Two distinct halo populations in the solar neighborhood

Context. In Papers I and II of this series, we have found clear indications of the existence of two distinct populations of stars in the solar neighborhood belonging to the metal-rich end of the halo metallicity distribution function. Based on high-resolution, high S/N spectra, it is possible to distinguish between "high-alpha" and "low-alpha" components using the [/Fe] versus [Fe/H] diagram. Aims. Precise relative ages and orbital parameters are determined for 67 halo and 16 thick-disk stars having metallicities in the range -1.4 < [Fe/H]< -0.4 to better understand the context of the two halo populations in the formation and evolution of the Galaxy. Methods. Ages are derived by comparing the positions of stars in the log T eff -log g diagram with isochrones from the Y 2 models interpolated to the exact [Fe/H] and [/Fe] values of each star. The stellar parameters have been adopted from the preceding spectroscopic analyses, but possible systematic errors in T eff and log g are considered and corrected. With space velocities from Paper I as initial conditions, orbital integrations have been carried out using a detailed, observationally constrained Milky Way model including a bar and spiral arms. Results. The "high-alpha" halo stars have ages 2-3 Gyr larger than the "low-alpha" ones, with some probability that the thick-disk stars have ages intermediate between these two halo components. The orbital parameters show very distinct differences between the "high-alpha" and "low-alpha" halo stars. The "low-alpha" ones have r max 's to 30-40 kpc, z max 's to 18 kpc, and e max 's clumped at values greater than 0.85, while the "high-alpha" ones, r max 's to about 16 kpc, z max 's to 6-8 kpc, and e max values more or less uniformly distributed over 0.4-1.0. Conclusions. A dual in situ-plus-accretion formation scenario best explains the existence and characteristics of these two metal-rich halo populations, but one remaining defect is that this model is not consistent regarding the r max 's obtained for the in situ "high-alpha" component; the predicted values are too small. It appears that Cen may have contributed in a significant way to the existence of the "low-alpha" component; recent models, including dynamical friction and tidal stripping, have produced results consistent with the present mass and orbital characteristics of Cen, while at the same time including extremes in the orbital parameters as great as those of the "low-alpha" component.