The photospheric solar oxygen project

Abstract

Context. The solar photospheric oxygen abundance is still widely debated. Adopting the solar chemical composition based on the "low" oxygen abundance, as determined with the use of three-dimensional (3D) hydrodynamical model atmospheres, results in a well-known mismatch between theoretical solar models and helioseismic measurements that is so far unresolved. Aims. We carry out an independent redetermination of the solar oxygen abundance by investigating the center-to-limb variation of the O i IR triplet lines at 777 nm in different sets of spectra.Methods. The high-resolution and high signal-to-noise solar center-to-limb spectra are analyzed with the help of detailed synthetic line profiles based on 3D hydrodynamical CO5BOLD model atmospheres and 3D non-LTE line formation calculations with NLTE3D. The idea is to exploit the information contained in the observations at different limb angles to simultaneously derive the oxygen abundance, A(O), and the scaling factor S H that describes the cross-sections for inelastic collisions with neutral hydrogen relative to the classical Drawin formula. Using the same codes and methods, we compare our 3D results with those obtained from the semiempirical Holweger-Müller model atmosphere as well as from different one-dimensional (1D) reference models. Results. With the CO5BOLD 3D solar model, the best fit of the center-to-limb variation of the triplet lines is obtained when the collisions by neutral hydrogen atoms are assumed to be efficient, i.e., when the scaling factor S H is between 1.2 and 1.8, depending on the choice of the observed spectrum and the triplet component used in the analysis. The line profile fits achieved with standard 1D model atmospheres (with fixed microturbulence, independent of disk position μ) are clearly of inferior quality compared to the 3D case, and give the best match to the observations when ignoring collisions with neutral hydrogen (S H = 0). The results derived with the Holweger-Müller model are intermediate between 3D and standard 1D. Conclusions. The analysis of various observations of the triplet lines with different methods yields oxygen abundance values (on a logarithmic scale where A(H) = 12) that fall in the range 8.74 < A(O) < 8.78, and our best estimate of the 3D non-LTE solar oxygen abundance is A(O) = 8.76 ± 0.02. All 1D non-LTE models give much lower oxygen abundances, by up to −0.15 dex. This is mainly a consequence of the assumption of a μ-independent microturbulence. An independent determination of the relevant collisional cross-sections is essential to substantially improve the accuracy of the oxygen abundance derived from the O i IR triplet.

Tracing the evolution of the Galactic bulge with chemodynamical modelling of alpha-elements

“…Bensby et al (2004) also used these lines to examine a sample of thick disk giants; (ii) the permitted OI λ 7771.96, 7774.18 and 7775.40 Å triplet lines measurable in dwarfs and subgiants (in supergiants, also the weaker triplet at λ 6156.03, 6156.80 and 6158.17 Å can be used); these lines are subject to strong NLTE effects. Recently, Steffen et al (2015) were able to take into account the NLTE effects together with a 3D model atmosphere for the Sun. These lines were observed for microlensed bulge dwarfs by Bensby et al (2013, and references therein); (iii) The infrared (IR) X 2 Π vibration-rotation transitions of OH lines in the H band, have measurable intensities down to [Fe/H] ∼ −3.0 for giants with effective temperatures lower than T eff ∼ 4300 K. This line is present in both dwarf and giant stars, being stronger in dwarfs of the same temperature.…”

“…In Table 3 we have gathered the solar oxygen and iron abundances used by these different authors, and have applied a correction to all of them, to be compatible with the value given by Steffen et al (2015) of A(O) = 8.76 and A(Fe) = 7.50. The reasoning is that if a higher A(O) value is assumed for the Sun, a lower value for the star will be given, therefore the correction has to be added to the oxygen abundances reported by the authors.…”

“…In Table 3 we also include the A(O) and A(Fe) solar abundances according to Allende-Prieto et al (2001), Asplund et al (2009), Grevesse & Sauval (1998), Lodders (2009), Kurucz (1993, and Steffen et al (2015).…”

“…Bensby et al (2013, and references therein) derived abundances for microlensed bulge dwarfs. They used the permitted 7774-7 Å oxygen triplet lines, which are strongly subject to NLTE effects (Kiselman 2001;Steffen et al 2015). The higher effective temperatures of turn-off dwarfs imply that there is no significant molecule formation, and in this case, it is the NLTE effects that cause a tendency to yield higher oxygen abundances.…”

The Galactic chemical evolution of oxygen inferred from 3D non-LTE spectral-line-formation calculations

“…Although the [OI] 630 nm line forms in LTE (Kiselman & Nordlund 1995), the effects of inhomogeneities and threedimensional structures in late-type stellar atmospheres on the [OI] 630 nm line in the metal-poor regime are significant (Nissen et al 2002;Collet et al 2007). The OI 777 nm lines are a classic case study of non-LTE line formation (Sedlmayr 1974;Kiselman 1993;Takeda 2003; Asplund et al 2004;Pereira et al 2009;Prakapavičius et al 2013;Steffen et al 2015).…”