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Consistent Iron Abundances Derived from Neutral and Singly-Ionized Iron Lines in Ultraviolet and Optical Spectra of Six Warm Metal-Poor Stars
Solar and Stellar Astrophysics · Submitted 18 May 2018
abstract compiled · 2026-09-23 · no language-model review
The authors' main claim 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.
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The abstract, sentence by sentence
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.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | Neutral Fe lines in metal-poor stars yield conflicting abundances depending on whether and how deviations from local thermodynamic equilibrium (LTE) are considered. | background 0.95 | 0.06 | 0.04 | asserted 1.00 |
| s1 | 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. | method 0.85 | 0.97 | 0.02 | asserted 1.00 |
| s2 | 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. | method 0.85 | 0.88 | 0.02 | asserted 1.00 |
| s3 | We calculate stellar parameters using methods that are largely independent of the spectra, adopting broadband photometry, color-temperature relations, Gaia parallaxes, and assumed masses. | method 1.00 | 0.94 | 0.02 | asserted 1.00 |
| s4 | 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. | method 1.00 | 0.96 | 0.03 | asserted 1.00 |
| s5 | 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. | result 1.00 | 0.97 | 0.75 | asserted 1.00 |
| s6 | This constrains the extent of any deviations from LTE that may occur within this parameter range. | interpretation 1.00 | 0.54 | 0.23 | asserted 0.59 |
| s7 | 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. | limitation 0.94 | 0.53 | 0.20 | hedged possibility 0.99 |
| s8 | 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. | result 0.78 | 0.93 | 0.05 | asserted 0.90 |
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Original 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.
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