Evidence of a possible supernova explosion that occured very near the Earth is presented by the presence of radioactive 26 Aluminum (26 Al). The time of the supernova explosion is placed at approximately 100,000 years ago and is partially supported by cosmic gamma ray measurements from both HEAO 3 and the Solar Maximum Mission

Clayton, D. D.

English

NASA Technical Reports Server

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2 5  I n t e r s t e l l a r  2 5 . ~ l  and Excess 316; Donald I). Clayton,  Rice l l n i - v c r s i t y ,  
Houston, '.-X 77251. 
When groups i n  A u s t r a l i a  and Cal tech  f i r s t  showed t h a t  aluminum-rich 
mini -a l s  v i t h i n  t h e  CaAl-rich i n c l u s i o n s  from t h e  Allende me teo r i t e  c a r r i e d  a n  
excess  of t h e  h e a v i e s t  i s o t o p e  of magnesium, 26Mg, i t  was concluded that  a 
supernova exp los ion  bes ide  t h e  forming s o l a r  system uust hav, peppered t h e  
s o l a r  cloud wi th  r a d i o a c t i v e  26A1. The i n f e r e n c e  was t h a t  l a te r ,  a f t e r  t h e  
s o l a r  system and t h e  me teo r i t e s  had assembled i n  m c h  t h e i r  p re sen t  form, 
t h a t  26A1 decayed t o  *%!g by nuc lea r  b e t a  decay wi th in  t h e  Allende minera ls .  
The highes: c o n c e n t r a t i o n s  observed sugges ted  Lhat 50 p a r t s  pe r  m i l l i o n  of 
aluminum vas t h e  r a d i o a c t i v e  mass-26 i so tope ,  whose h a l f l i f c  f o r  decay t o  26Mg 
i s  one m i l l l i o n  years .  The be l i eved  requirement  of a supernova accompanying 
t h e  b i r t h  of t h e  sun  de r lved  from t h e  b e l i e f  t h a t  t h e  i n t e r s t e l l a r  gas  worild 
not  normally have n e a r l y  enough r a d i o a c t i v e  2 6 A l  w i t h i n  i t ,  s o  t h a t  a synchro- 
n ized  thermonuclear  exp los ion  vas accepted  as t h e  sou rce  of t h e  cosmic r ad io -  
a c t i v e  " f a l l o u t "  t h a t  t h e  Allend2 minera ls  have recorded. 
Now t h i s  s i t u a t i o n  has  been v a s t l y  shaken by t h e  a c t u a l  d e t e c t i o n  of  
r a d i o a c t i v e  26Al i n  t h e  i n t e r s t e l l a r  medium today. The f i r s t  d e t e c t i o n ,  an 
h i s t o r i c  f i r s t  d e t e c t i o n  e v e r  of i n t e r s t e l l a r  r a d i o a c t i v i t y ,  was made by t h e  
HEAO 3 s p a c e c r a f t  [Mahoney e t  a l .  1984, Astro$hys.J., s 6 .  5781 which 
recorded a measarable  f l u x  of 1 8 0 3 k e V  gamma r a y s  s t r i k € n g  t h e  s o l a r  System. 
The 1809 keV gamma rays  are a well kncwn e l ec t romagne t i c  r a d i a t i o n  g iven  o f f  
fo l lowing  t h e  b e t a  decay of each 26Al nucleus.  The HEAO 3 s p a c e c r a f t  measured 
t h a t  about  480 such  gamma r a y s  impact a squa re  meter every  second, coming from 
t h e  gene ra l  d i r e c t i o n  o f  t h e  c e n t e r  of o u r  Galaxy, but  a t  a n  mknown d i s t a n c e .  
L inger ing  doubts  about  t h e  r e a l i t y  of t h i s  a s t o n i s h i n g  d iscovery  have now been 
removed by a conf i rma t ion  of i t s  co r rec tness .  The Solar Maxinum Mission t h a t  
was so dramat i ca l ly  r e p a i r e d  by t h e  S h u t t l e  a s t r o n a u t s  c a r r i e d  a gamma ray  
spec t rometer  t h a t  hsd s i n c e  February 1980 taken  u n i n t e n t i o n a l  p e r i o d i c  looks  
a t  ou r  Galactlc cen te r .  That spec t rometer  team (Share  e t  al .  1985, submi t ted  
t o  Astrophys. J.) conf i rms  about  400 gamma rays  p e r  squa re  meter p e r  second 
from t h e  d i r e c t i o n  of  t h e  Galactic c e n t e r ,  i n  d i r e c t  agreement wi th  t h e  HEAO 3 
measl:-ment. It can  now be  a s s e r t e d  u i t h o u t  r easonab le  doubt t h a t  some s o u r c e  
of  i d i o a c t i v e  26Al :€as in t h a t  g e n e r a l  d i r e c t i o n .  
