On the Solar Nickel and Oxygen Abundances

Determinations of the solar oxygen content relying on the neutral forbidden transition at 630 nm depend upon the nickel abundance, due to a Ni i blend. Here, we rederive the solar nickel abundance, using the same ab initio three-dimensional hydrodynamic model of the solar photosphere employed in the recent revision of the abundances of C, N, O, and other elements. Using 17 weak, unblended lines of Ni i together with the most accurate atomic and observational data available, we find log Ni = 6.17 ± 0.02(statistical) ± 0.05(systematic), a downward shift of 0.06–0.08 dex relative to previous abundances based on one-dimensional model atmospheres. We investigate the implications of the new nickel abundance for studies of the solar oxygen abundance based on the [O i ] 630 nm line in the quiet Sun. Furthermore, we demonstrate that the oxygen abundance implied by the recent sunspot spectropolarimetric study of Centeno & Socas-Navarro needs to be revised downward from log O = 8.86 ± 0.07 to 8.71 ± 0.10. This revision is based on the new nickel abundance, the application of the best available gf value for the 630 nm forbidden oxygen line, and a more transparent treatment of CO formation. Determinations of the solar oxygen content relying on forbidden lines now appear to converge around log O = 8.7.