Transcript UCLA 2004

The Astrophysical Origins of the
Short-Lived Radionuclides in the
Early Solar System
Steve Desch
November 30, 2004
UCLA - IGPP
with a shout-out to my ASU supernova posse:
Nicolas Ouellette, Jeff Hester, Laurie Leshin, Gary Huss
Outline
• Short-lived radionuclides:
– What are they?
– How are they measured?
• Possible sources:
– Inheritance
– Irradiation
– Injection
• “Aerogel” model:
– Astrophysical context
– SLR predictions
Short-Lived Radionuclides
“SLRs” = Radionuclides with
half-lives t1/2 < 16 Myr
Early Solar System SLRs Confirmed
by Isotopic Analyses of Meteorites:
41Ca
(t1/2 = 0.1 Myr) (Srinivasan et al. 1994, 1996)
36Cl (t
1/2 = 0.3 Myr) (Murty et al. 1997; Lin et al. 2004)
26Al (t
1/2 = 0.7 Myr) (Lee et al. 1976)
60Fe (t
1/2 = 1.5 Myr) (Tachibana & Huss 2003; Mostefaoui et al. 2004)
10Be (t
1/2 = 1.5 Myr) (McKeegan et al. 2000; Sugiura et al. 2001)
53Mn (t
1/2 = 3.7 Myr) (Birck & Allegre 1985)
107Pd (t
1/2 = 6.5 Myr) (Kelly & Wasserburg 1978)
182Hf (t
Myr) (Harper & Jacobsen 1994)
1/2 = 9
129I (t
1/2 = 15.7 Myr) (Jeffery & Reynolds 1961)
Isotopic analyses of meteorites
show they once held SLRs:
Excess 10B is from
decay of 10Be
Slope gives original
10Be/9Be ratio
“Natural”
10B / 11B ratio
McKeegan et al. (2000)
Initial Abundances of Confirmed SLRs:
Possibly 60Fe/56Fe = 1.6x10-6
irons
Unconfirmed SLRs:
7Be
(t1/2 = 57 days)
(Chaussidon et al. 2004)
63Ni
(t1/2 = 101 years) (Luck et al. 2003)
97Tc
(t1/2 = 2.6 Myr) (Yin& Jacobsen 1998)
99Tc
(t1/2 = 0.21 Myr) (Yin et al. 1992)
135Cs
(t1/2 = 2.3 Myr) (McCulloch & Wasserburg
1978; Hidaka et al. 2001)
205Pb
(t1/2 = 15 Myr) (Chen & Wasserburg 1981)
Chaussidon et al (2004)
Luck et al (2003)
Inheritance
Sun and Protoplanetary Disk may have inherited
SLRs as a result of Galactic processes:
Ongoing Galactic Nucleosynthesis
Supernovae, Wolf-Rayet winds, novae, etc., eject
newly created radionuclides into Galaxy
Galactic Cosmic Rays
Proton, alpha particle Galactic Cosmic Rays
(GCRs) spall ambient nuclei, producing SLRs
Some GCR nuclei are SLRs, get trapped in gas
that forms Solar System (Clayton & Jin 1995)
Ongoing Galactic Nucleosynthesis?
