Transcript Bergin, E.
The Fossil History of the Solar System: Links to Interstellar Chemistry Edwin A. Bergin University of Michigan Jeong-Eun Lee UCLA James Lyons UCLA Background: Oxygen Isotopes in the Solar System • 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 16O/17O 2600 Background: Oxygen Isotopes in the Solar System • 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 17 16 – so if you plot ( O/ O)/ (18O/16O) then the slope would be 1/2 • for more information see Clayton 1993, Ann. Rev. Earth. Pl. Sci. Oxygen Isotopes in Meteorites • In 1973 Clayton and co-workers discovered that calciumaluminum-rich inclusions (CAI) in primitive chondrite meteorites had anomalous oxygen isotopic ratios. • Definition: x O 16 source O X ( O) x 11000 O 16 O s tan dard SMOW = standard mean ocean water - (18O) = (17O) = -50 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 meteoritic results can be from mixing of 2 reservoirs Wither 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 Theory: Isotope Selective Photodissociation Continuum Dissociation -2 Photoabsorption Cross-Section (cm ) Line Dissociation 2 -17 10 6 4 2 -18 10 6 4 2 -19 10 6 4 130 140 150 160 170 180 (nm) van Dishoeck and Black 1988 H2O: Yoshino et al 1996+ How Does Isotope Selective Photodissociation Work? Line Dissociation van Dishoeck and Black 1988 CO Photodissociation and Oxygen Isotopes Av < 0.5 C16O + h -> C + 16O C18O + h -> C + 18O 0.5 < Av < 2 C16O C18O + h -> C + 18O 18O + gr -> H218Oice Av > 2 C16O C18O 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) – credible solution – mixing may only be active on surface where sufficient ionization is present – cannot affect Solar oxygen isotopic ratio • active on cloud surface and provided to disk (Yurimoto and Kuramoto 2004) – did not present a detailed model – can affect both Sun and disk Model • chemical-dynamical model of Lee, Bergin, and Evans 2004 – cloud mass of 1.6 M◉ – approximate pre-collapse evolution as a series of Bonner-Ebert solutions with increasing condensation on a timescale of 1 Myr – use Shu 1977 “inside-out” collapse model – examine evolution of chemistry in the context of physical evolution (i.e.. cold phase - star turn on - warm inner envelope) – vary strength of external radiation field -- parameterized as G0, where G0 = 1 is the standard interstellar radiation field. • two questions – what level of rare isotope enhancement is provided to disk? – what is provided to Sun? Time Density Gas shielding Basic Chemistry 18O Evolution with a Range of UV Enhancements 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 - it is 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. Particle Drift in Viscous Disks • Gas orbits more slowly than solids at a given radius – results in a headwind on particles that causes them to drift inwards • Drift velocity depends on size – small grains (<< 1 cm) are coupled to the gas – meter-sized particles are the most rapidly drifting – large planetesimals experience decreasing drift speeds with size • Inner nebula can be enriched in water vapor as icy bodies rapidly We are now seeing evidence for singificant advect inward and evaporate evolution in systems as young as 1 Myr… inside the snow line. dust (Bergin et al. 2004, Calvet et al. 2005; Furlan et al. 2006 Cuzzi & Zahnle 2004 Model Infall Model Infall Model Ice coated grains sink to midplane Infall make rocks, which feel headwind and fall into star 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). – time that rocks are formed and fractionation begins is a variable – after fractionation begins assume that water is enhanced over CO by a factor of 5 - 10 • constraints – meteoritic and planetary isotope ratios – the solar oxygen isotope ratios The Solar Oxygen Isotope Ratio 1.8x105 2.7x105 3.6x105 time fractionation starts G0 = 0.4 G0 = 10 G0 = 103 G0 = 105 • (18O)◉ = 50 per mil implies a slightly enhanced UV field (G0 = 10) with Mf 0.1 M◉ Mf = amount of solar mass affected by fractionation •M (f 18 -50 per mil implies a weak (Gbegins =O) 0.1 that fractionation 4 ax 105 yrs ◉ =assumes 0 = 1) or strong UV field (G0 = 105) with Mf 0.1 M◉ after collapse Oxygen Depletion in the Inner Disk • • Have 3 potential solutions with variable radiation field that depend on the solar value Either: – Sun formed in a cluster with an O star – Sun formed bathed in a weak to moderate UV field • What about the rocks? – over time the inner nebula becomes depleted in enriched water vapor and enhanced in CO vapor with low isotopic ratios – need a continuous source of replenishment of ices with highly enriched isotope ratios Looking Back in Time: 1 Myr Before the Sun was Born • The solar oxygen isotope ratio is uncertain – 2 disparate solutions - each with significant implications for the formation of our Solar System • Recently the presence of the extinct radionuclide 60Fe (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 • We suggest that oxygen isotopes provide independent supporting evidence for 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 What is Provided to the Disk? 1.8x105 2.7x105 3.6x105 time fractionation starts G0 = 0.4 G0 = 10 G0 = 103 G0 = 105 All relevant solutions G0 = 0.4, 10, and 105 can match solar C/O ratio if Mf 0.05 - 0.1 M◉