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Two distinct halo populations were found in the solar neighborhood by aseries of works. They can be clearly separated by [alpha\Fe] and several otherelemental abundance ratios including [Cu/Fe]. Very recently, a non-localthermodynamic equilibrium (non-LTE) study revealed that relatively largedepartures exist between LTE and non-LTE results in copper abundance analysis.We aim to derive the copper abundances for the stars from the sample of Nissenet al (2010) with both LTE and non-LTE calculations. Based on our results, westudy the non-LTE effects of copper and investigate whether the high-alphapopulation can still be distinguished from the low-alpha population in thenon-LTE [Cu/Fe] results. Our differential abundance ratios are derived from thehigh-resolution spectra collected from VLT/UVES and NOT/FIES spectrographs.Applying the MAFAGS opacity sampling atmospheric models and spectrum synthesismethod, we derive the non-LTE copper abundances based on the new atomic modelwith current atomic data obtained from both laboratory and theoreticalcalculations. The copper abundances determined from non-LTE calculations areincreased by 0.01 to 0.2 dex depending on the stellar parameters compared withthe LTE results. The non-LTE [Cu/Fe] trend is much flatter than the LTE one inthe metallicity range -1.6<[Fe/H]<-0.8. Taking non-LTE effects intoconsideration, the high- and low-alpha stars still show distinguishable copperabundances, which appear even more clear in a diagram of non-LTE [Cu/Fe] versus[Fe/H]. The non-LTE effects are strong for copper, especially in metal-poorstars. Our results confirmed that there are two distinct halo populations inthe solar neighborhood. The dichotomy in copper abundance is a peculiar featureof each population, suggesting that they formed in different environments andevolved obeying diverse scenarios.

Non-LTE analysis of copper abundances for the two distinct halo populations in the solar neighborhood