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We analyze a sample of 3,944 low-resolution (R ~ 2000) optical spectra fromthe Sloan Digital Sky Survey (SDSS), focusing on stars with effectivetemperatures 5800 < Teff < 6300 K, and distances from the Milky Way plane inexcess of 5 kpc, and determine their abundances of Fe, Ca, and Mg. We followedthe same methodology as in the previous paper in this series, derivingatmospheric parameters by chi2 minimization, but this time we obtained theabundances of individual elements by fitting their associated spectral lines.Distances were calculated from absolute magnitudes obtained by a statisticalcomparison of our stellar parameters with stellar-evolution models. Theobservations reveal a decrease in the abundances of iron, calcium, andmagnesium at large distances from the Galactic center. The median abundancesfor the halo stars analyzed are fairly constant up to a Galactocentric distancer ~ 20 kpc, rapidly decrease between r ~ 20 and r ~ 40 kpc, and flatten out tosignificantly lower values at larger distances, consistent with previousstudies. In addition, we examine the [Ca/Fe] and [Mg/Fe] as a function of Fe/Hand Galactocentric distance. Our results show that the most distant parts ofthe halo show a steeper variation of the [Ca/Fe] and [Mg/Fe] with iron. Wefound that at the range -1.6 < [Fe/H] < -0.4 [Ca/Fe] decreases with distance,in agreement with earlier results based on local stars. However, the oppositetrend is apparent for [Mg/Fe]. Our conclusion that the outer regions of thehalo are more metal-poor than the inner regions, based on in situ observationsof distant stars, agrees with recent results based on inferences from thekinematics of more local stars, and with predictions of recent galaxy formationsimulations for galaxies similar to the Milky Way.

Deep SDSS optical spectroscopy of distant halo stars II. Iron, calcium, and magnesium abundances