The a n a l y s i s  of t t e  magnitude of t h i s  gamma r a y  f l u x  and i t s  i m p l i c a t i o n s  
f o r  both :he o r i g i n  of t h e  elements  i n  exp los ions  of stars and f o r  t h e  o r i g i n  
of t h e  Allende minera ls  was undertaken by Clayton (1984, Astrophys. J., Z O ,  
141). He came t o  t h r e e  s t a r t l i n g  conclus ions :  (1 )  €f  t h e  z b A l  i s  s p r e a d  
unirormly throughout  t h e  i n t e r s t e l l a r  gas ,  i t s  c o n c e n t r a t i o n  of about  10 p a r t s  
pe r  m i l l l o i  of aluminum is r a t h e r  c l o s e  t o  t h e  f o s s i l  cvidpnce found i n  
Aller  q r  minera ls ,  sugges t ing  t h a t  t h e  requirement of a s p e c i a l  supernova 
t rCqger  t o  s o l a r  format ion  may have been unwarranted; ( 2 )  supernova exp los ions  
,*i not  adequate  t o  main ta in  t h i s  average  l e v e l  of i n t e r s t e l l a r  r a d i o a c t i v i t y ,  
ao t h a t  nova exo los ions  or  gas  s t reaming away from g i a n t  s t a r s  are  t h e  more 
l i k e l y  o r i g i n s  of t h e  r a d i o a c t i v e  aluminum. The i n e f f e c t i v e n e s s  of supernovae 
in mainta in ing  t h e  observed concen t r a t ion  a l s o  a rgues  a g a i n s t  i n p l i z a t i n g  a 
special  supernova wi th  t h e  s o l a r  o r i g i n .  These two conc lus ions ,  when added t o  
o t h e r  arguments t h a t  Clayton had advanced p r i o r  t o  t h e  d e t e c t i o n  of 76A1 gamma 
rays ,  kbve almost  e l imina ted  t h a t  supernova- t r igger  concept .  On t h e  o t h e r  
hand, Clayton ' s  t h i r d  conclus ion  was t h a t  ;f t h e  observed 26A1 Is no t  assumed 
t ') be spread  throughout  t h e  i n t e r s t e l l a r  medium, i t  could i n  f a c t  be l o c a l  
d e b r i s  from a s i n g l e  s.ipernova explos ion  t h a t  occur red  very near  t h e  e a r t h  
c n l y  abut  100,000 yea r s  ago, i n  which case i t  might g i v e  renewed emphasis t o  
t h e  concept of a s ia i la r  explosior ,  nea r  t h e  forming s o l a r  system 4.5 b i l l i o n  
https://ntrs.nasa.gov/search.jsp?R=19850018240 2020-03-22T19:08:14+00:00Z
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7 6  7 3  I n t e r s t e l l a r  A1 rind Excess - blg 
Cla) ton ,  I). I) .  
y e a r s  ago. Other  evidence bea r ing  on t h e  e x c i t i n g  p o s s i b i l i t y  t h a t  Ea r th  f s  
now loca ted  i n  a supernova remnant is being analyzed by Clayton and h i s  
co l l eagues ,  who r epor t ed  a t  t h e  "Galaxy and S o l a r  System" Meeting in Tucson i n  
January p o s s i b l e  ev idences  f o r  t h i s  r ecen t  nearby supernova, pr imar i l ;  l o n g  
s t and ing  puzz les  in t h e  cosmic r ad ia t ion .  
