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

Abstract

We analyze a sample of 3,944 low-resolution (R ~ 2000) optical spectra from the Sloan Digital Sky Survey (SDSS), focusing on stars with effective temperatures 5800 < Teff < 6300 K, and distances from the Milky Way plane in excess of 5 kpc, and determine their abundances of Fe, Ca, and Mg. We followed the same methodology as in the previous paper in this series, deriving atmospheric parameters by chi2 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. 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 the [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 the [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.

References in corpus (52)

• The Sloan Digital Sky Survey: Technical Summary
• The Seventh Data Release of the Sloan Digital Sky Survey
• The 2.5 m Telescope of the Sloan Digital Sky Survey
• SDSS-III: Massive Spectroscopic Surveys of the Distant Universe, the Milky Way Galaxy, and Extra-Solar Planetary Systems
• The Baryon Oscillation Spectroscopic Survey of SDSS-III
• The Ninth Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the SDSS-III Baryon Oscillation Spectroscopic Survey
• SEGUE: A Spectroscopic Survey of 240,000 stars with g=14-20
• The New Galaxy: Signatures of its Formation
• The Multi-Object, Fiber-Fed Spectrographs for SDSS and the Baryon Oscillation Spectroscopic Survey
• The Tenth Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the SDSS-III Apache Point Observatory Galactic Evolution Experiment
• Stellar Chemical Signatures And Hierarchical Galaxy Formation
• Two Stellar Components in the Halo of the Milky Way
• The Milky Way Tomography with SDSS: II. Stellar Metallicity
• Two distinct halo populations in the solar neighborhood. Evidence from stellar abundance ratios and kinematics
• The SEGUE Stellar Parameter Pipeline. I. Description and Initial Validation Tests
• Structure and Kinematics of the Stellar Halos and Thick Disks of the Milky Way Based on Calibration Stars from SDSS DR7
• The accretion origin of the Milky Way's stellar halo
• A Spectroscopic Study of the Ancient Milky Way: F- and G-Type Stars in the Third Data Release of the Sloan Digital Sky Survey
• The SEGUE Stellar Parameter Pipeline. II. Validation with Galactic Globular and Open Clusters
• Oxygen Abundances in Nearby FGK Stars and the Galactic Chemical Evolution of the Local Disk and Halo
• The stellar halo of the Galaxy
• The SEGUE Stellar Parameter Pipeline. III. Comparison with High-Resolution Spectroscopy of SDSS/SEGUE Field Stars
• Mapping the stellar structure of the Milky Way thick disk and halo using SEGUE photometry
• Two distinct halo populations in the solar neighborhood. II. Evidence from stellar abundances of Mn, Cu, Zn, Y, and Ba
• Galactic masers and the Milky Way circular velocity
• The Case for the Dual Halo of the Milky Way
• Quantifying Kinematic Substructure in the Milky Way's Stellar Halo
• Abundances and Kinematics of Field Stars II: Kinematics and Abundance Relationships
• The SEGUE Stellar Parameter Pipeline. IV. Validation with an Extended Sample of Galactic Globular and Open Clusters
• The Besancon Galaxy model renewed I. Constraints on the local star formation history from Tycho data
• The SEGUE Stellar Parameter Pipeline. V. Estimation of Alpha-Element Abundance Ratios From Low-Resolution SDSS/SEGUE Stellar Spectra
• Abundances for metal-poor stars with accurate parallaxes II. alpha-elements in the halo
• Stellar haloes of simulated Milky Way-like galaxies: Chemical and kinematic properties
• Galactic Stellar Populations in the Era of SDSS and Other Large Surveys
• The Stellar Metallicity Distribution Function of the Galactic Halo from SDSS Photometry
• Stellar haloes in Milky-Way mass galaxies: From the inner to the outer haloes
• The SEGUE K giant survey II: A Catalog of Distance Determinations for the SEGUE K giants in the Galactic Halo
• Galactic Globular and Open Clusters in the Sloan Digital Sky Survey. II. Test of Theoretical Stellar Isochrones
• Insight Into the Formation of the Milky Way Through Cold Halo Substructure. I. The ECHOS of Milky Way Formation
• On the alleged duality of the Galactic halo
• Global Properties of M31's Stellar Halo from the SPLASH Survey: II. Metallicity Profile
• The Metallicity Distribution Functions of SEGUE G and K dwarfs: Constraints for Disk Chemical Evolution and Formation
• Chemical abundances in 43 metal-poor stars
• Deep SDSS optical spectroscopy of distant halo stars I. Atmospheric parameters and stellar metallicity distribution
• Metal-Poor Stars and the Chemical Enrichment of the Universe
• Stellar Population Variations in the Milky Way's Stellar Halo
• Stellar halo streams in the Solar neighbourhood
• Chemical Abundances of the Outer Halo Stars in the Milky Way
• Does SEGUE/SDSS indicate a dual Galactic halo?
• Insight Into the Formation of the Milky Way Through Cold Halo Substructure. III. Statistical Chemical Tagging in the Smooth Halo
• Insight Into the Formation of the Milky Way Through Cold Halo Substructure. II. The Elemental Abundances of ECHOS
• Transformation from SDSS Photometric System to Johnson-Morgan-Cousins System in HK Survey

