Oxygen Isotopes Anomalies of the Sun and the Original Environment of the Solar System

1. Oxygen Isotopes Anomalies of the Sun and the Original Environment of the Solar System Jeong-Eun Lee UCLA

2. collaborators • Edwin A. Bergin (Univ. of Michigan) • James R. Lyons (UCLA)

4. The solar system and ISM • Matter from stars (stellar winds of red giant stars and supernova explosions) is expelled to ISM. • The molecular clouds are sites for star formation. • Extensive chemical and physical processing of materials in the Solar nebula and planetary bodies destroys the ISM heritage. • But: Asteroids and comets have escaped significant alteration by the reprocessing. Primitive bodies such as comets, meteorites, and IDPs possibly preserve the oldest solar system solids material to provide opportunities to probe the astrophysical environment when the Sun formed.

5. Isotopic Anomalies • Complete isotopic homogenization is expected from the chemical and physical processing of solar system materials. • Thus: any surviving presolar material will have an exotic isotopic composition, which could not have resulted from processes occurring in the solar system. Exotic Isotopic Ratios measured from IDPs, Meteorites, and Comets (connected to ISM): D/H, 15N/14N, 18O/16O (17O/16O)

6. Oxygen Isotopes • Oxygen isotope production 16O produced in stellar nucleosynthesis by He burning provided to ISM by supernovae rare isotopes 17O and 18O produced in CNO cycles novae and supernovae • Expected that ISM would have regions that are inhomogeneous • Is an observed galactic gradient (Wilson and Rood 1992) • Solar values 16O/18O  500 and 17O/18O  2600

7. Oxygen Isotopes • chemical fractionation can also occur in ISM • except for H, kinetic chemical isotopic effects are in general of order a few percent • distinguishes fractionation from nuclear sources of isotopic enrichment • almost linearly proportional to the differences in mass between the isotopes • Ex: a chemical process that produces a factor of x change in the 17O/16O ratio produces a factor of 2x change in the 18O/16O ratio • so if you plot (17O/16O)/ (18O/16O) then the slope would be 1/2

8. Oxygen Isotopes in Meteorites • In 1973 Clayton and co-workers discovered that calcium-aluminum-rich inclusions (CAI) in primitive meteorites had anomalous oxygen isotopic ratios.

9. Oxygen Isotopes in Meteorites • Earth, Mars, Vesta follow slope 1/2 line indicative of mass-dependent fractionation • primitive CAI meteorites (and other types) follow line with slope ~ 1 indicative of mass independent fractionation Terrestrial line Meteoritic line SMOW = standard mean ocean water

10. Oxygen Isotopes in Meteorites meteoritic results can be from mixing of 2 reservoirs Terrestrial line - 16O poor Meteoritic line - 16O rich Solar value? -The initial value of molecular cloud

11. Oxygen Isotopes in the Sun Considerable controversy regarding the Solar oxygen isotopic ratios. 2 Disparate Measurements: • 18O = 17O = -50 per mil • lowest value seen in meteorites • seen in ancient lunar regolith (exposed to solar wind 1-2 Byr years ago; Hachizume & Chaussidon 2005) • 18O = 17O = 50 per mil • contemporary lunar soil (Ireland et al. 2006) • differences are very difficult to understand. Huss 2006

12. Theory • stellar nucleosynthesis • lack of similar trend seen in other elements • chemical reactions that are non-mass dependent (Thiemens and Heidenreich 1983) • known to happen in the Earth’s atmosphere (for ozone) • no theoretical understanding of other reactions that can link to CO and H2O • photo-chemical CO self-shielding • suggested by Clayton 2002 at in the inner nebula at the edge of the disk (X point) • active on disk surface (Lyons and Young 2005) • active on cloud surface and provided to disk (Yurimoto and Kuramoto 2004)

13. Av > 2 Av < 0.5 0.5 < Av < 2 C16O + h -> C + 16O C16O C16O C18O + h -> C + 18O C18O + h -> C + 18O C18O 18O + gr -> H218Oice CO Photodissociation and Oxygen Isotopes

