Oxygen and Nitrogen Isotopes in the Sun and Solar Nebula

Abstract:

The record of light stable isotope signatures in early solar system materials provides key constraints on the formation environment, mixing processes, and loss processes that influenced the solar nebula. Considering first the Sun, the NASA Genesis mission showed that the solar wind has a ¹⁷O depletion that is slightly more negative than the isotopically lightest CAIs (McKeegan et al. 2011), but still only yields an inferred photospheric (and bulk) solar O isotope composition. Recent reanalysis of astronomical measurement of O isotopes from infrared CO lines (Ayres et al. 2013) argues for a similarity in d¹⁸O for the directly observed photosphere and the inferred photosphere from Genesis, in contrast to the wildly divergent astronomical values previously reported. The accuracy of the Ayres et al. results are limited by the accuracy of the IR oscillator strength models; I will report on the impact of new oscillator strength models for CO. Turning to the CAIs, the Genesis O isotope results are consistent with CO self-shielding as the mechanism for producing the CAI mixing line from a mixture of photo-produced H₂O (¹⁷O-enriched) and solar-like initial material (Clayton 2002; Lyons and Young 2005). Two other mechanisms have been proposed: ¹⁷O enrichment from 1) high temperature SiO reactions with OH (Chakraborty et al. 2013), and 2) transfer from low temperature O₃ formation on dust. I will critique both of these suggested mechanisms. Finally, N isotope ratios, which span a range in d¹⁵N values from ~-400 permil for the Sun (Marty et al. 2011) and solar nebula, to +1000s permil for hotspots in CB chondrites, are influenced by low temperature ion-molecule reactions, and potentially by photoprocesses. Heays et al. (2014) have shown that N₂ photolysis can produce very large ¹⁵N enrichments in product HCN in a static protoplanetary disk. I will present similar calculations for a turbulently mixed disk. Implications for cometary isotopic composition will be presented.