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Interstellar Chemistry: Exploring Links to Comets and Meteorites


This material is advected inwards and provided to the meteorite formation zone ... gas and grain advection in the disk must be decoupled in some way to enrich ... – PowerPoint PPT presentation

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Title: Interstellar Chemistry: Exploring Links to Comets and Meteorites

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
  • 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
  • Is an observed galactic gradient (Wilson and Rood
  • 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
  • so if you plot ?(17O/16O)/ ?(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
    calcium-aluminum-rich inclusions (CAI) in
    primitive chondrite meteorites had anomalous
    oxygen isotopic ratios.
  • Definition

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

Terrestrial line
Meteoritic line
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
  • ?18O ?17O 50 per mil
  • contemporary lunar soil (Ireland et al. 2006)
  • differences are very difficult to understand.

Huss 2006
Theory Isotope Selective Photodissociation
Line Dissociation
Continuum Dissociation
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
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

  • 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?

Basic Chemistry
?18O Evolution with a Range of UV Enhancements
  • large enhancements in ?18O and ?17O are provided
    to the disk at all radii in the form of water
  • This material is advected inwards and provided to
    the meteorite formation zone (r lt 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
  • results in a headwind on particles that causes
    them to drift inwards
  • Drift velocity depends on size
  • small grains (ltlt 1 cm) are coupled to the gas
  • meter-sized particles are the most rapidly
  • large planetesimals experience decreasing drift
    speeds with size
  • Inner nebula can be enriched in water vapor as
    icy bodies rapidly advect inward and evaporate
    inside the snow line.

We are now seeing evidence for singificant dust
evolution in systems as young as 1 Myr (Bergin
et al. 2004, Calvet et al. 2005 Furlan et al.
Cuzzi Zahnle 2004
  • Assume material provided at inner radius of our
    model (100 AU) is advected unaltered to the inner
  • Assume significant grain evolution has occurred
    and material fractionation has occurred (gas/ice
  • 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
  • ?(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
  • strong UV field (G0 105) with Mf ? 0.1 M?

Mf amount of solar mass affected by
fractionation Mf 0.1 assumes that fractionation
begins 4 x 105 yrs 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
  • The solar oxygen isotope ratio is uncertain
  • 2 disparate solutions - each with significant
    implications for the formation of our Solar
  • 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 (lt 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?
All relevant solutions G0 0.4, 10, and 105 can
match solar C/O ratio if Mf ? 0.05 - 0.1 M?