supernova
Stars form in the
spiral arms of
spiral galaxies
M 109
supernovae (and
Wolf-Rayet winds)
eject radionuclides
radionuclide-laden
gas orbits Galaxy
for ~100 Myr, until
next spiral arm
new stars form
with radionuclides
182Hf
129I
26Al
53Mn
60Fe
Harper (1996)
Ongoing Galactic Nucleosynthesis
•Could explain abundance of 129I, with ~100 Myr delay
•Could explain other SLRs (182Hf, 107Pd, even 53Mn), but not
without overproducing 129I
•Does NOT explain abundances of 26Al or 60Fe (even w/o delay)
Harper (1996); Wasserburg et al. (1996); Meyer & Clayton (2000)
•If 60Fe is attributed to ongoing Galactic nucleosynthesis, 53Mn,
182Hf and 129I vastly overproduced
Galactic Cosmic Rays
•Most GCRs are protons; other nuclei present in near-solar
proportions; spacecraft have accurately measured fluxes of
GCRs of different energies (10 MeV/n to > 10 GeV/n)
•Beryllium GCRs 106 times more abundant than solar
•Flux of 10Be GCRs is known and is large
•Fluxes of all GCRs probably factor of 2 higher 4.5 Gyr ago
Galactic Cosmic Rays
Galactic Cosmic Rays (GCRs)
follow magnetic field lines
Magnetic field lines observed to
converge in star-forming cores
Schleuning (1998)
GCRs funneled into cloud cores
Some GCRs mirrored
out of cloud core by B
fields
B fields funnel some
GCRs into cloud core
GCRs in cloud core
can be trapped if
column density ∑ is
high enough
Cloud core B fields,
Desch &
Mouschovias (2001)
Column Density ∑(t), Magnetic Field Strength B(t) calculated
(Desch & Mouschovias 2001; Desch, Connolly & Srinivasan 2004)
GCRs ionize gas passing through cloud core, lose energy, slow
down (Bethe formula)
Low-energy (< 100 MeV/n) 10Be GCRs are trapped when
∑ ~ 0.01 g cm-2
Desch, Connolly & Srinivasan (2004)
total 10Be/9Be
10Be
GCRs
trapped in
cloud core
10Be/9Be
in
meteorites
GCR protons spall
local CNO nuclei,
produce 10Be
Galactic Cosmic Rays
•10Be in meteorites entirely attributable to trapped 10Be GCRs
•Biggest uncertainty is GCR flux 4.5 Gyr ago (factor of 2);
probably all but at least half of 10Be is trapped GCRs
•Trapped GCRs do not explain any other SLR, but 10Be is
known to be decoupled from other SLRs (Marhas et al. 2002)
Inheritance –– Conclusions
•At least half, and probably all, 10Be is inherited
•129I may be inherited
•Other SLRs, especially 26Al and 60Fe, are not inherited.
Irradiation
Energetic particles (accelerated by solar flares within
the Solar System) may have irradiated material,
inducing nuclear reactions and creating SLRs
Solar flares accelerate p, 4He, 3He to E > 10 MeV/n
Particle fluxes ~105 times larger around T Tauri stars; in
1 Myr, 1048 (!) energetic particles emitted
Irradiation within the Disk
Gas and dust in the protoplanetary disk (~ 1 AU)
Irradiation within the Sun’s Magnetosphere
Solids only, inside ~ 0.1 AU
Irradiation in the Disk
If gas is present, energetic particles lose > 99% of their energy
ionizing gas, not inducing nuclear reactions (Nath & Biermann 1994)
Consider 26Al:
26Al
/ 27Al = 5 x 10-5 implies 1045
26Al
atoms in a 0.01 M disk
Only 1048 particles emitted in 1 Myr; only 1047 intercept disk
To make a 26Al atom by 26Mg(p,n)26Al, a proton must travel
through ∑ ~ 1.4 mH / (xMg26 ) > 3 x 106 g cm-2 of gas
But protons stopped by << 10 g cm-2 of gas (Bethe formula):
fewer than 1 proton in 105 reacts
Even including other energetic particles, other targets, can’t make
more than ~ 1042 26Al atoms
Similar results for other SLRs, including 10Be
Irradiation inside the Sun’s Magnetosphere
e.g., “X-wind” model
Shu et al. (2001)
very little gas -- it’s ionized and
part of the corona
only solids (CAIs)
are irradiated
a fraction of
the solids are
returned to
asteroid belt
Seven problems with the X-wind model:
1. Launching of solids from 0.1 AU to asteroid belt problematic:
winds probably launched from 1 AU, not 0.1 AU [Coffey et al.
