Download The Origin of Oxygen Isotopic Anomalies Seen in Primitive Meteorites

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.

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
 11000


 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
meteorites have oxygen
isotope ratios where the rare
isotopes are slightly more
abundant (50 per mil) than
16O.
Oxygen Isotopes in Meteorites
•
meteoritic results can
be from mixing of 2
reservoirs
- 16O poor
- 16O rich
•
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?
Continuum Dissociation
2
-2
Photoabsorption Cross-Section (cm )
Line Dissociation
-17
10
6
4
2
-18
10
6
4
2
-19
10
6
4
130
140
150
160
(nm)
170
180
How Does Isotope Selective Photodissociation Work?
Line Dissociation
CO Photodissociation and Oxygen Isotopes
Av < 0.5
CO + h -> C + O
C18O + h -> C + 18O
0.5 < Av < 2
CO
C18O + h -> C + 18O
18O
+ gr -> H218Oice
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 BonnerEbert 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
Pre-Stellar Core
0
2
8-10
8-10
Embedded Star
2
0
Cloud Depth (mag)
0
2
8-10
8-10
2
*calibrated to low mass core embedded in ISRF
0
Physical Evolution: Density and Velocity
Time steps for
inside-out collapse
t=5x105
t=1.6x105
t=105
t=5x104
t=2.5x104
t=104
t=5x103
t=2.5x103
t=0
Physical Evolution: Temperature
t=5x105
Heated by
ISRF
Evolution By Parcel
450
400
350
300
Density
250
200
150
100
Parcel #
Distance from the center
Dust temperature
Visual extinction
Basic Chemistry
18O Evolution Before and After Collapse
Before Collapse
105 years after collapse
Y=10-5, G0=0.7
Y=10-4, G0=0.7
Y=10-3, G0=0.7
Y=10-3, G0=1.7
Y=10-3, G0=3.4
18OSMOW (‰)
18O in Water Ice and CO vapor
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 AU
Green = 1000 AU
What is Provided to the Disk?
- 70% of mass in
model
contains
Red
= 200
AU enhancements (2.6 M◉).
- In disk model (Lyons
Young)
only 0.02% of total disk
Greenand
= 1000
AU
mass is affected.
Blue = 2000 AU
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:
•
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)
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
•
•
•
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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)