Detection of a galactic color gradient for blue horizontal-branch stars of the halo field and implications for the halo age and density distributions

Abstract

From analysis of a large sample of field blue horizontal-branch stars we find that the mean unreddened color, &lt;B-V&gt;<SUB>w</SUB>, in the color window -0.02 &lt;(B-V)<SUB>0</SUB> &lt; +0.18 increases outward in the Galaxy by ∼0.025 on 2 kpc &lt; R &lt; 12 kpc. On the basis of several tests of the sample, we conclude that this result is due neither to errors in reddening estimates nor to contamination of the sample by stars of the young disk or by the population described by Lance (1988b). <P />Calibration of the horizontal-branch (HB) parameter B/(B + R) with two theoretical, age-dependent isochrone parameters and an empirical correlation between &lt;B-V&gt;<SUB>w</SUB> and B/(B + R) suggest that the mean age of the field horizontal branch decreases outward in the halo systematically by a few Gyr. HB models appropriate for standard red giant evolution lead to a similar conclusion, but HB theory also permits an interpretation of the color gradient as a consequence of a systematic outward decrease in HB core mass at constant age, for which there is no plausible reason. <P />We calculate space densities of blue horizontal-branch (BHB) stars from our sample and recalculate space densities for RR Lyrae stars from published data. The RR Lyrae densities are well represented by a spheroidal power-law model in which flattening decreases outward. The power-law exponent for the RR Lyrae stars is n = -3.2±0.1. BHB stars in a more restricted distance range yield n = -3.5, as would be expected from the sign of the color gradient. <P />Predicted densities of BHB and RR Lyrae stars in the solar neighborhood, their ratio (6.5/1), and the abundance distribution and horizontal-branch morphologies of globular clusters are used to estimate the local density of red HB stars and the total local HB number density, 42 kpc<SUP>-3</SUP>. <P />Total HB populations for 44 globular clusters, derived from published photometry and the use of King models, establish that the visual luminosity of a globular cluster is closely proportional to the number of HB stars that it contains. The ratio is L<SUB>v</SUB>/N<SUB>HB</SUB> = 540±40 uncorrected for incompleteness of the counts that would decrease it, or possible radial stratification of HB stars in clusters that would increase it. <P />From the local field HB number density, L<SUB>v</SUB>/N<SUB>HB</SUB>, and an assumed M/L<SUB>v</SUB> = 2.5, we estimate the local halo mass density to be 5.7 × 10<SUP>4</SUP> M<SUB>sun</SUB> kpc<SUP>-3</SUP>, a value within the range estimated by other methods. The local observed density of metal-poor RR Lyrae stars unaccountably exceeds the number predicted by our model by a factor of at least 2.