Journal
ASTRONOMY & ASTROPHYSICS
Volume 585, Issue -, Pages -Publisher
EDP SCIENCES S A
DOI: 10.1051/0004-6361/201527491
Keywords
Galaxy: evolution; Galaxy: halo; line: formation; line: profiles; stars: abundances; stars: atmospheres
Categories
Funding
- National Key Basic Research Program of China [2014CB845700]
- National Natural Science Foundation of China [11321064, 11233004, 11390371, 11473033, U1331122]
- Chinese Academy of Sciences [6-1309001]
- Danish National Research Foundation [DNRF106]
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Context. 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. The study also showed that non-LTE effects of neutral copper vary with stellar parameters and thus affect the [Cu/Fe] trend. Aims. We aim to derive the copper abundances for the stars from the sample of Nissen & Schuster (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-a population can still be distinguished from the low-a population in the non-LTE [Cu/Fe] results. Methods. 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. Results. 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-a stars still show distinguishable copper abundances, which appear even more clear in a diagram of non-LTE [Cu/Fe] versus [Fe/H]. Conclusions. 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.
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