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The Oxygen Isotope Composition of the Sun: Implications for Oxygen Processing

This article by Trevor R. Ireland explores the oxygen isotope composition of the Sun and its implications on various astronomical processes. From the nuclear compositions sensitive to origin to the isotopic anomalies in solar nebula, the study delves into the complexities of oxygen processing in molecular clouds, star-forming regions, and the solar system. Key topics covered include the presence of presolar material, isotopic anomalies, nucleosynthetic additions, and photochemical fractionation. The article sheds light on the origin of oxygen isotope anomalies and the mechanisms at play in the solar nebula. Understanding the solar oxygen isotope composition is crucial for deciphering the evolution of the solar system.

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The Oxygen Isotope Composition of the Sun: Implications for Oxygen Processing

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  1. THE OXYGEN ISOTOPE COMPOSITION OF THE SUN IMPLICATIONS FOR OXYGEN PROCESSING IN MOLECULAR CLOUDS, STAR-FORMING REGIONS, AND THE SOLAR NEBULA. • Trevor R. Ireland • Planetary Science Institute and • Research School of Earth Sciences • The Australian National University • Canberra, Australia

  2. Introduction • Nuclear Astrophysics • H-R Diagram • Nuclear compositions sensitive to origin • Isotope Cosmochemistry • Solar System abundances mixtures of processes

  3. Presolar material • Premise • presolar material will differ from solar system in terms of its isotopic composition • SS is an average • any discrete component will likely deviate from average in one or more isotopic systems • presolar grains or presolar memory?

  4. Chemical Memory • Refractory Inclusions - earliest Hi-T objects Formed in solar system

  5. Isotopic Anomalies • Prior to 1973 • Isotopic anomalies only in noble gases, H • Hot homogeneous solar nebula • 1973 • O isotopic anomalies • Apparent enrichment in 16O by up to 4 % • nucleosynthetic component • hot-cold heterogeneous nebula

  6. Fe-group isotope anomalies • Anomalies in n-rich isotopes • commonly at 0.1 % • FUN inclusions at 1 % • hibonites at 10%

  7. Solar Nebula • Nucleosynthetic addition of 16O • Correlated with • 50Ti (n-rich isotope) • 26Al (halflife = 0.7 Ma) • Supernova injection • Trigger for collapse of molecular cloud into solar nebula

  8. Hibonite Inclusions • O isotopic anomalies do not scale with Ti isotopic anomalies • ∂50Ti: -70 to +270 ‰ • excess 16O: +40 to +60 ‰ • no sign of presolar 16O carriers

  9. Presolar Silicate • Imaging of IDP slices 16O 17O/16O Messenger et al. (2002) 0.25 µm forsterite 16O/17O = 440 17O: 5 times solar! TEM

  10. 17O/16O ozone 18O/16O air oxygen Source of Oxygen Isotope Anomalies • Nucleosynthetic (?) • Chemical Fractionation • Mass independent fractionation • Stratospheric oxygen-ozone • Airfall nitrates • Mechanism in solar nebula • AlO + O•, SiO + O• • CO - Predissociation

  11. 17O/16O 17O/16O H2O ice 18O/16O 18O/16O CO dust dust CO Inner cloud Originally, and outer cloud Photochemical Fractionation • Predissociation of CO in molecular cloud by UV • CO + hv ->C• + O• • Outer cloud • sufficient photons to dissociate C16O C17O C18O • but progressive self shielding of C16O (99.8 • Inner cloud • not enough photons to dissociate C16O • enrichment of dissociated 17O• and 18O• • 17O• and 18O• react with H (H2O ice) • residual CO gas 16O enriched

  12. Solar Nebula Fractionation • R. N. Clayton (2002) • Early solar UV causes predissociation • Dust becomes enriched in 17O, 18O • Dust recycled through disk • Nebula temperature incompatible with quantized absorption (?)

  13. Molecular Cloud Inheritance • Yurimoto and Kuramoto (2004) • Oxygen isotope fractionations inherited from the molecular cloud • 17O and 18O react with dust • altered dust • unreacted dust preserves original composition • refractory dust • Mixing of altered dust and refractory dust • CAI mixing line

  14. The solar oxygen isotope composition • Key to understanding solar system evolution • Photochemical predissociation (nebula or MC) predicts Sun should be most 16O enriched • Mass Independent Fractionation suggests solar close to terrestrial

  15. Directly sample solar wind

  16. Solar Wind on the Moon • Solar wind • sputtering by H, He (98% of solar wind) • isotopic mass fractionation in soil (Si, Ca, K, O...) • implantation • H, He, C, N, O, ... • Lunar soil predominantly silicates and oxides • Look for target mineral with low intrinsic oxygen • metal grains

  17. Oxygen isotopes in metal • A natural Genesis Experiment • little intrinsic oxygen • surface oxidation? • implanted solar wind O • Target is 50 µm, 70 ng spherule • analyze 1 ng of Fe • less than 10 pg of oxygen • with variable composition

  18. SHRIMP • Sensitive High Resolution Ion MicroProbe

  19. Δ17O = +27 ‰ Δ16O = -53 (±5, 2σ) ‰ SW Oxygen isotopes? • high surface concentration of oxygen • mass fractionated • contamination • exotic oxygen at depth (20 nm +) • 16O fractionated

  20. Solar or Local Origin? • IS solar wind • solar wind is representative of Sun • primordial nebula composition • solar wind is not representative of Sun or nebula • late stage infall to Sun (nebula water) • IS NOT solar wind • local lunar phenomena (?) • mass independent fractionation or another source

  21. If Solar Composition... • What happened? • Sun appears inconsistent with any model • Some indications for low 16O in • cometary water • high dust/gas enrichments (Wiens et al. 1999) • Need to explain why Sun is heaviest composition observed • Molecular Cloud Inheritance • Solar Nebula Processing

  22. 17O/16O 17O/16O H2O ice H2O ice average average 18O/16O 18O/16O dust dust CO Molecular Cloud Inheritance • Model of Yurimoto & Kuramoto (2004) • Sun is lightest composition • Gas - Dust Fractionation • remove C16O • increases average isotopic weight • but, requires removal of >80% of C16O gas

  23. Sun Earth +5% CAI +11% Solar Nebula Process • Nebula Photochemistry • requires 16O enrichment of most refractory solids • Mass independent fractionation • c.f stratospheric oxygen • production of isotopically light reactant • reaction scheme? • Outer solar system water • e.g. cometary • but where’s the solar wind?

  24. Consequences • Refractory Inclusions can not be solar condensates • how do (REE) volatility fractionations occur? • consistent with preservation of isotopic anomalies in residues • Planets highly fractionated from Solar Composition • terrestrial planets (very small component of SS) • Cometary source • where’s the solar wind? • Local lunar phenomenon • mass independent fractionation occurring in nebula

  25. Synopsis - more Q than A • Measurement of solar wind in lunar grains reveals a new oxygen isotopic component in the solar system • The solar composition is apparently the heaviest (most 16O depleted) component yet measured • Difficult to reconcile with any current models of solar system formation-evolution • Analysis of Genesis, Stardust, Hayabusa samples needed to resolve nature of solar wind

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