Consistent Iron Abundances Derived from Neutral and Singly-Ionized Iron Lines in Ultraviolet and Optical Spectra of Six Warm Metal-Poor Stars

Abstract

Neutral Fe lines in metal-poor stars yield conflicting abundances depending on whether and how deviations from local thermodynamic equilibrium (LTE) are considered. We have collected new high resolution and high signal-to-noise ultraviolet (UV) spectra of three warm dwarf stars with [Fe/H] = -2.9 with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. We locate archival UV spectra for three other warm dwarfs with [Fe/H] = -3.3, -2.2, and -1.6, supplemented with optical spectra for all six stars. We calculate stellar parameters using methods that are largely independent of the spectra, adopting broadband photometry, color-temperature relations, Gaia parallaxes, and assumed masses. We use the LTE line analysis code MOOG to derive Fe abundances from hundreds of Fe I and Fe II lines with wavelengths from 2290 to 6430 Angstroms. The [Fe/H] ratios derived separately from Fe I and Fe II lines agree in all six stars, with [Fe II/H] - [Fe I/H] ranging from +0.00 +/- 0.07 to -0.12 +/- 0.09 dex, when strong lines and Fe I lines with lower excitation potential < 1.2 eV are excluded. This constrains the extent of any deviations from LTE that may occur within this parameter range. While our result confirms non-LTE calculations for some warm, metal-poor dwarfs, it may not be generalizable to more metal-poor dwarfs, where deviations from LTE are predicted to be larger. We also investigate trends of systematically lower abundances derived from Fe I lines in the Balmer continuum region (3100-3700 Angstroms), and we conclude that no proposed explanation for this effect can fully account for the observations presently available.

Accepted for publication in the Astrophysical Journal (18 pages, 4 figures, 9 tables)

References in corpus (19)

• The NumPy array: a structure for efficient numerical computation
• A grid of MARCS model atmospheres for late-type stars I. Methods and general properties
• Gaia Data Release 1. Summary of the astrometric, photometric, and survey 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
• A Search for Stars of Very Low Metal Abundance. VI. Detailed Abundances of 313 Metal-Poor Stars
• Synthetic Stellar Photometry - I. General considerations and new transformations for broad-band systems
• Both accurate and precise gf-values for FeII lines
• Non-LTE line formation of Fe in late-type stars - III. 3D non-LTE analysis of metal-poor stars
• Silver and palladium help unveil the nature of a second r-process
• New Extremely Metal-Poor Stars in the Galactic Halo
• APASS Landolt-Sloan BVgri photometry of RAVE stars. I. Data, effective temperatures and reddenings
• Fe I Oscillator Strengths for the Gaia-ESO Survey
• Fe I Oscillator Strengths for Transitions from High-lying Even-Parity Levels
• Three Ancient Halo Subgiants: Precise Parallaxes, Compositions, Ages, and Implications for Globular Clusters
• Lithium abundances of halo dwarfs based on excitation temperature. I. LTE
• Improved V II log($gf$) Values, Hyperfine Structure Constants, and Abundance Determinations in the Photospheres of the Sun and Metal-poor Star HD 84937
• Constraining cosmic scatter in the Galactic halo through a differential analysis of metal-poor stars
• New Fe I level energies and line identifications from stellar spectra

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