The Astrophysical Origins of the Short-Lived Radionuclides in the Early Solar System Steve Desch September 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 t 1/2 < 16 Myr 41 Ca (t 1/2 = 0.1 Myr) (Srinivasan et al. 1994, 1996) 36 Cl (t 1/2 = 0.3 Myr) (Murty et al. 1997; Lin et al. 2004) 26 Al (t 1/2 = 0.7 Myr) (Lee et al. 1976) 60 Fe (t 1/2 = 1.5 Myr) (Tachibana & Huss 2003; Mostefaoui et al. 2004) 10 Be (t 1/2 = 1.5 Myr) (McKeegan et al. 2000; Sugiura et al. 2001) 53 Mn (t 1/2 = 3.7 Myr) (Birck & Allegre 1985) 107 Pd (t 1/2 = 6.5 Myr) (Kelly & Wasserburg 1978) 182 Hf (t 1/2 = 9 Myr) (Harper & Jacobsen 1994) 129 I (t 1/2 = 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” 10 B / 11 B ratio Excess 10 B is from decay of 10 Be Slope gives original 10 Be/ 9 Be ratio McKeegan et al. (2000)

Initial Abundances of Confirmed SLRs: Possibly 60 Fe/ 56 Fe = 1.6x10 -6 irons

Unconfirmed SLRs: 7 Be (t 1/2 = 57 days) (Chaussidon et al. 2004) 63 Ni (t 1/2 = 101 years) (Luck et al. 2003) 97 Tc (t 1/2 = 2.6 Myr) (Yin& Jacobsen 1998) 99 Tc (t 1/2 = 0.21 Myr) (Yin et al. 1992) 135 Cs (t 1/2 = 2.3 Myr) (McCulloch & Wasserburg 1978; Hidaka et al. 2001) 205 Pb (t 1/2 = 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

60 Fe 26 Al Harper (1996) 182 Hf 129 I 53 Mn

Jacobsen (2005) 41 Ca, 26 Al, 60 Fe 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 129 I and longer-lived radio- nuclides with ~100 Myr delay consistent with Galactic dynamics. Definitely does not explain 41 Ca, 26 Al or 60 Fe abundances [Harper (1996); Wasserburg et al. (1996); Meyer & Clayton (2000); Jacobsen (2005)]. If the majority of 60 Fe really was due to ongoing Galactic nucleosynthesis, 53 Mn, 107 Pd, 182 Hf and 129 I 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 10 6 times more abundant than expected from solar abundances (i.e., 1 in 10 3 instead of 1 in 10 9 ). Flux of 10 Be 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) 10 Be GCRs are trapped when ∑ ~ 0.01 g cm -2

10 Be/ 9 Be in meteorites Desch, Connolly & Srinivasan (2004) GCR protons spall local CNO nuclei, produce 10 Be 10 Be GCRs trapped in cloud core total 10 Be/ 9 Be

Galactic Cosmic Rays 10 Be in meteorites entirely attributable to trapped 10 Be GCRs Biggest uncertainty is GCR flux 4.5 Gyr ago (factor of 2); probably all but at least half of 10 Be is trapped GCRs Trapped GCRs do not explain any other SLR, but 10 Be is known to be decoupled from other SLRs (Marhas et al. 2002) Inheritance –– Conclusions At least half, and probably all, 10 Be is inherited 129 I may be inherited Other SLRs, esp. 41 Ca, 26 Al and 60 Fe, 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, 4 He, 3 He to E > 10 MeV/n Particle fluxes ~10 5 times larger around T Tauri stars; in 1 Myr, (!) 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 26 Al: 26 Al / 27 Al = 5 x implies Al atoms in a 0.01 M  disk Only particles emitted in 1 Myr; only intercept disk To make a 26 Al atom by 26 Mg(p,n) 26 Al, a proton must travel through ∑ ~ 1.4 m H / (x Mg26  ) > 3 x 10 6 g cm -2 of gas But protons stopped by << 10 g cm -2 of gas (Bethe formula): fewer than 1 proton in 10 5 reacts (Clayton & Jin 1995) Even including other energetic particles, other targets, can’t make more than ~ Al atoms Similar results for other SLRs, including 10 Be 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 O 2, but “reconnection ring” is >10 4 times more oxidizing than solar [using values in Shu et al. (2001) ] 3.Concordant production of 26 Al, 41 Ca 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 26 Al or 41 Ca at meteoritic levels will overproduce 10 Be, using best-case scenario [ Gounelle et al. (2001) ] and new measured reaction rate for 3 He( 24 Mg,p) 26 Al [ Fitoussi et al. (2004) ], especially if most 10 Be 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 36 Cl) requires T < 970 K to condense [ Lodders (2003) ] 6.Many other SLRs cannot be produced by spallation, including 60 Fe, 107 Pd and 182 Hf [ 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 7 Be, 10 Be, 41 Ca, 26 Al, 53 Mn, if irradiation occurs in Sun’s magnetosphere (to minimize ionization energy losses) Confirmation of 7 Be would demand irradiation Concordant production of 41 Ca, 26 Al difficult, 10 Be probably overproduced, and 36 Cl hard to condense 60 Fe, 107 Pd, 182 Hf (and 36 Cl?) 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 [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 wind Eskimo nebula: after AGB winds expose white dwarf

Problems with the AGB Scenario: 1.AGB stars do produce 41 Ca, 36 Cl, 26 Al, 60 Fe, 107 Pd, 135 Cs and 205 Pb [ Wasserburg et al. 1994, 1995, 1996, 1998; Gallino et al. 1998, 2004 ]. But they do not produce 129 I, 53 Mn, or 182 Hf. 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 10 Be, 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 ] Order-of-magnitude agreement sufficient, considering real supernova ejecta highly heterogeneous Supernovae do produce all the confirmed SLRs: 41 Ca, 36 Cl, 26 Al, 53 Mn, 60 Fe, 107 Pd, 182 Hf, 129 I. 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 km/s Requires a lot of intervening gas, but travel times t ~ 10 5 yr

Are these conditions met? Preceding state must include H II region! cloud core low-density, ionized gas ionization front = sharp density discontinuity shock shocked gas UV photons ~ 0.2 pc supernova progenitor n ~ 10 cm -3 dense molecular gas n ~ 10 4 cm -3

cloud core supernova ejecta ionization front = sharp density discontinuity

cloud core ejecta V ej ~ 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. 53 Mn, 182 Hf) 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” Model Very 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 26 Al ( 26 Al/ 27 Al = 5 x ) is explained by homogeneous injection of 5 x M  of a 25 M  supernova’s ejecta into a minimum-mass (0.01 M  ) disk. 5 x 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 –10 51 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

Canonical Run

Reverse Shock

Disk Stripping

Stripping and Mixing: KH Rolls

EnergyInjection efficiency 26 Al/ 27 Al 0.25 foe0.9%9 x foe (canonical)0.7%7 x foe0.6%6 x Disk massInjection efficiency 26 Al/ 27 Al 0.1 x min. mass1.2%1.2 x Min. mass (canonical)0.7%7 x x min. mass0.8%8 x DistanceInjection efficiency 26 Al/ 27 Al 0.1 pc2.4%7 x pc (canonical)0.7%7 x pc0.5%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: 10 Be likely inherited (trapped cosmic rays), 129 I may be inherited, but no others, especially not 60 Fe! Irradiation: would be necessary for 7 Be, but overproduces 10 Be, can’t explain 182 Hf, 107 Pd, ( 36 Cl?), and especially 60 Fe! Injection: AGB star can’t explain 53 Mn, 182 Hf, 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!