Metal abundances of RR Lyrae variables in selected galactic star fields. II. The Lick astrograph fields near the North Galactic Pole

Revised periods, light curves, and metal abundances (Preston Δs's and corresponding [Fe/H] values) are given for nearly 40 RR Lyrae variables of the Lick North Galactic Pole survey. The most distant stars lie at a height z above the galactic plane of 20--25 kpc. The data give no evidence for composition gradients in the RR Lyraes apart from the composition difference between the variables in the halo (<Δs>=7.7, <[Fe/H]>=-1.5) and the disk (<Δs>=5.0, <[Fe/H]>=-1.0). We show from a statistical argument that the sample of stars is too small to detect an abundance gradient over the 20-kpc interval in z unless the space density of RR Lyraes in Oosterhoff group I falls off about four times faster than the RR Lyraes of group II. Difficulties in inferring the halo abundance gradient for common subdwarfs, based on studies of halo RR Lyraes, are cited. It is not clear whether the paucity of RR Lyraes with [Fe/H]< or approx. =-2.2 is related to a corresponding lack of extremely metal-poor subdwarfs or due to an effect of stellar evolution