The solar photospheric abundance of carbon

Abstract

<i>Context. <i/>The analysis of the solar spectra using hydrodynamical simulations, with a specific selection of lines, atomic data, and method for computing deviations from local thermodynamical equilibrium, has led to a downward revision of the solar metallicity, <i>Z<i/>. We are using the latest simulations computed with the CO5BOLD code to reassess the solar chemical composition. Our previous analyses of the key elements, oxygen and nitrogen, have not confirmed any extreme downward revision of <i>Z<i/>, as derived in other works based on hydrodynamical models.<i>Aims. <i/>We determine the solar photospheric carbon abundance with a radiation-hydrodynamical CO5BOLD model and compute the departures from local thermodynamical equilibrium by using the Kiel code.<i>Methods. <i/>We measured equivalent widths of atomic lines on high-resolution, high signal-to-noise ratio solar atlases of disc-centre intensity and integrated disc flux. These equivalent widths were analysed with our latest solar 3D hydrodynamical simulation computed with CO5BOLD. Deviations from local thermodynamic equilibrium we computed in 1D with the Kiel code, using the average temperature structure of the hydrodynamical simulation as a background model.<i>Results. <i/>Our recommended value for the solar carbon abundance relies on 98 independent measurements of observed lines and is <i>A<i/>(C)=8.50 <i>±<i/> 0.06. The quoted error is the sum of statistical and systematic errors. Combined with our recent results for the solar oxygen and nitrogen abundances, this implies a solar metallicity of <i>Z<i/> = 0.0154 and <i>Z<i/>/<i>X<i/> = 0.0211.<i>Conclusions. <i/>Our analysis implies a solar carbon abundance that is about 0.1 dex higher than what was found in previous analyses based on different 3D hydrodynamical computations. The difference is partly driven by our equivalent width measurements (we measure, on average, larger equivalent widths than the other work based on a 3D model), in part because of the different properties of the hydrodynamical simulations and the spectrum synthesis code. The solar metallicity we obtain from the CO5BOLD analyses is in slightly better agreement with the constraints of helioseismology than the previous 3D abundance results.