NLTE Analysis of Copper Lines in Different Stellar Populations^∗

Abstract

The copper abundances of 29 metal-poor stars are determined based on the high resolution, high signalto-noise ratio spectra from the UVES spectragraph at the ESO VLT telescope. Our sample consists of the stars of the Galactic halo, thick-and thin-disk with [Fe/H] ranging from ∼ −3.2 to ∼ 0.0 dex. The non-local thermodynamic equilibrium (NLTE) effects of Cu I lines are investigated, and line formation calculations are presented for an atomic model of copper including 97 terms and 1089 line transitions. We adopted the recently calculated photo-ionization cross-sections of Cu I, and investigated the hydrogen collision by comparing the theoretical and observed line profiles of our sample stars. The copper abundances are derived for both local thermodynamic equilibrium (LTE) and NLTE based on the spectrum synthesis methods. Our results show that the NLTE effects for Cu I lines are important for metal-poor stars, in particular for very metal-poor stars, and these effects depend on the metallicity. For very metal-poor stars, the NLTE abundance correction reaches as large as ∼ +0.5 dex compared to standard LTE calculations. Our results indicate that [Cu/Fe] is under-abundant for metal-poor stars (∼ −0.5 dex) when the NLTE effects are included.

Star Formation Timescales of the Halo Populations from Asteroseismology and Chemical Abundances*

“…(Prantzos et al 2018). Therefore, we tentatively conclude that the star formation timescales of the low-/high-α populations and abundance differences of Cu and Y in Figures 13, 14 We note that Cu abundance can significantly be affected by the NLTE effect (Andrievsky et al 2018;Shi et al 2018). For example, Shi et al (2018) reported that the [Cu/Fe] abundance of HD 122563 is underestimated in LTE analysis by 0.3 −0.4 dex compared to NLTE analysis when the absorption line at 5105 Å is used.…”

“…Therefore, we tentatively conclude that the star formation timescales of the low-/high-α populations and abundance differences of Cu and Y in Figures 13, 14 We note that Cu abundance can significantly be affected by the NLTE effect (Andrievsky et al 2018;Shi et al 2018). For example, Shi et al (2018) reported that the [Cu/Fe] abundance of HD 122563 is underestimated in LTE analysis by 0.3 −0.4 dex compared to NLTE analysis when the absorption line at 5105 Å is used. Although the trend with metallicity could change when studied with an NLTE analysis, the discussion presented here is mostly based on the relative abundance difference between the low-α and high-α populations at a given metallicity, which should be less affected by the NLTE effect (e.g., Yan et al 2016).…”

Detailed Iron-peak Element Abundances in Three Very Metal-poor Stars*

“…From the resonance lines in the three program stars, we derived a mean abundance of log 0.6, or [Cu/H] −3.5. However, it is possible that these very low abundances may be severe underestimates, as some studies (e.g., Andrievsky et al 2018, Shi et al 2018) have argued that LTE-based abundances should be corrected upward by large factors, typically by at least +0.5 dex in very metal-poor stars.…”

“…Thirdly, in many instances only a handful of lines are employed to derive individual elemental abundances. Finally, significant questions have been raised about whether local thermodynamic equilibrium (LTE) can adequately describe the ionization equilibrium, and large upward corrections have been proposed to LTE-based abundances from neutral species (e.g., Bergemann et al (2010), Andrievsky et al (2018) and Shi et al (2018)).…”

Purveyors of fine halos

“…Our own values and the cited literature values were derived under the assumption of LTE. It appears that the Cu abundances are affected by NLTE effects (Andrievsky et al 2018;Shi et al 2018), although different model-atoms and NLTE codes provide different results, especially at low metallicity. This notion is reinforced by the observed deviation between Cu i and Cu ii abundances observed by Roederer & Barklem (2018).…”