Line formation in solar granulation VI. [Cl], Cl, CH and C₂ lines and the photospheric C abundance

The solar photospheric carbon abundance has been determined from [C _I], C_I, CH vibration-rotation, CH A-X electronic and C₂ Swan electronic lines by means of a time-dependent, 3D, hydrodynamical model of the solar atmosphere. Departures from LTE have been considered for the C_I lines. These turned out to be of increasing importance for stronger lines and are crucial to remove a trend in LTE abundances with the strengths of the lines. Very gratifying agreement is found among all the atomic and molecular abundance diagnostics in spite of their widely different line formation sensitivities. The mean value of the solar carbon abundance based on the four primary abundance indicators ([C _I], C_I, CH vibration-rotation, C₂ Swan) is log ∈_C = 8.39 ± 0.05, including our best estimate of possible systematic errors. Consistent results also come from the CH electronic lines, which we have relegated to a supporting role due to their sensitivity to the line broadening. The new 3D based solar C abundance is significantly lower than previously estimated in studies using ID model atmospheres.