High-resolution spectroscopy of extremely metal-poor stars from SDSS/SEGUE. III. Unevolved Stars with $[Fe/H]\lesssim -3.5$

Abstract

We present elemental abundances for eight unevolved extremely metal-poor stars with $T_{\mathrm{eff}}>5500\mathrm{K}$, among which seven have $[Fe/H]<-3.5$. The sample is selected from the Sloan Digital Sky Survey / Sloan Extension for Galactic Understanding and Exploration (SDSS/SEGUE), and our previous high-resolution spectroscopic follow-up with the Subaru Telescope (Aoki et al.). Several methods to derive stellar parameters are compared, and no significant offset in the derived parameters is found in most cases. From an abundance analysis relative to the standard extremely metal-poor star G 64-12, an average Li abundance for stars with $[Fe/H]<-3.5$ is $A(\mathrm{Li})=1.90$, with a standard deviation of $\sigma =0.10$ dex. This result confirms that lower Li abundances are found at lower metallicity, as suggested by previous studies, and demonstrates that the star-to-star scatter is small. The small observed scatter could be a strong constraint on Li-depletion mechanisms proposed for explaining the low Li abundance at lower metallicity. Our analysis for other elements obtained the following results: i) A statistically significant scatter in $[X/Fe]$ for Na, Mg, Cr, Ti, Sr, and Ba, and an apparent bimodality in $[Na/Fe]$ with a separation of $\sim 0.8\mathrm{dex}$, ii) an absence of a sharp drop in the metallicity distribution, and iii) the existence of a CEMP-$s$ star at $[Fe/H]\simeq -3.6$ and possibly at $[Fe/H]\simeq -4.0$, which may provide a constraint on the mixing efficiency of unevolved stars during their main-sequence phase.

Accepted for publication in AJ

References in corpus (31)

• The Gaia mission
• Cosmic Star Formation History
• Gaia Data Release 1. Summary of the astrometric, photometric, and survey properties
• Galactic chemical evolution: Carbon through Zinc
• Empirical Color Transformations Between SDSS Photometry and Other Photometric Systems
• The SEGUE Stellar Parameter Pipeline. I. Description and Initial Validation Tests
• An extremely primitive halo star
• The SEGUE Stellar Parameter Pipeline. II. Validation with Galactic Globular and Open Clusters
• CH in stellar atmospheres: an extensive linelist
• The SEGUE Stellar Parameter Pipeline. III. Comparison with High-Resolution Spectroscopy of SDSS/SEGUE Field Stars
• Near-UV Observations of HD221170: New Insights into the Nature of r-Process-Rich Stars
• First stars VII. Lithium in extremely metal poor dwarfs
• The elemental composition of the Sun III. The heavy elements Cu to Th
• The intermediate neutron-capture process and carbon-enhanced metal-poor stars
• Both accurate and precise gf-values for FeII lines
• The stellar content of the Hamburg/ESO survey. IV. Selection of candidate metal-poor stars
• TOPoS: II. On the bimodality of carbon abundance in CEMP stars. Implications on the early chemical evolution of galaxies
• Binarity in Carbon-Enhanced Metal-Poor stars
• Improved Laboratory Transition Probabilities for Neutral Chromium and Re-determination of the Chromium Abundance for the Sun and Three Stars
• First stars IX -Mixing in extremely metal-poor giants. Variation of the 12C/13C, [Na/Mg] and [Al/Mg] ratios
• HE 1327-2326, an unevolved star with [Fe/H]<-5.0. II. New 3D-1D corrected abundances from a VLT/UVES spectrum
• Sulphur and zinc abundances in Galactic halo stars revisited
• NLTE determination of the sodium abundance in a homogeneous sample of extremely metal-poor stars
• Carbon-Enhanced Metal-Poor Stars. III. Main-Sequence Turn-Off Stars from the SDSS/SEGUE Sample
• High-Resolution Spectroscopic Study of Extremely Metal-Poor Star Candidates from the SkyMapper Survey
• Exploring the Origin of Lithium, Carbon, Strontium and Barium with four new Ultra Metal-Poor Stars
• From First Stars to the Spite Plateau: a Possible Reconciliation of Halo Stars Observations with Predictions from Big Bang Nucleosynthesis
• Lithium abundances in CEMP stars
• First Stars XI. Chemical composition of the extremely metal-poor dwarfs in the binary CS 22876--032
• Experimental Mg I oscillator strengths and radiative lifetimes for astrophysical applications on metal-poor stars - New data for the Mg I b triplet
• G64-12 and G64-37 are Carbon-Enhanced Metal-Poor Stars

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• The metal-poor end of the Spite plateau. II. Detailed chemical investigation
• A Critique of the Spite Plateau, and the Astration of Primordial Lithium
• Search for Extremely Metal-Poor Stars with GEMINI-N/GRACES I. Chemical-Abundance Analysis
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• A coherent view of Li depletion and angular momentum transport to explain the Li plateau -- from Population II to Population I stars
• Progress in Nuclear Astrophysics of East and Southeast Asia
• A chemodynamical analysis of bright metal-poor stars from the HESP-GOMPA survey -- Indications of a non-prevailing site for light r-process elements
• Initial Li Abundances in the Protogalaxy and Globular Clusters Based upon the Chemical Separation and Hierarchical Structure Formation
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