Radioactivity in the early solar system

Maria Lugaro

(Monash University)

Amanda Karakas (ANU, Australia)

Mark van Raai (Utrecht, NL)

Anibal Garcia-Hernandez (Tenerife, Spain)

Joseph Trigo-Rodriguez (Barcelona, Spain)

Lee et al. 1977

coarse-grained chondrule

includingdifferent mineral phases

CaAl2Si2O8

(Ca,Mg,Fe)SiO3

24Mg 79%

25Mg 10%

26Mg 11%

27Al 100%

26Al0.7My

= Al/Mg

Most primitive meteoritic inclusions, e.g. Calcium-aluminium rich (CAI)s +

chondrules, show this “canonical” value.

THE PROBLEM:

26Al 26Mghigh-energy

photons(1.8 MeV rays)

• Source of energy for the differentiation of planetesimals (Grimm & McSween 1993),• which are believed to be the source of much of Earth’s water (Morbidelli et al. 2000), • with consequences on the habitability of the Earth.

Is the presence of 26Al special or common in young planetary systems?

Radioactivity in the early Solar System:

T1/2(Myr)

41Ca 0.10 36Cl 0.3 26Al 0.710Be 1.660Fe 2.253Mn 5.3107Pd 9.4182Hf 13

etc.

60Fe same or up to 300 times more than the average observed today in the

interstellar medium.

Half-life also a difficult measurement: it was given as 1.5 Myr a year ago.

Sh

ort

-liv

ing

nu

clei

26Al ~ 4-30 times more than average observed today in the interstellar

medium (COMPTEL observations).

Time delay between isolation of the protosolar cloud and formation of first

rocks ~ 1 Myr (?) 26Al > 8 times more.

Produced in the early Solar System?The X-wind model (Lee et al. 1998)

Irradiation by accelerated particles from the young Sun results in spallation reactions producing radioactive nuclei.

• YES for 10Be, which cannot be made in stars, BUT 60Fe cannot be synthesized by spallation.

• If 26Al was produced by spallation, it could be homogeneously distributed only over a relatively small (~ Earth mass) rocky reservoir (Duprat and Tatischeff 2007).

• Recent measurements of the 24Mg(3He,p)26Al cross section exclude this origin for 26Al (Fitoussi et al. 2008).

Produced in a nearby star?

1. Produce the right inventory of short-living radioactive nuclei

2. Be proposed with a scenario for the birth of the Sun where it is plausible for them to be there, in the right place and at the right time

The formation of the Sun is ONE event, but is it special?

Candidate stellar sources must:

Produced in a nearby supernova?1. Do supernovae produce the right inventory of short-living

radioactive nuclei?Yes, provided it was

not a common supernova but a faint supernova with a lot (~ 3 to 11 Msun) of

inner material - containing the 56Ni that

powers supernova light curves - getting

mixed and falling back into a black hole.

Otherwise we get way too much 53Mn (Meyer 2005, Takigawa et al.

2008).(The mass location that divides the part of the star that collapses in the remnant from the part expelled

- or injected?)

Produced in a nearby supernova?2. Do supernovae come with a scenario for the birth of the Sun

where it is plausible for them to be there, in the right place and at the right time?

• Many solar-type stars form in the vicinity of a massive star (Orion)• Hester and Desch 2005: low-mass stars form in the compressed gas at the edges of H II regions produced by the ionization from massive stars and their disks are polluted by supernova ejecta. • By comparing the timescale of disk dissipation (< 6 Myr) to the main-sequence lifetime, one can derive that the polluting star must have had a minimum mass of ~40 Msun.• The cluster where the Sun formed must have had at least 6000 members, in order to include at least one 40 Msun star. Could planetary orbits have remained unperturbed in such a rich cluster? (Adams & Laughlin 2001)• From a detailed probability analysis accounting for cluster number distribution, cluster expansion, the initial mass function, disk lifetimes, and SNII timescales,Williams and Gaidos (2007) and Gounelle and Meibom (2008) conclude that supernova enrichment of protostellar disks “is a highly unlikely event, affecting less than about a 1% of all stars in the Galaxy”.

Produced in a nearby AGB star?1. Do asymptotic giant branch (AGB) stars produce the right

inventory of short-living radioactive nuclei?

Schematic out-of-scale picture of AGB star structure.

time

Proton captures at the base of the convective envelope make 26Al via 25Mg+p

Neutron captures in the thermal pulses, with 22Ne+ as neutron source, make 41Ca and 60Fe.

Monash stellar nucleosynthesis models

A group within the Centre for Stellar and Planetary Astrophysics (CSPA)

The SINS group at Monash

Who are SINners?• Permanent Staff

– John Lattanzio

• Monash Fellow– Maria Lugaro

• Post-Docs– Richard Stancliffe– Ross Church– Herbert Lau

• Post-Grads– Carolyn Doherty– George Angelou

• Honorary Sinners– Amanda Karakas (ANU)– Simon Campbell (Barcelona)– Sandro Chieffi (Rome)

We have our own beer!

Some current specific programs• Super-AGB stars

– With Lionel Siess (Belgium) and Pilar Gil-Pons (Barcelona)

• Slow Neutron Capture Nucleosynthesis– With Sergio Cristallo, Oscar Straniero, Roberto Gallino (Italy)

• 3D Hydro in Stars– With Djehuty and David Dearborn and Peter Eggleton (LLNL)– Discovered mechanism of “deep-mixing”?

