The Astrophysical Origins of the Short-Lived Radionuclides in the

Early Solar System

Steve DeschSeptember 15, 2006

U. of Toronto

with a shout-out to my ASU supernova posse:

Jeff Hester, Nicolas Ouellette, Carola Ellinger

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 (2006)

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

Jacobsen (2005)

41Ca, 26Al, 60Fe definitely not inherited from ISM.

More sophisticated mixing models show

Predicted ratios if only sources are type II SNe.

Ongoing Galactic Nucleosynthesis

•Could explain abundance of 129I and longer-lived radio-nuclides with ~100 Myr delay consistent with Galactic dynamics.

•Definitely does not explain 41Ca, 26Al or 60Fe abundances [Harper (1996); Wasserburg et al. (1996); Meyer & Clayton (2000); Jacobsen (2005)].

•If the majority of 60Fe really was due to ongoing Galactic nucleosynthesis, 53Mn, 107Pd, 182Hf and 129I would be 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 nuclei and energies (10 MeV/n to > 10 GeV/n)

•Beryllium GCRs 106 times more abundant than expected from solar abundances (i.e., 1 in 103 instead of 1 in 109).

•Flux of 10Be GCRs is known and is large

•Fluxes of all GCRs scale linearly with star formation rate, which was almost certainly a 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, ∑ taken from 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, esp. 41Ca, 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 (Clayton & Jin 1995)

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

Six 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)]

Six 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)]

Many of these problems pertain to any model of irradiation in the Sun’s magnetosphere

Irradiation –– Conclusions•Energetic-particle irradiation occurs and can produce 7Be, 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 [wind possibly triggers collapse of cloud core] (Wasserburg et al. 1994)

Nearby (Type II) Supernova

Contaminates Sun’s molecular cloud core and triggers its collapse (Cameron & Truran 1977) ... or ....

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

Supernova and Star Formation

•Meteoritic values require Solar System disk to be 0.01% SN ejecta

•Requires supernova < 10 pc away, ~ 1 Myr before CAIs formed (see Fields et al. 2007)

•What are the odds our Solar System “happened” be near supernova? Like case of AGB star: too low. There must be a causal connection.

•One way in which SN could be causally connected is if 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 a lot of intervening gas, but travel times t ~ 105 yr

Are these conditions met? Preceding state must include H II region!

cloud core

low-density, ionized gas

ionization front = sharp density discontinuity

shock

shoc

ked

gas

UV photons

~ 0.2 pc

supernova progenitor

n ~ 10 cm-3

dense molecular gas

n ~ 104 cm-3

cloud core

supernova

ejecta

ionization front = sharp density discontinuity

cloud core

ejecta

Vej ~ 2000 km/s

cloud core

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

The actual ejecta (and SLRs) do not penetrate into cloud: they bounce! (Hester et al. 1994)

Does this gas contain any radioactivities?

Injection –– Conclusions so far...

•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

•Alternative supernova scenario...

“Aerogel” ModelVery close (< 1 pc) supernova injected SLRs into the Solar System, after it had formed a disk (Gold 1977; Clayton 1977; Chevalier 2000; Ouellette et al 2005)

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

Same scenario even more likely for disks observed in Carina Nebula, with sixty O stars [Smith et al. (2003)], or NGC 6611 [Oliveira et al. 2005] or NGC 6357 [Healy et al. 2007, in prep]

Ejecta dust grains penetrate disk, evaporate on entry, but leave SLRs lodged in disk like aerogel: “Aerogel Model”

Initial abundance of 26Al (26Al/27Al = 5 x 10-5) is explained by homogeneous injection of 5 x 10-6 M of a 25 M supernova’s ejecta into a minimum-mass (0.01 M) disk.

5 x 10-6 M is the ejecta mass intercepted by a 40 AU-radius disk 0.2 pc from a 25 M supernova

But will a disk this close survive? Will ejecta be mixed in?

To answer these questions, we have written a 2-D hydro code based on the Zeus algorithms.

Includes tensor artificial viscosity and a cooling term.

Canonical Simulation• Disk

– Minimum mass (0.01 M) disk truncated at 30 AU

– Disk allowed to dynamically relax for 1000 years

– Final radius ~ 40 AU

• Supernova

– 0.3 pc away

– 1051 ergs (1 foe) explosion energy

– 20 M ejected isotropically with time dependence of density and velocity consistent with Matzner & McKee (1999) and uniform-density star*

– Isotopic composition assumed homogeneous, that of 25 M supernova from Woosley & Weaver (1995)

Relaxed Disk

QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.

Canonical Run

Reverse Shock

Disk Stripping

Stripping and Mixing: KH Rolls

Energy Injection efficiency 26Al/27Al

0.25 foe 0.9% 9 x 10-8

1 foe (canonical) 0.7% 7 x 10-8

4 foe 0.6% 6 x 10-8

Disk mass Injection efficiency 26Al/27Al

0.1 x min. mass 1.2% 1.2 x 10-6

Min. mass (canonical) 0.7% 7 x 10-8

10 x min. mass 0.8% 8 x 10-9

Distance Injection efficiency 26Al/27Al

0.1 pc 2.4% 7 x 10-6

0.3 pc (canonical) 0.7% 7 x 10-8

0.5 pc 0.5% 1 x 10-8

• Protoplanetary disks will survive nearby supernova explosions

• Gas-phase supernova ejecta is mixed into the disk, but with low efficiency (~ 1%), too low to explain SLR ratios

• Dust injection is the best candidate for SLR injection and will be the subject of future work– Preliminary calculations show the dust will travel roughly 100 AU

before being deviated by the bow shock, and will be mixed in with ~ 100% efficiency

• All SLRs inferred from meteorites were in solid phase...

Aerogel Model: Conclusions

Allowing supernova ejecta to be injected inhomogeneously allows an almost perfect match to meteoritic abundances

Conclusions

•Inheritance: 10Be likely inherited (trapped cosmic rays), 129I may be inherited, but no others, especially not 60Fe!

•Irradiation: would be necessary for 7Be, but overproduces 10Be, can’t explain 182Hf, 107Pd, (36Cl?), and especially 60Fe!

•Injection: AGB star can’t explain 53Mn, 182Hf, and 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; at first cut is consistent with data. Refinements underway!