The Astrophysical Origins of the Short-Lived Radionuclides in the

Early Solar System

Steve DeschNovember 30, 2004

UCLA - IGPP

with a shout-out to my ASU supernova posse:

Nicolas Ouellette, Jeff Hester, Laurie Leshin, Gary Huss

Outline

• “Aerogel” model:– Astrophysical context– SLR predictions

• Short-lived radionuclides:– What are they? – How are they measured?

• Possible sources:– Inheritance– Irradiation – Injection

“SLRs” = Radionuclides with half-lives t1/2 < 16 Myr

41Ca (t1/2 = 0.1 Myr) (Srinivasan et al. 1994, 1996) 36Cl (t1/2 = 0.3 Myr) (Murty et al. 1997; Lin et al. 2004) 26Al (t1/2 = 0.7 Myr) (Lee et al. 1976) 60Fe (t1/2 = 1.5 Myr) (Tachibana & Huss 2003; Mostefaoui et al. 2004) 10Be (t1/2 = 1.5 Myr) (McKeegan et al. 2000; Sugiura et al. 2001) 53Mn (t1/2 = 3.7 Myr) (Birck & Allegre 1985) 107Pd (t1/2 = 6.5 Myr) (Kelly & Wasserburg 1978)

182Hf (t1/2 = 9 Myr) (Harper & Jacobsen 1994)

129I (t1/2 = 15.7 Myr) (Jeffery & Reynolds 1961)

Early Solar System SLRs Confirmed by Isotopic Analyses of Meteorites:

Short-Lived Radionuclides

Isotopic analyses of meteorites show they once held SLRs:

“Natural” 10B / 11B ratio

Excess 10B is from decay of 10Be

Slope gives original 10Be/9Be 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)

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)

Inheritance

Ongoing Galactic Nucleosynthesis?

M 109

supernova

Stars form in the spiral arms of spiral galaxies

new stars form with radionuclides

radionuclide-laden gas orbits Galaxy for ~100 Myr, until next spiral arm

supernovae (and Wolf-Rayet winds) eject radionuclides

60Fe

26Al

Harper (1996)

182Hf129I

53Mn

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

Schleuning (1998)

Galactic Cosmic Rays

Galactic Cosmic Rays (GCRs) follow magnetic field lines

Magnetic field lines observed to converge in star-forming cores

GCRs funneled into cloud cores

Cloud core B fields, Desch & Mouschovias (2001)

B fields funnel some GCRs into cloud core

Some GCRs mirrored out of cloud core by B fields

GCRs in cloud core can be trapped if column density ∑ is high enough

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

10Be/9Be in meteorites

Desch, Connolly & Srinivasan (2004)

GCR protons spall local CNO nuclei, produce 10Be

10Be GCRs trapped in cloud core

total 10Be/9Be

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.

IrradiationEnergetic 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

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 in the Disk

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

InjectionStellar 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 Asymptotic-Giant Branch stage

SLRs created within star are dredged up to the surface and ejected in a powerful windEskimo nebula: after AGB

winds expose white dwarf

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

(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

•Supernovae do produce all the confirmed SLRs: 41Ca, 36Cl, 26Al, 53Mn, 60Fe, 107Pd, 182Hf, 129I.

Cassiopeia A supernova remnant

Meyer et al (2003), LPSC abstract

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

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

Vanhala & Boss (2002)

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

Requires some intervening gas, travel times t~105 yr

Problems with the Supernova Trigger Model:

Environment in which supernovae occur is important!!

cloud core

low-density, ionized gas

ionization frontshock

shoc

ked

gas

UV photons

~ 0.2 pc

supernova progenitor

n ~ 10 cm-3

dense molecular gas

n ~ 104 cm-3

This gas already shocked –– no “cloud cores”

cloud core

low-density, ionized gas

ionization frontshock

shoc

ked

gas

UV photonssupernova progenitor

n ~ 10 cm-3

dense molecular gas

n ~ 104 cm-3

~ 2 pc

∑ ~ 0.03 g cm-2

cloud core

shoc

ked

gas

supernova

~ 2 pc

∑ ~ 0.03 g cm-2

ejecta

cloud core

shoc

ked

gas

~ 2 pc

∑ ~ 0.03 g cm-2

ejecta

∑ej ~ 10-4 g cm-2

Vej ~ 5000 km/s

cloud core

Ejecta transfers its momentum: shock propagates to cloud core, but is slowed to < 20 km/s

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

Protostars with disks

Orion Nebula

1 Ori C: 40 M O6 star; will supernova in 1-2 Myr

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 aYUV420 codec decompressor

are needed to see this picture.

The special case of Nickel 63

Luck et al (2003) tentatively claim evidence for live 63Ni (t1/2 = 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