Oxygen lines in solar granulation^*

II. Centre-to-limb variation, NLTE line formation, blends, and the solar oxygen abundance

¹ Research School of Astronomy and Astrophysics, Australian National University, Cotter Rd., Weston, ACT 2611, Australia e-mail: tiago@mso.anu.edu.au
² The Institute for Solar Physics of the Royal Swedish Academy of Sciences, AlbaNova University Center, 106 91 Stockholm, Sweden
³ Max-Planck-Institut für Astrophysik, Postfach 1317, 85741 Garching b. München, Germany

Received: 7 July 2009
Accepted: 9 September 2009

Abstract

Context. There is a lively debate about the solar oxygen abundance and the role of 3D models in its recent downward revision. These models have been tested using high-resolution solar atlases of flux and disk-centre intensity. Further testing can be done using centre-to-limb variations.

Aims. Using high-resolution and high S/N observations of neutral oxygen lines across the solar surface, we seek to test that the 3D and 1D models reproduce their observed centre-to-limb variation. In particular we seek to assess whether the latest generation of 3D hydrodynamical solar model atmospheres and NLTE line formation calculations are appropriate to derive the solar oxygen abundance.

Methods. We use our recent observations of O i 777 nm, O i 615.81 nm, [O i] 630.03 nm, and nine lines of other elements for five viewing angles of the quiet solar disk. We compared them with the predicted line profiles from the 3D and 1D models computed with the most up-to-date line formation codes and line data and allowing for departures of LTE. The centre-to-limb variation of the O i 777 nm lines is also used to obtain an empirical correction for the poorly known efficiency of the inelastic collisions with H i.

Results. The 3D model generally reproduces the centre-to-limb observations of the lines very well, particularly the oxygen lines. From the O i 777 nm lines we find that the classical Drawin recipe slightly overestimates H i collisions ( agrees best with the observations). The limb observations of the O i 615.82 nm line allow us to identify a previously unknown contribution of molecules for this line, prevalent at the solar limb. A detailed treatment of the [O i] 630.03 nm line that includes the recent nickel abundance shows that the 3D modelling closely agrees with the observations. The derived oxygen abundances with the 3D model are 8.68 (777 nm lines), 8.66 (630.03 nm line), and 8.62 (615.82 nm line).

Conclusions. These additional tests have reinforced the trustworthiness of the 3D model and line formation for abundance analyses.