14. CO Self-Shielding Models • active in the inner nebula at the edge of the disk (Clayton 2002) • only gas disk at inner edge, cannot make solids as it is too hot • active on disk surface and mixing to midplane (Lyons and Young 2005) • mixing may only be active on surface where sufficient ionization is present • cannot affect Solar oxygen isotopic ratio

15. CO Self-Shielding Models • active on cloud surface and provided to disk (Yurimoto and Kuramoto 2004) • did not present a detailed model • can affect both Sun and disk

16. Model • chemical-dynamical model of Lee, Bergin, and Evans 2004 • use Shu 1977 “inside-out” collapse model • cloud mass of 3.6 M◉ • approximate pre-collapse evolution as a series of Bonner-Ebert solutions with increasing condensation • examine evolution of chemistry in the context of physical evolution • model updated to include CO fractionation and isotopic selective photodissociation • two questions • what level of rare isotope enhancement is provided to disk? • what is provided to Sun?

17. Temperature and Density Evolution in the Model

19. 18O Evolution with a Range of UV Enhancements

20. Issues • large enhancements in 18O and 17O are provided to the disk at all radii in the form of water ice. • This material is advected inwards and provided to the meteorite formation zone (r < 4 AU). • BUT: - the gas has an opposite signature - enriched in 16O in the form of CO - gas and grain advection in the disk must be decoupled in some way to enrich inner disk in heavy oxygen isotopes relative to 16O • Icy grains drift inward due to gas drag(Cuzzi et al . 2004) • Gas orbits more slowly than solids at a given radius • results in a headwind on particles that causes them to drift inwards

21. Model • Assume material provided at inner radius of our model (100 AU) is advected unaltered to the inner disk • Assume significant grain evolution has occurred and material fractionation has occurred (gas/ice segregation) in the disk. • time this fractionation begins is a variable • after fractionation begins assume that water is enhanced over CO by a factor of 5 - 10 • constraints • the solar oxygen isotope ratios • the solar C/O ratio - need to assume (C/O)initial > (C/O)◉

22. time fractionation starts 1.8x105 2.7x105 3.6x105 G0 = 0.4 G0 = 10 G0 = 103 G0 = 105 The Solar Oxygen Isotope Ratio  (18O)◉ = 50 per mil implies a slightly enhanced UV field (G0 = 10) with Mf ~ 0.1 M◉ (18O)◉ = -50 per mil implies a weak (G0 = 1) or a strong UV field (G0 = 105) with Mf ~ 0.1 M◉ Mf = amount of solar mass affected by fractionation Mf = 0.1 if fractionation begins 4 x 105 yrs after collapse

23. G0 = 0.4 G0 = 10 G0 = 103 G0 = 105 The solar C/O ratio 2.7x105 3.6x105 time fractionation starts 1.8x105 All relevant solutions G0 = 0.4, 10, and 105 can match solar C/O ratio if Mf ~ 0.05 - 0.1 M◉

24. More constraints on G0 • Have 3 potential solutions with variable radiation field that depend on the solar value More constraints? • meteoritic and planetary isotope ratios • water ices in comets… Go=105 !!!

25. Looking Back in Time: Before the Sun was Born Our model of oxygen isotopes suggests the presence of a massive O star in the vicinity of the forming Sun 1 million years before collapse and that the Solar value is (18O)◉ = -50 per mil.

26. Looking Back in Time: Before the Sun was Born • Recently the presence of the extinct radionuclide60Fe (1/2 = 1.5 Myr) is inferred in meteorites with varied composition (Tachibana & Huss 2003; Mosteraoui et al. 2005; Tachibana et al. 2006) • cannot be produced by particle irradiation • abundance consistent with production in nucleosynthesis in a Type II supernova or an intermediate-mass AGB star and provided to the solar system before formation • probability of an encounter between Sun and intermediate mass AGB star is low (< 3 x 10-6; Tachibana et al. 2006) • taken as strong evidence that Sun formed in a stellar cluster near an O star