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Evaluating CO Self-Shielding and the Oxygen Isotopic Evolution of the Solar Protoplanetary Disk with Astrochemical Reaction Networks

Introduction: Self shielding by CO in the early stages of evolution of the solar protoplanetary disk may be the explanation for the anomalous array of O/O and O/O in primitive solar system materials (the slope-1 line on a plot of δO vs. δO) [1, 2]. Thorough evaluation of the various hypotheses that CO photochemistry is the source of the ∆O variability in the solar system requires detailed astrochemical calculations involving oxygen isotopologues of all Obearing species and transport of chemical species between disk reservoirs. Here I present reaction network and reservoir exchange calculations that constrain the conditions under which CO self shielding can produce ∆O variability like that observed in the inner solar system. Models: The kinetic reaction network was constructed from the UMIST rate99 database by adding oxygen isotopologues (MO and MO) for each Obearing species MO. Reaction rates for the O isotopologues were adjusted to allow for mass-dependent isotope fractionation. Oxygen isotope exchange reactions were added, as well as several grain surfacemediated reactions. The calculations include CO and H2 mutual and self shielding, and were performed for an ultraviolet flux field defined by a parameterized circumstellar disk model [3]. For the results shown here, 546 species and 7603 reactions comprising a closed set were considered. The resulting stiff set of ODEs were solved using the Livermore Solver for Ordinary Differential Equations (LSODE). The results of the reaction network calculations were used to evaluate the isotopic consequences of exchange of mass between four reservoirs in the disk and its environs: 1. molecular cloud; 2. disk surfaces; 3. inner disk; and 4. the star. Results: UV flux. Isotopic effects of CO self shielding are most pronounced where the optical depth for CO is ~ 4. Comparison of rates of CO photodissociation with vertical illumination by a uniform uv flux of 200x local ISM with uv illumination along lines of site from the central star with a similar flux at 100 AU (low end of estimates for a T-Tauri star field) shows only small differences at large radial distances R from the star. Differences become substantial as uv flux rises sharply in the latter case with proximity to the star. Time scales of reaction. At R > 10 AU the net result of CO photodissociation is wholesale conversion of CO gas to H2O water ice. The formation of water ice occurs without mediation on grain surfaces. The time scale for water ice formation from CO is on the order of 10 yrs (Fig. 2). At R ~ 10 AU a steady state is achieved at disk surfaces such that reactions that produce CO are balanced by the conversion of CO to H2O ice. At R ≤ 5 AU reaction time scales are too rapid to preserve the isotopic effects of CO photodissociation and self shielding due to the high uv flux. A: CO photodissociation rate (cm s) stellar site line, 200 Habing at 100 AU