Deep SDSS optical spectroscopy of distant halo stars

II. Iron, calcium, and magnesium abundances^⋆,⋆⋆

¹ Instituto de Astrofísica de Canarias, vía Láctea, 38205 La Laguna, Tenerife Spain
e-mail: emmafa_ext@iac.es
² Universidad de La Laguna, Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain
³ Research School of Astronomy and Astrophysics, The Australian National University, ACT 2611 Weston, Australia
⁴ Department of Physics and JINA Center for the Evolution of the Elements, University of Notre Dame, Notre Dame, IN 46556, USA
⁵ Institut Utinam, CNRS UMR6213, Université de Franche-Comté, Observatoire de Besançon, 25000 Besançon, France
⁶ Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA 16802, USA
⁷ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
⁸ Department of Astronomy and Space Science, Chungnam National University, 305-764 Daejeon, Republic of Korea
⁹ Apache Point Observatory, PO Box 59, Sunspot, NM 88349-0059, USA

Received: 3 December 2014
Accepted: 20 February 2015

Abstract

Aims. We analyze a sample of 3944 low-resolution (R ~ 2000) optical spectra from the Sloan Digital Sky Survey (SDSS), focusing on stars with effective temperatures 5800 ≤ T_eff ≤ 6300 K, and distances from the Milky Way plane in excess of 5 kpc, and determine their abundances of Fe, Ca, and Mg.

Methods. We followed the same methodology as in the previous paper in this series, deriving atmospheric parameters by χ² minimization, but this time we obtained the abundances of individual elements by fitting their associated spectral lines. Distances were calculated from absolute magnitudes obtained by a statistical comparison of our stellar parameters with stellar-evolution models.

Results. The observations reveal a decrease in the abundances of iron, calcium, and magnesium at large distances from the Galactic center. The median abundances for the halo stars analyzed are fairly constant up to a Galactocentric distance r ~ 20 kpc, rapidly decrease between r ~ 20 and r ~ 40 kpc, and flatten out to significantly lower values at larger distances, consistent with previous studies. In addition, we examine [Ca/Fe] and [Mg/Fe] as a function of [Fe/H] and Galactocentric distance. Our results show that the most distant parts of the halo show a steeper variation of [Ca/Fe] and [Mg/Fe] with iron. We found 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 opposite trend is apparent for [Mg/Fe]. Our conclusion that the outer regions of the halo are more metal-poor than the inner regions, based on in situ observations of distant stars, agrees with recent results based on inferences from the kinematics of more local stars, and with predictions of recent galaxy formation simulations for galaxies similar to the Milky Way.