STATISTICAL EQUILIBRIUM OF COPPER IN THE SOLAR ATMOSPHERE

Abstract

Non-local thermodynamic equilibrium (NLTE) line formation for neutral copper in the one-dimensional solar atmospheres is presented for the atomic model, including 96 terms of Cu i and the ground state of Cu ii. The accurate oscillator strengths for all the line transitions in model atom and photoionization cross sections were calculated using the R-matrix method in the Russell–Saunders coupling scheme. The main NLTE mechanism for Cu i is the ultraviolet overionization. We find that NLTE leads to systematically depleted total absorption in the Cu i lines and, accordingly, positive abundance corrections. Inelastic collisions with neutral hydrogen atoms produce minor effects on the statistical equilibrium of Cu i in the solar atmosphere. For the solar Cu i lines, the departures from LTE are found to be small, the mean NLTE abundance correction of ∼0.01 dex. It was found that the six low-excitation lines, with excitation energy of the lower level Eexc ⩽ 1.64 eV, give a 0.14 dex lower mean solar abundance compared to that from the six Eexc > 3.7 eV lines, when applying experimental gf-values of Kock & Richter. Without the two strong resonance transitions, the solar mean NLTE abundance from 10 lines of Cu i is log ε☉(Cu) = 4.19 ± 0.10, which is consistent within the error bars with the meteoritic value 4.25 ± 0.05 of Lodders et al. The discrepancy between Eexc = 1.39–1.64 eV and Eexc > 3.7 eV lines can be removed when the calculated gf-values are adopted and a mean solar abundance of log ε☉(Cu) = 4.24 ± 0.08 is derived.