A t  Lunar and P lane ta ry  Science 16,  Clayton argued t h a t  t h e  i n t e r p r e t a t i o n  
c c  t h e  excess 26Mg i n  Allende aluminum-rich mine ra l s  depends upon t h e  c o r r e c t  
i n t e r p r e t a t i o n  of t h e  26Al gamma-ray l i ne .  The c o r r e c t  i n t e r p r e t a t r o n  of  t h e  
gamma r a y s  r e q u i r e s  b e t t e r  in format ion  on t h e i r  angu la r  d i s t r i b u t i o n .  aecause  
of t h e i i  wide viewing ang le s  and s e r e n d i p i t o u s  measurements, t h e  HEAO 3 and 
S o l a r  Maximm Mission teams have been r e l u c t a n t  t o  be very  p r e c i s e  about  the 
angu la r  d i s t r i b u t i o n ,  Although they  d e s c r i b e  i t  as being " c o n s i s t e n t  wi th"  
r a d i o a c t i v i t y  concent ra ted  i n  t h e  g a l a c t i c  p lane  having p2ak i n t e n s i t y  n e a r  
t h e  Galactic c e n t e r ,  they  s t o p  s h o r t  of c l a imiag  t h a t  t h e  d a t a  r e q u i r e s  t h a t  
i n t e r p r e t a t i o n .  The s i n g l e  supernova neighbor  could happen t o  be toward t h e  
gene ra l  Galactic c e n t e r ,  but  i t  would occupy a large c i r c u l a r  area on t h e  sky ,  
perhaps a 30" Eo 90" cone, and i t  would he most u n l i k e l y  t h a t  t h e  c e n t e r  of 
such a nearby t i s t r i b u t i o n  would happen t o  l i e  in t h e  p lane  of t h e  Galaxy. If 
t h e  observ ing  teams can s u c c e s s f u l l y  show t h a t  t h e  l a t i  Jde d i s t r i b u t i o n  i s  
narrow and cen te red  on t h e  p lane ,  Ciayton s a i d  t h a t  we w i l l  be fo rced  t o  
accep t  t h e  i n t e r p r e t a t i o n  of t h e  26A1 concen t r a t ion  as a g e n e r a l  f e a t u r e  of 
t h e  gas  as w e l l  as  r e a f f i r m i n g  t e  need of a nonsupernova source  (x., novaz) 
of t h e  young 26Al. 
To a i d  t h i s  i n t e r p r e t a t i o n ,  Clayton presented  t h e o r e t i c a l  angu la r  d i s t r i -  
bu t ions  and i s o t o p i c  concen t r a t ions  t h a t  would be c o n s i s t e n t  w i th  t h e  observed 
gamma ray  f lux .  He and Le i s ing  (1985, Astrophys. J., in p r e s s )  made s e v e r a l  
d i f f e r e n t  assumptions about  t h c  d i s t r i b u t i o n  of t h e  product ion  of 2 6 A l  i n  t h e  
hopes cha t  a t  least  Gamma Ray Observatory,  i f  no t  HE.40 3 or Solar Max, w i l l  be  
a b l e  t o  i d e n t i f y  t h e  a c t u a l  angu la r  d i s t r i b u t i o n .  The f o u r  cases  they  
modelled assumed 2 6 A l  product ion  p ropor t iona l  t o ,  r e s p e c t i v e l y :  ( 1 )  t h e  mass 
d i s t r i b u t i o n  of co ld  molecular  c louds ;  ( 2 )  t h e  t o t a l  mass d i s t r i b u t i o n  of  
i n t e r s t e l l a r  matter;  (3)  t h e  r a t e  of o p t i c a l  emission from t h e  s u r f a c e  of  t h e  
d i sk ;  o r  ( 4 )  t h e  d i s t r i b u t i o n  of Galactic nova=. T h e i r  r e s u l t s  o f f e r  good 
hope of recogniz ing  t h e  t r u e  s i t u a t i o n .  Easiest t o  recognize  would be 26Al 
product ion by novae because i t  would be much more s t r o n g l y  peaked toward t h e  
Galactic c-!iter than wnrrl! tPle o the r s .  The cor responding  Galactic concentra-  
t i o n  near t h e  sun would be t h e  lowest  of t hose  p o s s i b i l i t i e s ,  however, o n l y  
about  2 atoms of 2 6 A 1  per  m i l l i o n  Al atoms. The h i g h e s t  concen t r a t ion ,  
10 26Al atcros per m i l l i o n  Al atoms, e x i s t s  i n  molecular  c louds  i f  t h e  produc- 
t i o n  occurs  p r imar i ly  w i t h i n  them, as i t  would i f  massive stars are i t s  
primary source.  
