Is the Sun Lighter Than the Earth? Isotopic Co in the Photosphere, Viewed Through the Lens of Three-Dimensional Spectrum Synthesis

We consider the formation of solar infrared (2–6 μm) rovibrational bands of carbon monoxide (CO) in CO5BOLD 3D convection models, with the aim of refining abundances of the heavy isotopes of carbon ( 13 C) and oxygen ( 18 O, 17 O), to compare with direct capture measurements of solar wind light ions by the Genesis Discovery Mission. We find that previous, mainly 1D, analyses were systematically biased toward lower isotopic ratios (e.g., R 23 ≡ 12 C/ 13 C), suggesting an isotopically "heavy" Sun contrary to accepted fractionation processes that were thought to have operated in the primitive solar nebula. The new 3D ratios for 13 C and 18 O are R 23 = 91.4 ± 1.3 ( R ⊕ = 89.2) and R 68 = 511 ± 10 ( R ⊕ = 499), where the uncertainties are 1σ and "optimistic." We also obtained R 67 = 2738 ± 118 ( R ⊕ = 2632), but we caution that the observed 12 C 17 O features are extremely weak. The new solar ratios for the oxygen isotopes fall between the terrestrial values and those reported by Genesis ( R 68 = 530, R 67 = 2798), although including both within 2σ error flags, and go in the direction favoring recent theories for the oxygen isotope composition of Ca–Al inclusions in primitive meteorites. While not a major focus of this work, we derive an oxygen abundance, O ∼ 603 ± 9 ppm (relative to hydrogen; log ∼ 8.78 on the H = 12 scale). The fact that the Sun is likely lighter than the Earth, isotopically speaking, removes the necessity of invoking exotic fractionation processes during the early construction of the inner solar system.