The Meaning of 60Fe: A Nearby Supernova Injected

Short-Lived Radionuclides into our Protoplanetary Disk

Steve Desch

Nicolas Ouellette

Jeff Hester

Laurie Leshin

Arizona State University

Outline• Our early Solar System contained 60Fe

• It wasn’t created by in-situ spallation...

• It wasn’t inherited from our molecular

cloud...

• It wasn’t injected by an AGB star...

• It had to come from a supernova...

• It was probably injected directly into

our protoplanetary disk

Early Solar System Contained 60Fe

60Fe/56Fe ~ 2 - 5 x 10-7

Semarkona + Bishunpur pyroxene chondrules

(Tachibana et al 2005)

60Fe/56Fe ~ 9 x 10-7

Semarkona troilites + magnetites (Mostefaoui

et al 2005)

60Fe/56Fe ~ 7 x 10-7

Semarkona troilites + pyroxenes (Mostefaoui et

al 2004)

60Fe/56Fe ~ 3 - 4 x 10-7

Bishunpur + Krymka troilites (Tachibana &

Huss 2003)

Tachibana & Huss (2003)

60Fe/56Fe ~ 5 x 10-7

60Fe wasn’t created by spallationIrradiation of rocky material at 0.1 AU in principle can produce radionuclides like 60Fe

Only 64Ni(p,p)60Fe reaction can happen, but 64Ni is rare and cross section is < 0.1 mbarn

Predicted yields:

60Fe/56Fe ~ 10-11

(Lee et al 1998; Leya et al 2003)

Shu et al (1996)

60Fe wasn’t inherited from our molecular cloud

Harper (1996)

Supernovae, Wolf-Rayet winds, novae, AGB stars maintain steady-state levels of 60Fe, etc., in the Galaxy

60Fe

26Al

129I

Steady-state “average” abundance of 60Fe is 60Fe/56Fe ~ 3 x 10-7 (Harper 1996) or 60Fe/56Fe ~ 3 x 10-8 (Wasserburg et al 1996)

But then 129I/127I ~ 10-2, 182Hf/180Hf ~ 10-3, etc.

60Fe wasn’t inherited from our molecular cloud

Our molecular cloud was isolated from sources of radionuclides for >> 107 yr (Wasserburg et al 1996; Harper 1996)

Much of the radionuclides produced by supernovae go into hot phase, and don’t enter molecular clouds for ~108 yr

M 109

supernova

new star-forming molecular clouds

In the meantime, 60Fe completely decays

60Fe wasn’t injected by an AGB starAGB outflows do eject 60Fe, but only for ~ 1 Myr, after star has evolved for > 2 Gyr

AGB stars randomly distributed in Galaxy: no reason to associate them with star-forming regions

Odds per 1 Myr that a parcel of gas in a molecular cloud will be contaminated with AGB material:

< 3 x 10-6

(Kastner & Myers 1994)

AGB stars also do not eject appreciable 53Mn, 182Hf (Gallino et al 1998)

60Fe is ejected by supernovae associated with star-forming regions!Live 60Fe discovered in ocean crust 3 Myr old (Knie et al 2004)

Only plausible source is supernovae in Scorpius-Centaurus star-forming region (Maiz-Appelaniz 2001; Fields et al 2004)

P. Frisch, U. Chicago

60Fe

Fe

60Fe will be injected into disks associated with those supernovae

~ 0.2 pc

1 Ori C: 40 M star will supernova in < 1 Myr

protoplanetary disks

HST image, Orion Nebula

Sufficient 60Fe can be injected without destroying disk

Iron likely in form of dust grains: gas-phase Fe disappeared from SN 1987A ejecta at same time (1-2 years post-explosion) that 10-3 M of dust formed (Colgan et al 1994)

Mass of 60Fe ejected by 25 M supernova ~ 8 x 10-6 M (Woosley & Weaver 1995)

Fraction intercepted by 30 AU radius disk at 0.3 pc away ~ (30 AU)2 / 4 (0.3 pc)2 ~ 6 x 10-8

Mixed with 0.01 M of solar composition

material, 60Fe / 56Fe ~ 1 x 10-6

Sufficient 60Fe can be injected without destroying disk

Worst-case scenario: fast cooling

Ouellette et al (2005), in prep

QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.

Sufficient 60Fe can be injected without destroying disk

Ouellette et al (2005), in prep

With realistic cooling

QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.

Conclusions60Fe in early solar system could not be inherited, could not be produced by spallation, almost certainly was not from AGB star

Only plausible source of the 60Fe is a nearby supernova

Protoplanetary disks are observed near (~ 0.3 pc) massive stars that will soon go supernova

Supernova ~0.3 pc away can inject 60Fe-bearing dust grains into a protoplanetary disk, yielding 60Fe/56Fe ~ 10-6 in the disk material, without destroying disk