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• The origin of accreted stellar halo populations in the Milky Way using APOGEE, $\text{Gaia}$, and the EAGLE simulations
• Chemical tagging with APOGEE: Discovery of a large population of N-rich stars in the inner Galaxy
• Gaia FGK benchmark stars: abundances of alpha and iron-peak elements
• Overview of the DESI Milky Way Survey
• Abundance to age ratios in the HARPS-GTO sample with Gaia DR2: Chemical clocks for a range of [Fe/H]
• The radial profile and flattening of the Milky Way's stellar halo to $80$kpc from the SEGUE K-giant Survey
• A Tail of Two Populations: Chemo-dynamics of the Sagittarius Stream and Implications for its Original Mass
• Chemo-dynamics of outer halo dwarf stars, including \textit{Gaia}-Sausage and \textit{Gaia}-Sequoia candidates
• Disentangling the Galactic Halo with APOGEE: II. Chemical and Star Formation Histories for the Two Distinct Populations
• The Pristine survey III: Spectroscopic confirmation of an efficient search for extremely metal-poor stars
• The Fractions of Inner- and Outer-Halo Stars in the Local Volume
• Chemical Cartography. I. A Carbonicity Map of the Galactic Halo
• Identifying Sagittarius Stream Stars By Their APOGEE Chemical Abundance Signatures
• Elemental Abundances in M31: A Comparative Analysis of Iron and Alpha Element Abundances in the Outer Disk, Giant Stellar Stream, and Inner Halo of M31
• Weak Galactic Halo--Fornax dSph Connection from RR Lyrae Stars
• Follow-up observations of extremely metal-poor stars identified from SDSS
• An equatorial ultra iron-poor star identified in BOSS
• Disk stars in the Milky Way detected beyond 25 kpc from its center
• Chemical trends in the Galactic Halo with APOGEE data
• J0815+4729: A chemically primitive dwarf star in the Galactic Halo observed with Gran Telescopio Canarias
• Solar and Stellar Photospheric Abundances
• Stellar chemo-kinematics of the Cetus dwarf spheroidal galaxy
• Chemical Cartography. II. The Assembly History of the Galactic Stellar Halo Traced by Carbon-Enhanced Metal-Poor Stars
• The assembly history of the Galactic inner halo inferred from alpha-patterns
• The metal-rich halo component extended in z: a characterization with Gaia DR2 and APOGEE
• Metallicity and Kinematics of the Galactic halo from the LAMOST sample stars
• The Metallicity Gradient and Complex Formation History of the Outermost Halo of the Milky Way
• What is the Milky Way outer halo made of? High resolution spectroscopy of distant red giants
• Dependence of Galactic Halo Kinematics on the Adopted Galactic Potential
• Differential rotation of the halo traced by the K-giant stars
• Deep SDSS Optical Spectroscopy of Distant Halo Stars. III. Chemical analysis of extremely metal-poor stars
• Automated Pipelines for Spectroscopic Analysis
• Galactic Stellar Populations from Photometric Metallicity Distribution Functions
• Tomography of stellar halos: what does anisotropy in a stellar halo tell us?
• A Hint of Three-section Halo as Seen from the APOGEE DR14
• rrlfe: Software for Generating and Applying Metallicity Calibrations for RR Lyrae Variable Stars Across a Wide Range of Phases and Temperatures