  Two distinct halo populations were found in the solar neighborhood by a
series of works. They can be clearly separated by [alpha\Fe] and several other
elemental abundance ratios including [Cu/Fe]. Very recently, a non-local
thermodynamic equilibrium (non-LTE) study revealed that relatively large
departures 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 Nissen
et al (2010) with both LTE and non-LTE calculations. Based on our results, we
study the non-LTE effects of copper and investigate whether the high-alpha
population can still be distinguished from the low-alpha population in the
non-LTE [Cu/Fe] results. Our differential abundance ratios are derived from the
high-resolution spectra collected from VLT/UVES and NOT/FIES spectrographs.
Applying the MAFAGS opacity sampling atmospheric models and spectrum synthesis
method, we derive the non-LTE copper abundances based on the new atomic model
with current atomic data obtained from both laboratory and theoretical
calculations. The copper abundances determined from non-LTE calculations are
increased by 0.01 to 0.2 dex depending on the stellar parameters compared with
the LTE results. The non-LTE [Cu/Fe] trend is much flatter than the LTE one in
the metallicity range -1.6<[Fe/H]<-0.8. Taking non-LTE effects into
consideration, the high- and low-alpha stars still show distinguishable copper
abundances, 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-poor
stars. Our results confirmed that there are two distinct halo populations in
the solar neighborhood. The dichotomy in copper abundance is a peculiar feature
of each population, suggesting that they formed in different environments and
evolved obeying diverse scenarios.
