Deep SDSS optical spectroscopy of distant halo stars^⋆

I. Atmospheric parameters and stellar metallicity distribution

¹ Instituto de Astrofísica de Canarias, vía Láctea, 38205 La Laguna, Tenerife, Spain
e-mail: callende@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 Astronomy, New Mexico State University, Las Cruces NM 88003, USA
⁵ Department of Astronomy, Case Western Reserve University, Cleveland OH 44106, USA
⁶ Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA 16802, USA
⁷ National Optical Astronomy Observatory, Tucson AZ 85719, USA
⁸ JINA (Joint Institute for Nuclear Astrophysics), Michigan State University, East Lansing MI 48824, USA
⁹ Apache Point Observatory, PO Box 59, Sunspot NM 88349-0059, USA
¹⁰ Lund Observatory, Box 43, 221 00 Lund, Sweden
¹¹ Department of Astronomy, University of Virginia, Charlottesville VA 22903, USA
¹² Institut Utinam, CNRS UMR6213, Université de Franche-Comté, Observatoire de Besançon, 25009 Besançon, France

Received: 23 April 2014
Accepted: 4 June 2014

Abstract

Aims. We analyze a sample of tens of thousands of spectra of halo turnoff stars, obtained with the optical spectrographs of the Sloan Digital Sky Survey (SDSS), to characterize the stellar halo population “in situ” out to a distance of a few tens of kpc from the Sun. In this paper we describe the derivation of atmospheric parameters. We also derive the overall stellar metallicity distribution based on F-type stars observed as flux calibrators for the Baryonic Oscillations Spectroscopic Survey (BOSS).

Methods. Our analysis is based on an automated method that determines the set of parameters of a model atmosphere that reproduces each observed spectrum best. We used an optimization algorithm and evaluate model fluxes by means of interpolation in a precomputed grid. In our analysis, we account for the spectrograph’s varying resolution as a function of fiber and wavelength. Our results for early SDSS (pre-BOSS upgrade) data compare well with those from the SEGUE Stellar Parameter Pipeline (SSPP), except for stars with log  g (cgs units) lower than 2.5.

Results. An analysis of stars in the globular cluster M 13 reveals a dependence of the inferred metallicity on surface gravity for stars with log g < 2.5, confirming the systematics identified in the comparison with the SSPP. We find that our metallicity estimates are significantly more precise than the SSPP results. We also find excellent agreement with several independent analyses. We show that the SDSS color criteria for selecting F-type halo turnoff stars as flux calibrators efficiently excludes stars with high metallicities, but does not significantly distort the shape of the metallicity distribution at low metallicity. We obtain a halo metallicity distribution that is narrower and more asymmetric than in previous studies. The lowest gravity stars in our sample, at tens of kpc from the Sun, indicate a shift of the metallicity distribution to lower abundances, consistent with what is expected from a dual halo system in the Milky Way.