The Fossil History of the Solar System: Links to Interstellar

Chemistry

Edwin A. Bergin University of

Michigan

Jeong-Eun Lee

UCLAJames LyonsUCLA

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

– 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:

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δ(X O) =

xO16O

⎛ ⎝ ⎜ ⎞

⎠ ⎟source

xO16O

⎛ ⎝ ⎜ ⎞

⎠ ⎟s tan dard

⎛

⎝

⎜ ⎜ ⎜

⎞

⎠

⎟ ⎟ ⎟− 1

⎧

⎨ ⎪

⎩ ⎪

⎫

⎬ ⎪

⎭ ⎪

1000

SMOW = standard mean ocean water - δ(18O) = δ(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

Terres

trial

line

Meteoritic line

Wither the Sun?

Considerable controversy regarding the Solar oxygen isotopic ratios.

2 Disparate Measurements: δ18O = δ17O = -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)

δ18O = δ17O = 50 per mil– contemporary lunar soil

(Ireland et al. 2006) differences are very

difficult to understand.

Huss 2006

Theory: Isotope Selective Photodissociation

4

610

-19

2

46

10-18

2

46

10-17

2

Photoabsorption Cross-Section (cm

-2)

180170160150140130

λ ( )nm

Line Dissociation Continuum Dissociation

van Dishoeck and Black 1988H2O: Yoshino et al 1996+

How Does Isotope Selective Photodissociation Work?

Line Dissociation

van Dishoeck and Black 1988

CO Photodissociation and Oxygen Isotopes

0.5 < Av < 2

C18O + h -> C + 18O

C16O

18O + gr -> H218Oice

C16O + h -> C + 16O

C18O + h -> C + 18O

Av < 0.5 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

δ18O Evolution with a Range of UV Enhancements

Issues

• large enhancements in δ18O and δ17O 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 advect inward and evaporate inside the snow line.

Cuzzi & Zahnle 2004

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

Model

Infall

Model

Infall

Model

Infall

Ice coated grainssink to midplane

make rocks, whichfeel 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

Mf = amount of solar mass affected by fractionationMf = 0.1 assumes that fractionation begins 4 x 105 yrsafter collapse

G0 = 0.4

G0 = 10

G0 = 103

G0 = 105

• δ(18O)◉ = 50 per mil implies a slightly enhanced UV field (G0 = 10) with Mf 0.1 M◉

• δ(18O)◉ = -50 per mil implies a weak (G0 = 1) or a strong UV field (G0 = 105) with Mf 0.1 M◉

1.8x1052.7x105 3.6x105 time fractionation starts

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 δ(18O)◉ = -50 per mil

What is Provided to the Disk?

G0 = 0.4

G0 = 10

G0 = 103

G0 = 105

All relevant solutions G0 = 0.4, 10, and 105 can matchsolar C/O ratio if Mf 0.05 - 0.1 M◉

1.8x1052.7x1053.6x105 time fractionation starts