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 (¹³C) and oxygen (¹⁸O, ¹⁷O), 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 ≡ ¹²C/¹³C), 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 ¹³C and ¹⁸O 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 ¹²C ¹⁷O 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, ε_O ∼ 603 ± 9 ppm (relative to hydrogen; log ε ∼ 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.