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

June 2018 • 2018ApJ...860..125R

Authors • Roederer, Ian U. • Sneden, Christopher • Lawler, James E. • Sobeck, Jennifer S. • Cowan, John J. • Boesgaard, Ann Merchant

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 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.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under NASA contract NAS 5-26555. These observations are associated with program GO-14232. Some data presented in this paper were obtained from the Barbara A. Mikulski Archive for Space Telescopes (MAST). These data are associated with programs GO-7402, GO-9049, and GO-14161. Other data have been obtained from the European Southern Observatory (ESO) Science Archive Facility. These data are associated with programs 66.D-0636(A), 67.D-0439(A), 67.D-0554(A), 68.B-0475(A), 68.D-0094(A), 072.B-0585(A), and 095.D-0504(A). This research has also made use of the Keck Observatory Archive (KOA), which is operated by the W.M. Keck Observatory and the NASA Exoplanet Science Institute (NExScI), under contract with NASA. This work has also made use of data collected from McDonald Observatory of the University of Texas at Austin.

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Jennifer Sobeck

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