Volume 585, January 2016
|Number of page(s)||8|
|Published online||23 December 2015|
Non-LTE analysis of copper abundances for the two distinct halo populations in the solar neighborhood⋆
Key Laboratory of Optical Astronomy, National Astronomical Observatories,
Chinese Academy of Sciences,
20A Datun Road, Chaoyang District,
2 University of Chinese Academy of Sciences, No.19A Yuquan Road, 100049, Beijing, PR China
3 Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark
Received: 1 October 2015
Accepted: 23 October 2015
Context. Two distinct halo populations were found in the solar neighborhood by a series of works. They can be clearly separated by [α/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-α population can still be distinguished from the low-α 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-α 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.
Key words: Galaxy: evolution / Galaxy: halo / line: formation / line: profiles / stars: abundances / stars: atmospheres
Based on observations made with the FIbre fed Echelle Spectrograph (FIES) at the Nordic Optical Telescope (NOT) on La Palma, and on data from the European Southern Observatory ESO/ST-ECF Science Archive Facility (programs 65.L-0507, 67.D-0086, 67.D-0439, 68.D-0094, 68.B-0475, 69.D-0679, 70.D-0474, 71.B-0529, 72.B-0585, 76.B-0133 and 77.B-0507).
© ESO, 2015
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