The Predominance of Self-shielding in Laboratory Co Photolysis Experiments

Introduction: CO self-shielding is presently the leading hypothesis for explaining the CAI oxygen isotope mixing line [1]. Preliminary analyses of GENESIS solar wind samples show that the solar wind ions have ΔO ~ -20 ‰ [2], similar to the isotopically lightest CAIs. Barring non-mass-dependent fractionation during solar wind acceleration and ion passage through the GENESIS ion concentrator, the preliminary GENESIS results support a self-shielding scenario for the solar nebula. Recent oxygen isotope measurements of CO2 produced during CO photodissociation in the laboratory have been interpreted as evidence that self-shielding is not significant during CO photolysis, and is therefore not relevant to understanding solar system formation [3]. The authors arrive at this conclusion based on the massive and wavelength-dependent fractionation in CO2 they measured. I argue here that the laboratory CO photolysis results are, in fact, primarily a result of CO self-shielding, and I present line-by-line model simulations of 2 of their 4 (by wavelength) experiments. A critique of the Chakraborty et al. paper has been submitted [4] but will not be discussed in detail here. Simulation of experiments: The photolysis experiments [3] were carried out at the ALS synchrotron using the direct undulator beam (FWHM ~ 2 nm, photon flux ~ 5x10 ph s from 91-110 nm). The beam entered a phototube containing a CO column density of 4-12x10 cm. CO2, formed by recombination of O and CO, was collected on cold fingers near the end of the phototube. Experiments were performed with the beam centered at 94.12, 97.03, 105.17 and 107.61 nm. The experiments are continuous flow, and the residence time of CO in the tube is ~ 1 second. I have simulated the experiments at 105.17 and 107.61 nm by converting the code I used to compute self-shielding at the nebular surface [5] to a phototube geometry. Turbulent diffusion in the disk has been replaced by molecular diffusion of CO in the tube, with the diffusion coefficient given as ~ / 3 CO D ac ,