Photodissociation of CO isotopologues: Models of laboratory experiments and implications for the solar nebula

CO photodissociation in the solar nebula and/or parent cloud has been proposed to be the mechanism responsible for forming the 16 O‐poor reservoir of the calcium‐aluminum‐rich inclusion (CAI) mixing line. However, laboratory experiments on CO photolysis found a wavelength dependence in the oxygen isotope ratios of the product O atoms, which was interpreted as proof that CO photolysis was not a viable mechanism. Here, I report photochemical simulations of these experiments using line‐by‐line CO spectra to identify the origin of the wavelength dependence. At long wavelengths (>105 nm), the line‐by‐line spectra for isotopic CO can explain the experimental data with a combination of C 16 O self‐shielding and reduced dissociation probabilities for C 18 O. At short wavelengths, the greater number of diffuse bands increases the importance of mass‐dependent fractionation, lowering the slope to below unity. The line‐by‐line isotopic spectra are then applied to CO photodissociation in a model solar nebula. Three FUV sources are considered (1) HD 303308, an O4 star in Carina; (2) HD 36981, a B5 star in Orion; and (3) TW Hydrae, a T Tauri star of 10 Myr age. Using reduced dissociation probabilities for C 18 O based on the photolysis experiments yields nebular water slopes approximately 0.95–1.0 for HD 303308 and TW Hya, and approximately 0.8–1.5 for HD 36981. For the central protostar case (TW Hya) with a simplified treatment of the 2‐D radiative transfer, slopes approximately 0.95–1.0 are obtained, independent of the C 18 O dissociation probability. Greatly improved measurements of the C 17 O and C 18 O cross sections and dissociation probabilities are in progress.