  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.
