Interstellar Chemistry: Exploring Links to Comets and Meteorites

1
The Fossil History of the Solar System Links to
Interstellar Chemistry
Edwin A. Bergin University of Michigan
Jeong-Eun Lee UCLA
James Lyons UCLA
2
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

3
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.

4
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
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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

Terrestrial line
Meteoritic line
6
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
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Theory Isotope Selective Photodissociation
Line Dissociation
Continuum Dissociation
van Dishoeck and Black 1988
H2O Yoshino et al 1996
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How Does Isotope Selective Photodissociation Work?
Line Dissociation
van Dishoeck and Black 1988
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CO Photodissociation and Oxygen Isotopes
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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

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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?

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Basic Chemistry
13
?18O Evolution with a Range of UV Enhancements
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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 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.

15
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 (ltlt 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 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.
2006
Cuzzi Zahnle 2004
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Model
Infall
17
Model
Infall
18
Model
Infall
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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

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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 assumes that fractionation
begins 4 x 105 yrs after collapse
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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

22
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 (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

23
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?