| In order to study the structure of the Galaxy and learn about its formation history, we have derived accurate abundances from low-resolution spectra for 302 nearby ab-type RR Lyraes. Radial velocities were derived from the spectra and distances were computed from apparent magnitudes and reddenings taken from the literature.; We find an abrupt change in the kinematics at (Fe/H) = {dollar}-{dollar}1.0; below this value, the slow Galactic rotation and broad velocity dispersions of the halo are apparent, while the more metal-rich stars have the fast rotation and small dispersions characteristic of the thick disk. The kinematic properties and spatial distribution of the disk stars with (Fe/H) {dollar}>{dollar} {dollar}-{dollar}0.45 are consistent with there being a mixture of thick disk and old thin disk populations at these abundances.; The rotational velocity and velocity dispersion are independent of abundance in the halo, supporting chaotic pictures of halo formation such as the fragment accretion picture suggested by Searle & Zinn. Our observations are not consistent with the Sandage & Fouts picture of uniform collapse, spin-up and enrichment in the early Galaxy. The space density of the local halo RR Lyraes is larger than predicted by Preston et al.; either the halo is flatter than Preston estimated, or a two component halo exists. The rotational velocities of the most metal-poor stars ((Fe/H) {dollar}<{dollar} {dollar}-{dollar}1.9) in this and other non-kinematically-selected samples are prograde, and are not influenced by metal-poor disk stars as suggested by Majewski. Perhaps the two-component, Old/Younger Halo model recently proposed by Zinn can account for Majewski's retrograde halo rotation at large Z-heights. The metallicity distribution of the field halo RR Lyraes is broader than those of the RR Lyraes in the inner and outer halo globular clusters; if age is the dominant "second parameter" controlling HB morphology in the halo, this suggests the bulk of the local field halo stars span an age range of at least several billion years, and that a segment of these stars may be younger than the outer halo clusters by 1-2 Gyrs. |
