Astrophys. J., 860, 125-125 (2018/June-3)
Consistent iron abundances derived from neutral and singly ionized iron lines in ultraviolet and optical spectra of six warm metal-poor stars.
ROEDERER I.U., SNEDEN C., LAWLER J.E., SOBECK J.S., COWAN J.J. and BOESGAARD A.M.
Abstract (from CDS):
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 Å. 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 Å), and we conclude that no proposed explanation for this effect can fully account for the observations presently available.
© 2018. The American Astronomical Society. All rights reserved.
stars: abundances - stars: atmospheres - stars: individual - stars: population II
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<Available at CDS (J/ApJ/860/125): table2.dat table4.dat table7.dat>
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