Download 18 O

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 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

17
16
– so if you plot ( O/ O)/ (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
meteoritic results can be
from mixing of 2 reservoirs
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
Theory: Isotope Selective Photodissociation
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
170
180
(nm)
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
Av < 0.5
C16O + h -> C + 16O
C18O + h -> C + 18O
0.5 < Av < 2
C16O
C18O + h -> C + 18O
18O
+ gr -> H218Oice
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
18O Evolution with a Range of UV Enhancements
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 < 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
We
are
now
seeing
evidence for singificant
advect inward and evaporate
evolution
in systems
as young as 1 Myr…
inside the snow
line.
dust
(Bergin et al. 2004, Calvet et al. 2005;
Furlan
et al. 2006
Cuzzi
& Zahnle
2004
Model
Infall
Model
Infall
Model
Ice coated grains
sink to midplane
Infall
make rocks, which
feel 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
1.8x105 2.7x105 3.6x105
time fractionation
starts
G0 = 0.4
G0 = 10
G0 = 103
G0 = 105
• (18O)◉ = 50 per mil implies a slightly enhanced
UV field (G0 = 10) with Mf  0.1 M◉
Mf = amount of solar mass affected by fractionation
•M
(f 18
-50 per mil
implies
a weak (Gbegins
=O)
0.1
that
fractionation
4 ax 105 yrs
◉ =assumes
0 = 1) or
strong
UV field (G0 = 105) with Mf  0.1 M◉
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 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
(18O)◉ = -50 per mil
What is Provided to the Disk?
1.8x105 2.7x105 3.6x105
time fractionation
starts
G0 = 0.4
G0 = 10
G0 = 103
G0 = 105
All relevant solutions G0 = 0.4, 10, and 105 can match
solar C/O ratio if Mf  0.05 - 0.1 M◉