(2004)]; trajectories very sensitive to particle size [Shu et al. (1996)]
2. CAIs formed in near-solar f O2, but “reconnection ring” is >104
times more oxidizing than solar [using values in Shu et al. (2001)]
3. Concordant production of 26Al, 41Ca requires Fe,Mg silicate
mantle to surround Ca,Al-rich core, but real minerals do not
separate this way (e.g., Simon et al. 2002)
4. Production of 26Al or 41Ca at meteoritic levels will overproduce
10Be, using best-case scenario [Gounelle et al. (2001)] and new
measured reaction rate for 3He(24Mg,p)26Al [Fitoussi et al. (2004)],
especially if most 10Be is inherited [Desch et al. (2004)]. [See also
Marhas & Goswami (2004)]
Seven problems with the X-wind model (continued):
5. Temperatures inside magnetosphere at least 750 K, and usually
> 1200 K [Shu et al. (1996)]. Chlorine (including 36Cl) requires
T < 970 K to condense [Lodders (2003)]
6. Many other SLRs cannot be produced by spallation, including
60Fe, 107Pd and 182Hf [Gounelle et al. (2001); Leya et al. (2003)] and
63Ni [Leya et al. (2003)]
7. Siting of 26Al must be in small grains, not CAIs: type 6 OCs
heated to ~1200 K, must have had abundant 26Al, yet OCs have
almost no CAIs [Ouellette & Desch (2005, in prep)]
Many of these problems pertain to any model of
irradiation in the Sun’s magnetosphere
Irradiation –– Conclusions
•Energetic-particle irradiation occurs and can produce
10Be, 41Ca, 26Al, 53Mn, if irradiation occurs in Sun’s
magnetosphere (to minimize ionization energy losses)
•Confirmation of 7Be would demand irradiation
•Concordant production of 41Ca, 26Al difficult, 10Be
probably overproduced, and 36Cl hard to condense
•60Fe, 107Pd, 182Hf (and 36Cl?) demand external source
Injection
Stellar nucleosynthesis products ejected by an evolved
star and enter the Solar System material shortly before,
or soon after, Solar System formation:
AGB star
Contaminates Sun’s molecular cloud
(Wasserburg et al. 1994)
Nearby (Type II) Supernova
Contaminates Sun’s molecular cloud core and
triggers its collapse (Cameron & Truran 1977)
Injects into already-formed protoplanetary disk...
AGB Star
Stars at least as
massive as the Sun at
the ends of their lives
enter AsymptoticGiant Branch stage
Eskimo nebula: after AGB
winds expose white dwarf
SLRs created within star are dredged up
to the surface and ejected in a powerful
wind
Problems with the AGB Scenario:
1. AGB stars do produce 41Ca, 36Cl, 26Al, 60Fe, 107Pd, 135Cs and 205Pb
[Wasserburg et al. 1994, 1995, 1996, 1998; Gallino et al. 1998, 2004].
But they do not produce 129I, 53Mn, or 182Hf.
2. AGB stars are extremely unlikely to be associated with the early
Solar System. Kastner & Myers (1994) conservatively calculate
probability of contamination of Sun’s molecular cloud core at
< 3 x 10-6
Supernovae
•Supernovae do produce all the confirmed SLRs: 41Ca, 36Cl, 26Al,
53Mn, 60Fe, 107Pd, 182Hf, 129I.
(Except for 10Be, which is
known to have a separate
origin.)
•Relative abundances of
SLRs in outermost ~18 M
of a 25 M supernova match
meteoritic values very well
[Meyer et al. 2003]
•Order-of-magnitude
agreement sufficient,
considering real supernova
ejecta highly heterogeneous
Cassiopeia A supernova remnant
time delay
= 0.9 Myr
Meyer et al (2003), LPSC abstract
time delay
= 0.9 Myr
Meyer et al (2003), LPSC abstract
time delay
= 0 Myr
Meyer et al (2003), LPSC abstract
time delay
= 0.4 Myr
Meyer et al (2003), LPSC abstract
Supernova and Star Formation
•Meteoritic values require Solar System to be ~10-4 SN ejecta
•Requires supernova < 10 pc away, ~ 1 Myr before CAIs formed
•What are the odds our Solar System “happened” be near supernova?
Like case of AGB star: too low.
