On the Relative Ages of the $\alpha$-Rich and $\alpha$-Poor Stellar   Populations in the Galactic Halo
  We study the ages of $\alpha$-rich and $\alpha$-poor stars in the halo using a sample of F and G dwarfs from the Sloan Digital Sky Survey (SDSS). To separate stars based on [$\alpha$/Fe], we have developed a new semi-empirical spectral-index based method and applied it to the low-resolution, moderate signal-to-noise SDSS spectra. The method can be used to estimate the [$\alpha$/Fe] directly providing a new and widely applicable way to estimate [$\alpha$/Fe] from low-resolution spectra. We measured the main-sequence turnoff temperature and combined it with the metallicities and a set of isochrones to estimate the age of the $\alpha$-rich and $\alpha$-poor populations in our sample. We found all stars appear to be older than 8 Gyr confirming the idea that the Galactic halo was formed very early on. A bifurcation appears in the age-metallicity relation such that in the low metallicity regime the $\alpha$-rich and $\alpha$-poor populations are coeval while in the high metallicity regime the $\alpha$-rich population is older than the $\alpha$-poor population. Our results indicate the $\alpha$-rich halo population, which has shallow age-metallicity relation, was formed in a rapid event with high star formation, while the $\alpha$-poor stars were formed in an environment with a slower chemical evolution timescale. 
Keith Hawkins, Paula Jofre, Gerry Gilmore, Thomas Masseron
astro-ph.GA astro-ph.SR