• Carbon Enhanced Metal Poor Stars (CEMPs)– Stancliffe on APD

• Abundances in Globular Clusters– Theoretical and some observational programs (with David

Yong)

• Binary Stars and Population Synthesis– With Robert Izzard (Belgium) and Chris Tout (UK)

Current Priorities• The Australian Network for Nuclear

Astrophysics (ANNA)– “Elizabeth and Frederick White Conference on Nuclear

Astrophysics in Australia”: 23-24-25 August 2009, Shine Dome, Canberra

– Bidding for Australia to host the 12th “Nuclei in the Cosmo” conference in 2012

• ASA/ANITA/SINS Summer School 2010 on Stellar Nucleosynthesis: 1 week for

postgrads/postdocs in January 2010 on nuclear reactions, theory of low-mass and high-mass stars, stellar

abundances, meteoritic stardust grains.

1/DIL=330t = 0.53 My

3.2d-05

2.6d-06

1.5d-08

5.d-05

2.d-062.d-07

1.5.d-08

ESS

1.5d-02

1.0d-03

1.6d-04

6.5 M, Z=0.02

Produced in a nearby AGB stars?1. Do AGB stars produce the right inventory of short-living

radioactive nuclei?

Yes! Trigo-Rodriguez, Garcia-Hernandez,

Lugaro, Karakas et al. 2009, Meteoritic & Planetary Science

(except for 36Cl which is a problem for any scenario

nuclear physics?)

Produced in a nearby AGB star?2. Do AGB stars come with a scenario for the birth of the Sun

where it is plausible for them to be there, in the right place and at the right time?

• Shock waves from AGB winds may be capable of triggering the collapse of the proto-solar cloud (Boss 1995). • But the AGB scenario has been dismissed because Kastner and Myers (1994) estimated observationally that any giant molecular cloud located within 1 kpc of the Sun has only about a 1% probability of encountering an AGB star in a 1 Myr period, which implies that AGB stars are relatively rare near star-forming regions today. • However, a detailed re-analysis of this point is needed because the statistics of Kastner and Myers (1994) are very poor and completely dominated by only two stars. • Kastner and Myers (1994) also state that “There is a significant (~70%) probability at the present epoch for a given cloud to be visited by an AGB star in ~108 yr”. (?) If this encounter “triggers multiple star formation, then AGB-induced proto-stars should exist in every molecular cloud” (Boss 1995).

Summary• All current scenarios are problematic:

Nucleosynthesis in the solar system is plausible, but does not seem to make enough radioactive nuclei, while stellar nucleosynthesis can make enough radioactive nuclei, but does not have a plausible scenario (...are we special?)

• 60Fe important discriminant between different scenarios: it seems to be made only in supernovae and AGB stars. Measurements are difficult!

• Stellar nucleosynthesis models need to be as accurate as (humanly!) possible and test all related nuclear and mixing uncertainties.

Comparison with -ray observation of 60Fe in the interstellar medium

• ~ 0.3 Msun of 60Fe are observed in the interstellar medium (COMPTEL+INTEGRAL observations)

• The mass of gas+dust in the Milky Way is ~ 5-10 109 Msun

• The average mass fraction of 60Fe in the Milky Way ~3-6 10-11

• The 60Fe/56Fe ratio in the early solar system is 2 10-7 - 2 10-6 (?????)• The mass fraction of 60Fe in the solar system is ~ 10-3 (60Fe int x2)• So, the mass fraction of 60Fe in the early solar system was ~ 10-10 - 2 10-9

60Fe in the early solar system ~ 2-140 more than the average observed today in the Milky Way.

26Al in the interstellar medium • ~ 3.1+/-0.9 Msun of 26Al in the

interstellar medium (-ray

COMPTEL Knödlseder 1999)• The mass of gas+dust in the

Milky Way is ~ 5-10 109 Msun• The average mass fraction of

26Al in the Milky Way ~2-8 10-10

• The canonical 26Al/27Al ratio in the early solar system is 5 10-5

• The mass fraction of 27Al in the solar system is ~ 6 10-5 (27Al int. x2)• So, the mass fraction of 26Al in the early solar system was ~ 3 10-9

26Al in the early solar system ~ 4-30 more than the average observed today in the interstellar medium.

Time delay between isolation of the protosolar cloud and formation of first rocks ~ 1 Myr (?) 26Al > 8 more.

• YES for 10Be, which cannot be made in stars,

• BUT 60Fe cannot be synthesized by spallation.

• Shielding of CAI cores by Fe-Mg-rich mantles is needed to get the right 41Ca and 26Al abundances (Gounelle et al. 2001) ( heterogeneities?)

• If 26Al was produced by spallation, it should be homogeneously distributed over a relatively small (~ Earth mass) rocky reservoir (Duprat and Tatischeff 2007).

• Data indicates that the production of 10Be is decoupled from that of 41Ca and 26Al (Marhas et al. 2002)

• Recent measurements of the 24Mg(3He,p)26Al cross section ( 3 times smaller than before) exclude this origin for 26Al (Fitoussi et al. 2008).

Produced in the early Solar System?

What about Super-AGB stars, Wolf-Rayet stars, ...?

• They can certainly make 26Al (Siess & Arnould 2008, Arnould et al. 2006)

• It seems more problematic to make 60Fe... (if we need to make it at all!)

proton diffusion

13Cn)16O

22Nen)25Mg

time

During the s process:

Nn~ 107 n/cm3 n >>

Except for branching points, which open for higher Nn

if half life > a few days

HBB