Line formation in solar granulation VII. CO lines and the solar C and O isotopic abundances

Abstract

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 space-based observations from the ATMOS space shuttle experiment. We employed weak12C16O,13C16O and12C18O lines from the fundamental (Δv = 1) and first overtone (Au = 2) bands to determine the solar carbon abundance, as well as the12C/13C and16O/18O isotopic ratios. A weighted solar carbon abundance of log εC = 8.39 ± 0.05 was found. We note with satisfaction that the derived abundance is identical to our recent 3D determination based on C I, [C I], C2 and CH lines, increasing our confidence in the accuracy of both results. Identical calculations were carried out using ID models, but only the 3D model was able to produce abundance agreement between different CO lines and the other atomic and molecular diagnostics. Solar12C/13C and16O/18O ratios were measured as 86.8-3.7+3.9 (δ13C = 30-44+46) and 479-28+29 (δ18O = 41-59+67), respectively. These values may require current theories of solar system formation, such as the CO self-shielding hypothesis, 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. Whilst the line asymmetries (bisectors) were reasonably well reproduced, line strengths predicted on the basis of C and O abundances from other diagnostics were weaker than observed. 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 4000 K. This shortcoming of the 3D model atmosphere is not surprising, given that it was never intended to properly describe such high layers.