Re: Nuclear Physics, Supernovae

Enrico,
The question you ask involves multiple disciplines and is in fact one of
the faster growing fields of physics today.  Please bear with me as I first
cover the relevant topics in individual fields and then try to pull them
together.

First, there is the issue of solar system formation.  The prevailing theory
for a very long time was that in the early stages of the solar nebula,
basically all matter was melted and completely mixed until it was
heterogeneous, even down to isotopic composition.  Thus, if you measured
26Al abundances in any chunk of solar system material, and measured 27Al
abundances in the same chunk, the ratio 26Al/27Al would be a constant,
predictable value.   Nowadays, we know that some early material survived
the "melting pot," and was incorporated into meteorites and other planetary
bodies as they formed.

As far as I can tell, what you describe is an early study in a field of
meteoritics that deals with presolar materials.  By studying meteorites,
particularly small pieces of meteorites, it was discovered among other
things that some of the samples had isotopically anomalous 26Al/27Al
ratios, indicating that they contained material which somehow survived the
solar nebula.  Of special interest at the time were parts of meteorites
known as Calcium-Aluminum-rich Inclusions (CAI's).  (Actually, the 26Al
abundances were inferred from 26Mg, which is the daughter product via beta+
decay in 26Al.  Of course, you already noted this fact.)

So now we get to the big question: Now that we know that the material is
presolar in nature (it originated before the solar system formed), where
did it come from?  One thing of interest in the case of 26Al is that is has
a half life of 0.73 million years.  Considering the fact that evidence for
its presence was found inside meteoritic material (and considering other    
isotopes with relatively short half-lives like 41Ca with a half life of 0.1
million years), we know that the process which formed the 26Al took
place less than 1 million years before the formation of the meteorite's
parent body (the planetoid which broke up early on and from which that
particular meteorite came from).

So what was the process that formed the 26Al?  We need to consider a field
which only really originated around 1950, which is the field of
nucleosynthesis in astrophysical sources.  Elements and isotopes don't just
appear out of nowhere. They are built up through various nuclear processes
within stars and other astrophysical sources.  The Sun is currently fusing
hydrogen atoms together to create helium.  When it comes to much heavier
elements and isotopes which don't occur via fusion processes, it becomes
necessary to plug individual neutrons or protons into the nuclei to create
new isotopes.  (That is one way that scientists have created new elements
in the laboratory.  They take preexisting elements, bombard them with
neutrons until they attach to the nuclei, and let them decay into a new
element.)

26Al is produced by a number of processes, and so simply its presence
implies nothing.  For instance, it is produced by proton capture from 25Mg
in AGB (asymptotic giant branch) stars.  However, what makes this case so
special is the unusual excess of 26Al, implying unusual circumstances. 
Within supernovae, 26Al is produced within the hydrogen-burning zone, but
it is also produced in large quantities by explosive nucleosynthesis in the
shock waves that pass through ejected supernova material. 
(Wherein neutrons are added rapidly to lighter isotopes to create heavier
ones.)  Wolf-Rayet stars and some other sources are also believed to
produce 26Al in similar quantities. 
Let's put the pieces together: an astrophysical source which produced much
26Al relatively recently (and thus not too far away), yet is not seen
today.  It all points to a supernova event.

The field of presolar materials is a rapidly growing field pursuing new and
exciting techniques of data acquisition.  It is the only field in which we
can study astrophysical sources (stars, supernovae, etc) by directly
studying samples from them in the laboratory.  The current state of the art
uses TEM, SEM and ion microprobes and can detect and study individual
presolar dust grains on the order of 1 micron (10^-6 meters) in size,
embedded in otherwise normal meteorite material.  The data gathered from
these grains directly puts limits on parameters dealing with conditions
within stars, supernovae and other sources.  Amazingly, somethng so small
can tell us so much about something so big.

I hope this answers your question reasonably well.

---Bob Macke
MIT S.B. '96 Physics
 in St. Louis Ph.D. Candidate, Physics

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REFERENCES:

 McDonnell Center for the Space Sciences

MacPherson, et. al., "The Distribution of aluminum-26 in the early Solar
System - A reappraisal", Meteoritics 30, 365-386 (1995).

Bernatowicz, T.J., and Zinner, E. ed, Astrophysical Implications of the
Laboratory Study of Presolar Materials, American Institute of Physics,
Woodbury NY, 1997.  (See esp. Zinner, E. "Presolar Materials in Meteorites:
An Overview" and Meyer, B.S. "Supernova Nucleosynthesis")

Burbidge, E. M., Burbidge, G.R., Fowler, W.A., and Hoyle, F., Rev. Mod.
Phys., 29, 547 (1957).