The Origin of Oxygen Isotopic Anomalies Seen in Primitive Meteorites

Edwin A. Bergin (U. Mich) 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– 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.

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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

• meteorites have oxygen isotope ratios where the rare isotopes are slightly more abundant (50 per mil) than 16O.

Terrestr

ial line

Met

eorit

ic lin

e

Oxygen Isotopes in Meteorites

• meteoritic results can be from mixing of 2 reservoirs

Terrestr

ial line

Met

eorit

ic lin

e- 16O rich

- 16O poor

• thought 16O poor state in gas (Clayton 1993, etc.)

Theory

• stellar nucleosynthesis– lack of similar trend seen in outer 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)

How Does Isotope Selective Photodissociation Work?

4

610

-19

2

46

10-18

2

46

10-17

2

Photoabsorption Cross-Section (cm

-2)

180170160150140130

λ ( )nm

Line Dissociation Continuum Dissociation

How Does Isotope Selective Photodissociation Work?

Line Dissociation

CO Photodissociation and Oxygen Isotopes

0.5 < Av < 2

C18O + h -> C + 18O

CO

18O + gr -> H218Oice

CO + h -> C + O

C18O + h -> C + 18O

Av < 0.5 Av > 2

CO

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– use Shu 1977 “inside-out” collapse model– approximate pre-collapse evolution as a series of

Bonner-Ebert solutions with increasing condensation on a timescale of 1 Myr

– examine evolution of chemistry in the context of physical evolution (i.e.. cold phase - star turn on - warm inner envelope)

– 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

Chemical Evolution

Cloud Depth (mag) *calibrated to low mass core embedded in ISRF

0 2 8-10 8-10 2 0 0 2 2 08-10 8-10

Pre-Stellar Core Embedded Star

Physical Evolution: Density and Velocity

t=2.5x103

t=5x103

t=104

t=2.5x104

t=5x104

t=105

t=1.6x105

t=5x105

t=0

Time steps for inside-out collapse

Physical Evolution: Temperature

Heated by

ISRF

t=5x105

Evolution By Parcel

100

150200

250300

350400

450

Distance from the center

Density

Dust temperature Visual extinction

Parcel #

Basic Chemistry

δ18O Evolution Before and After Collapse

Before Collapse 105 years after collapseY=10-5, G0=0.7Y=10-4, G0=0.7Y=10-3, G0=0.7Y=10-3, G0=1.7Y=10-3, G0=3.4

δ18O in Water Ice and CO vapor

δ18 O

SMO

W (

‰)

What is Provided to the Disk?

Red = 200 AU

What is Provided to the Disk?

Red = 200 AU

shift original 16O/17O to 2580 from 2600

What is Provided to the Disk?

Red = 200 AUGreen = 1000 AU

What is Provided to the Disk?

Red = 200 AUGreen = 1000 AUBlue = 2000 AU

- 70% of mass in model contains enhancements (2.6 M◉).- In disk model (Lyons and Young) only 0.02% of total disk mass is affected.

Can this Work?

• Material provided to outer disk - at 100 to 1000 AU– advect to 1 AU in < 105 yrs– will not undergo loss of ice either by radiative

heating or an accretion shock

• Midplane of disk is seeded with isotopic enhancements simply by collapse.

• Cuzzi & Zahnle (2004) showed that drifting ice grains with enhancements evaporate at snow line, enriching gas with heavy isotopes

• Grains and gas provide reservoir for chondrite formation.

Wither the Sun?

Some controversy regarding the Solar oxygen isotopic ratios.

Estimates are: 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)

Huss 2006

The Pre-History of the Sun

Based on our model (preliminary):

• if the Sun is at the low end then any massive O star in the vicinity must not have been present 1 Myr before the Sun was born.– or need an additional 9

magnitudes of extinction• If Sun at high end then

massive O star cannot have been nearby but some UV enhancement is needed. – with 9 magnitudes of

extinction O star could have been within 0.1 pc.

• This model can easily account for trends in both Sun and disk!

Before Collapse

Interstellar Origin of Meteoritic Isotopic Anomalies

• Isotopic selective photodissociation in the outer layers of the solar nebula can seed the forming planetary disk with anomalies consistent with observed meteoritic trends.

• Model can also account for the unknown Solar ratios (if enhanced above -50 per mil).

• Sets new constraints on the presence of a massive star near the forming solar system.

• Results to appear in Lee, Bergin, and Lyons (2006) - to be submitted…

Interstellar Origin of Meteoritic Oxygen Isotopic Anomalies

Edwin A. Bergin (U. Mich) Jeong-Eun Lee (UCLA)James Lyons (UCLA)