaxy (Reidel, Dordrecht) (1987)] although our discussion refers only to the region outside the solar circle. For Z5 kpc, a classical spherical halo dominates which follows a Rgal-3.5 space-density law and which has a HB morphology like that of the globular cluster M3 (i.e., approximately equal numbers of BHB and RR Lyrae stars). Closer to the galactic plane, there is an additional component with a much flatter galactic distribution (scale height 2.2 kpc near the Sun). The stars of the two components do not have significantly different metallicity distributions but do have slighty different distributions of the parameter which measures the steepness of the Balmer jump; this is the only physical criterion (independent of spatial or kinematic considerations) which distinguishes between the two components. If present estimates of the local RR Lyrae star space density are correct, then the ratio of BHB stars to RR Lyrae stars is higher in the flatter halo component. The flat component would then have a bluer HB morphology (which could be interpreted as making it older) than the spherical component. In the solar neighborhood about 80 percent of the BHB stars come from the flat component and about 20 percent from the spherical component. More than half of the AF stars with V13.0 and (B-V)0+0.23 are not BHB stars but have surface gravities that are more like those expected for main sequence stars. Their measured metallicities lie in the range -0.2[Fe/H]-2.3. The more metal-poor of these stars are probably similar to the blue metal-poor stars that have been discussed by Preston et al. [AJ, 108, 538 (1994)] which, while they probably include globular cluster blue stragglers as a subset, must also comprise stars of other types.
