A Spectroscopic Study of the Ancient Milky Way: F‐ and G‐Type Stars in the Third Data Release of the Sloan Digital Sky Survey

Abstract

We perform an analysis of spectra and photometry for 22,770 stars included in the third data release (DR3) of the Sloan Digital Sky Survey (SDSS). We derive atmospheric parameters and distances. Our analysis procedures are throughly checked using three recently published spectroscopic libraries of nearby stars and alternative methods. The SDSS sample covers a range in stellar brightness of 14 <3 kpc show a much more compact metallicity distribution, with a maximum at [Fe/H]~=-0.7, and a median Galactic rotation lagging the local standard of rest by 63 km s-1. A comparison of color indices and metal abundances with isochrones indicates that no significant star formation has taken place in the halo in the last ~11 Gyr, but there are thick-disk stars that are at least 2 Gyr younger. We find the metallicities of thick-disk stars to be nearly independent of Galactocentric distance between 5 and 14 kpc from the Galactic center, in contrast with the marked gradients found in the literature for the thin disk. No vertical metallicity gradient is apparent for the thick disk, but we detect a gradient in its rotational velocity of -16+/-4 km s-1 kpc-1 between 1 and 3 kpc from the plane. We estimate that among the stars in our sample there are over 2000 with an iron abundance [Fe/H] <-3.