Clayton emphasized t h a t  t h e  i s o t o p i c  concen t r a t ion  2 6 A l / 2 7 A l  = 10 p a r t s  
pe r  mi l l i on ,  which i s  c o n s i s t e n t  wi th  t h e  gamma-ray f l u x  f o r  t h e  f i r s t  t h r e e  
of t hese  d i s t r i b u t i o n s ,  l i e s  squa re ly  between bu t  d i s t i n c t  from t h e  concentra-  
t i o n  50 p a r t s  p e r  m i l l i o n  seen  in sone,  bu- not  a l l ,  aluminum-rich Allemie 
minera ls  and t h e  rmah smaller concen t r a t ion  less than  1 p a r t  p e r  m i l l i o n  t h a t  
could be maintained t h e r e  by supernova explos ions .  Thus t h e  observed  Allende 
concen t r a t ions  of excess 26Mg are s t i l l  t o o  l a r g e  t o  be i n t e r p r e t e d  as be ing  
t h e  average i n t e r s t e l l a r  concen t r a t ion ,  so t h a t  i f  t h e  26A.l was a c t u a l l y  once 
a l i v e  i n  t h e  Allende minerals seen today there-mmst have been some s o u r c e  
of  26Al enhancement i n  Lhe s o l a r  c loud as i t  was c o l l a p s i n b  t o  form t h e  s o l a r  
I n t e r s t e l l a r  2 6 A l  and Excess 26Mg 
Clayton, D. D. 
system. Whether t h i s  contaminatio.1 r e s u l t e d  from a nearby nova o r  r ed  g i a n t ,  
Its ejecta w i l l  ha\., conta ined  o t h e r  nuc lea r  c l u e s  t h a t  mst be d e t a i l e d  and 
Sought. 
Clay ton ' s  f i n a l  remarks reopened h i s  a s s a u l t  [ s e e  f o r  example h n a r  
Planet .  Sci. ,  2, 115 (1992); Astro h s. J. 2, 374 (1981)) on t h e  b e l i e f  
whatever t h e  sou rce  of t h e  26Al observed by today 's  gamma r a d i a t i o n ,  t hose  
o b j e c t s  are n e c e s s a r i l y  e j e c t i n g  4 N, per  m i l l i o n  yea r s  i n t o  t h e  i n t e r s t e l l a r  
medium. By c o n t r a s t  supernova eject  24#N, of new s t a b l e  aluminum and stars 
r e i n j e c t  60 H, of o l d  J t a b l e  aluminilm over  t h e  same m i l l i o n  years .  Arguing 
t h a t  a l l  of t h e s e  ejecta condense i n r o  r e f r a c t o r y  aluminum-rich s o l i d s  as they 
l eave  t h e i r  r e s p e c t i v e  sources ,  t h a t  t h e  26Al  decays t o  26Mg wi th in  t h e  
r e s u l t i n g  d x t u r e  of w e l l  mixed dus t  g r a i n s ,  Clayton showed t h a t  t h e  
r a t i o  26Mg/Al  = 0.05 r e s u l t s  w i th in  t h e  aluminum-rich d u s t ,  f u l l y  a thousand 
times g r e a t e r  t han  t h e  c o r r e l a t i o n  t h a t  l a te r  s u r v i v e s  i n  t h e  alurninum-ri:h 
minerals .  This h igh  i n t e r s t e l l a r  c o r r e l a t i o n  of excess  ' *6& with  A 1  could not 
have been t o t a l l y  removed by evapora t ion ,  because t h e  Allende minera ls  c a r r y  
o t h e r  i s o t o p i c  anomalies t h a t  demonstrate  t h a t  they were not  evaporated 
t o t a l l y  a t  any stage p r i o r  t:, t h e i r  assembly. Thus Qty ton  concludes t h a t  
t h e  26Mg-Al c o r r e l a t i o n  pbserved i n  Allende minera ls  is a m a n i f e s t a t i o n  of a 
cosmic chemical memory. He l a i d  down a cha l l enge  t o  minera l  chemists  t o  
demonstrate  how t h i s  memory su rv ives  t h e  proceRs of minera l  form?tion and 
s u b s t a n t i a l  e x t r a c t i o n  of magnesium from them. 
t h a t  t h e  zbAl  was ac tua l - ly  a l i v e  ---i%T i n  t e ende minerals .  He po in ted  o u t  t h a t  
11 


Interstellar Al-26 and excess Mg-26

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