Transcript Oxygen Isotopes Anomalies in the Solar System and the G0

Oxygen Isotopes Anomalies
of
the Sun and the Original
Environment of the Solar System
Jeong-Eun Lee
UCLA
collaborators
 Edwin A. Bergin (Univ. of Michigan)
 James R. Lyons (UCLA)
The solar system and ISM
 Matter from stars (stellar winds of red giant stars
and supernova explosions) is expelled to ISM.
 The molecular clouds are sites for star formation.
 Extensive chemical and physical processing of
materials in the Solar nebula and planetary bodies
destroys the ISM heritage.
 But:
Asteroids and comets have escaped significant
alteration by the reprocessing.
Primitive bodies such as comets, meteorites, and IDPs
possibly preserve the oldest solar system solids material
to provide opportunities to probe the astrophysical
environment when the Sun formed.
Isotopic Anomalies
 Complete isotopic homogenization is expected
from the chemical and physical processing of
solar system materials.
 Thus:
any surviving presolar material will have an
exotic isotopic composition, which could not
have resulted from processes occurring in the
solar system.
Exotic Isotopic Ratios measured from IDPs,
Meteorites, and Comets (connected to ISM):
D/H, 15N/14N, 18O/16O (17O/16O)
Oxygen Isotopes
 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 17O/18O  2600
Oxygen Isotopes
 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 ratio
 so if you plot (17O/16O )/ (18O/16O) then the
slope would be 1/2
Oxygen Isotopes in Meteorites
 In 1973 Clayton and
co-workers discovered
that calciumaluminum-rich
inclusions (CAI) in
primitive meteorites
had anomalous oxygen
isotopic ratios.
Oxygen Isotopes in Meteorites
 Earth, Mars, Vesta
 x O
 



  16O 
 
X
source
( O)   x
 11000


 O 16 
 


O


s tan dard 
follow slope 1/2 line
indicative of massdependent
fractionation
 primitive CAI
meteorites (and other
types) follow line
with slope ~ 1
indicative of mass
independent
fractionation
SMOW = standard mean ocean water
Oxygen Isotopes in Meteorites
meteoritic results
can be from mixing
of 2 reservoirs
- 16O poor
- 16O rich
Solar value?
-The initial value
of molecular
cloud
Oxygen Isotopes in 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
 stellar nucleosynthesis
 lack of similar trend seen in other 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)
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)
 mixing may only be active on surface where
sufficient ionization is present
 cannot affect Solar oxygen isotopic ratio
CO Self-Shielding Models
 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
 cloud mass of 3.6 M◉
 approximate pre-collapse evolution as a series of Bonner-
Ebert solutions with increasing condensation
 examine evolution of chemistry in the context of physical
evolution
 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?
Temperature and Density
Evolution in the Model
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 - 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
Icy grains drift inward due to gas drag (Cuzzi et al . 2004)
Gas orbits more slowly than solids at a given radius
–results in a headwind on particles that causes them to drift inwards
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) in the disk.
 time this fractionation begins is a variable
 after fractionation begins assume that water is
enhanced over CO by a factor of 5 - 10
constraints
the solar oxygen isotope ratios
the solar C/O ratio - need to assume (C/O)initial > (C/O)◉
The Solar Oxygen Isotope Ratio
1.8x105 2.7x105 3.6x105
time fractionation
starts
G0 = 0.4
G0 = 10
G0 = 103
G0 = 105
M
amount
solar
affected
by enhanced
fractionation
 f (=18O)
per
mil mass
implies
a slightly
◉ = 50of
5
MfUV
= 0.1
if fractionation
4 xM
10
field
(G0 = 10) withbegins
Mf ~ 0.1
◉ yrs after
collapse
(18O)◉ = -50 per mil implies a weak (G0 = 1) or a
strong UV field (G0 = 105) with Mf ~ 0.1 M◉
The solar C/O ratio
1.8x1052.7x105 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◉
More constraints on G0
 Have 3 potential solutions
with variable radiation
field that depend on the
solar value
More constraints?
 meteoritic and planetary
isotope ratios
 water ices in comets…
Go=105 !!!
Looking Back in Time: Before the Sun was Born
Our model of oxygen isotopes suggests
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.
Looking Back in Time: Before the Sun was Born
 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