Metal abundances of RR Lyrae variables in selected Galactic star fields. V - The Lick Astrographic fields at intermediate Galactic latitudes
Additional Document Info
We derive S metallicities for 60 RR Lyrae variables in three new Lick Astrographic Survey fields at intermediate Galactic latitudes: (l, b) = (86, +35), (52, +35), and (18, +25). S values are also given for five nearby field RR Lyraes without previously published values, and improved S values are listed for 24 nearby RR Lyraes. We have combined these data with the data from the previous Lick surveys and the work of Saha and Oke to study the nature of the Galactic halo in the galactocentric distance range 4-30 kpc based on 171 RR Lyraes. We find that the nature of the metallicity of the Galactic halo changes at the solar circle. Outside the solar circle, the metallicity gradient is zero as a function of R or |z|. The average S outside the solar circle corresponds to <[Fe/H]> = -1.65 with a dispersion of 0.30 dex. Inside the solar circle, the gradient in [Fe/H] is -0.06 dex kpc-1. In comparison with the globular cluster population, outside the solar circle the field RR Lyraes and globular clusters have the same mean metallicity and dispersion, while inside the solar circle the field RR Lyraes are more metal-rich than the globular clusters. However, when the field RR Lyraes are compared with the RR Lyraes in globular clusters, we find that the metallicity distribution is the same over the whole distance range. On the assumption that the two populations are the same, we derive a ratio of globular cluster mass to halo mass of 0.02, a luminous halo mass of 9 108 M in the range 4-25 kpc, and a local space density for luminous halo matter of 5.5 1.8 10-5 M pc-3. The ratio of solar neighborhood metal-rich to metal-poor RR Lyraes is shown to be consistent with the probability of formation of an RR Lyrae in the thick disk versus the halo population. Finally, we show that the field RR Lyraes obey a periodshift-metallicity relationship similar to that in the globular clusters, and that the field RR Lyraes also exhibit the two "Oosterhoff" groups seen in the clusters.