A Differential Abundance Analysis of Very Metal-Poor Stars

Abstract

We have performed a differential, line-by-line, chemical abundance analysis, ultimately relative to the Sun, of nine very metal-poor main sequence halo stars, near [Fe/H]=$-$2 dex. Our abundances range from $-2.66\le [Fe/H]\le -1.40$ dex with conservative uncertainties of 0.07 dex. We find an average [$\alpha$/Fe]$=0.34\pm 0.09$ dex, typical of the Milky Way. While our spectroscopic atmosphere parameters provide good agreement with HST parallaxes, there is significant disagreement with temperature and gravity parameters indicated by observed colors and theoretical isochrones. Although a systematic underestimate of the stellar temperature by a few hundred degrees could explain this difference, it is not supported by current effective temperature studies and would create large uncertainties in the abundance determinations. Both 1D and $⟨$3D$⟩$ hydrodynamical models combined with separate 1D non-LTE effects do not yet account for the atmospheres of real metal-poor MS stars, but a fully 3D non-LTE treatment may be able to explain the ionization imbalance found in this work.

18 pages, 13 tables, 5 figures, Accepted in ApJ

References in corpus (15)

• A grid of MARCS model atmospheres for late-type stars I. Methods and general properties
• The Dartmouth Stellar Evolution Database
• New constraints on the chemical evolution of the solar neighbourhood and Galactic disc(s). Improved astrophysical parameters for the Geneva-Copenhagen Survey
• alpha-, r-, and s-process element trends in the Galactic thin and thick disks
• Gaia Data Release 1: Astrometry - one billion positions, two million proper motions and parallaxes
• New Grids of ATLAS9 Model Atmospheres
• A Search for Stars of Very Low Metal Abundance. VI. Detailed Abundances of 313 Metal-Poor Stars
• A non-LTE study of neutral and singly-ionized calcium in late-type stars
• The Tycho-Gaia astrometric solution. How to get 2.5 million parallaxes with less than one year of Gaia data
• A new abundance scale for the globular cluster 47 Tuc
• Carbon and Oxygen in Nearby Stars: Keys to Protoplanetary Disk Chemistry
• A Comparison of Stellar Elemental Abundance Techniques and Measurements
• Manganese Abundances in Cluster and Field Stars
• The Evolution of Oxygen and Magnesium in the Bulge and Disk of the Milky Way
• Testing Metal Poor Stellar Models and Isochrones with HST Parallaxes of Metal Poor Stars

Cited by in corpus (8)

• The R-Process Alliance: First Release from the Northern Search for r-Process Enhanced Metal-Poor Stars in the Galactic Halo
• High-precision stellar abundances of the elements - methods and applications
• Absolute Ages and Distances of 22 GCs using Monte Carlo Main-Sequence Fitting
• Constraining cosmic scatter in the Galactic halo through a differential analysis of metal-poor stars
• Constraints on the Distance Moduli, Helium and Metal Abundances, and Ages of Globular Clusters from their RR Lyrae and Non-Variable Horizontal-Branch Stars. I. M3, M15, and M92
• The Absolute Age of M92
• High-precision chemical abundances of Galactic building blocks. The distinct chemical abundance sequence of Sequoia
• Constraints on the Distance Moduli, Helium and Metal Abundances, and Ages of Globular Clusters From Their RR Lyrae and Non-Variable Horizontal-Branch Stars. III. M55 and NGC 6362