•Supernova must be causally linked to Solar System formation:
perhaps the SN shock triggered the collapse of our cloud core [Cameron
(1963), Cameron & Truran (1977)]: “supernova trigger” model
Supernova shock
can inject right
amounts of
SLRs, and
trigger collapse
of cloud core if...
Supernova shock
can be slowed to
20 - 50 km/s
Vanhala & Boss (2002)
Requires some
intervening gas,
travel times
t~105 yr
Problems with the Supernova Trigger Model:
Environment in which supernovae occur is important!!
low-density, ionized gas
dense molecular gas
n ~ 104 cm-3
shocked gas
n ~ 10 cm-3
cloud
core
shock
UV photons
ionization front
~ 0.2 pc
supernova
progenitor
This gas already shocked –– no “cloud cores”
low-density, ionized gas
dense molecular gas
n ~ 104 cm-3
shocked gas
n ~ 10 cm-3
cloud
core
UV photons
supernova
progenitor
~ 2 pc
shock
ionization front
∑ ~ 0.03 g cm-2
cloud
core
shocked gas
supernova
ejecta
~ 2 pc
∑ ~ 0.03 g cm-2
cloud
core
shocked gas
ejecta
∑ej ~ 10-4 g cm-2
Vej ~ 5000 km/s
~ 2 pc
∑ ~ 0.03 g cm-2
Ejecta transfers its
momentum: shock
propagates to cloud
core, but is slowed
to < 20 km/s
cloud
core
The actual ejecta (and
SLRs) do not penetrate
into cloud: they bounce!
(Hester et al. 1994)
Injection –– Conclusions
•Injection by AGB stars highly unlikely, and cannot
explain all isotopes anyway (esp. 53Mn, 182Hf)
•Injection by supernovae explains all isotopes well, but
causal link to Solar System formation must be explained
•Supernova trigger viable, but needed conditions may
not exist where supernovae happen
“Aerogel” Model
Very close (< 1 pc) supernova injected SLRs into the
Solar System, after it had formed a disk
1 Ori C: 40 M O6 star;
will supernova in 1-2 Myr
Protostars
with disks
Orion Nebula
When 1 Ori C goes supernova, all the disks in the Orion Nebula
will be pelted with radioactive ejecta
Even more true for the disks observed in Carina Nebula, with
sixty O stars [Smith et al. (2003)], many other H II regions
Ejecta dust grains penetrate disk, evaporate on entry, but leave
SLRs lodged in disk like aerogel: “Aerogel Model”
Potential Problems with the Aerogel Model:
Q: Won’t the disks be destroyed by the supernova shock?
A: No, disks are tightly bound to protostar
30-AU disks > 0.3 pc from supernova definitely survive
10-AU disks > 0.1 pc from supernova definitely survive
[Chevalier (2000); Ouellette & Desch (2004)]
Q: Isn’t the disk too small for it to intercept enough SLRs?
A: No,we are mixing only with ~ 0.01 M of disk material
A 30-AU disk 0.15 pc from a 25 M supernova, or 0.4 pc
. from a 60 M supernova ends up with 26Al/27Al = 5x10-5
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
The special case of Nickel 63
Luck et al (2003)
tentatively claim
evidence for live
63Ni (t
1/2 = 101 yr)
in the early Solar
System
Easily explained by
aerogel model, since
travel times < 100 yr
No other models can explain this result: if live 63Ni is
confirmed, it’s proof for the aerogel model
Conclusions
•Inheritance: 10Be likely inherited (trapped cosmic rays),
129I may be inherited, but no others, especially not 60Fe!
•Irradiation: may be necessary for 7Be, but overproduces
10Be, can’t explain 182Hf, 107Pd, (36Cl?), and especially 60Fe!
•Injection: AGB star can’t explain 53Mn, 182Hf, is very
unlikely; supernova can explain all SLRs if link to Solar
System formation made; supernova trigger viable but may
not pertain to real supernova environments
•Aerogel Model: Inevitable in supernova environments;
future modeling will test it; 63Ni may prove it