Line formation in solar granulation VII. CO lines and the solar C and O   isotopic abundances
  CO spectral line formation in the Sun has long been a source of consternation for solar physicists, as have the elemental abundances it seems to imply. We modelled solar CO line formation using a realistic, ab initio, time-dependent 3D radiative-hydrodynamic model atmosphere. Results were compared with observations from the space-based ATMOS experiment. We employed weak 12C16O, 13C16O and 12C18O lines from the fundamental and first overtone bands to determine the solar carbon abundance, as well as the 12C/13C and 16O/18O isotopic ratios. A weighted carbon abundance of log epsilonC = 8.39 +-0.05 was found. We note with satisfaction that the derived abundance is identical to our recent 3D determination based on CI, [C I], C2 and CH lines. Identical calculations were carried out using 1D models, but only the 3D model was able to produce abundance agreement between different CO lines and the other atomic and molecular diagnostics. Solar 12C/13C and 16O/18O ratios were measured as 86.8+3.9-3.7 (delta13C = 30+46-44) and 479+29-28 (delta18O = 41+67-59), respectively. These values may require current theories of solar system formation to be revised. Excellent agreement was seen between observed and predicted weak CO line shapes, without invoking micro- or macroturbulence. Agreement breaks down for the strongest CO lines however, which are formed in very high atmospheric layers. The simplest explanation is that temperatures are overestimated in the highest layers of the 3D simulation. Thus, our analysis supports the presence of a COmosphere above the traditional photospheric temperature minimum, with an average temperature of less than 4000K. The shortcoming of the model atmosphere is not surprising, given that it was never intended to properly describe such high layers. 
Patrick C. Scott, Martin Asplund, Nicolas Grevesse and A. Jacques   Sauval