As discussed in the Significance
of LiBeB section, lithium, beryllium and boron production are not accounted
for in stellar nucleosynthesis and Big Bang nucleosynthesis models.
The LiBeB elements are so fragile that they are destroyed as soon as they
are created in stars and in supernovae. They have to be produced
by other means.
The interstellar medium is, in Earth terms, a vacuum,
but on a cosmic scale, it is rich with matter. Shortly after the
Big Bang, the most massive of the early stars exploded as supernova, ejecting
heavier elements which then enriched the interstellar medium from which
subsequent generations of stars were born. If there were some way
that the elements in the interstellar medium could be split into other
elements, or "spalled," then we would have a method by which LiBeB could
be produced. Cosmic rays, of course, are the missing link.
They are incredibly abundant, transit galaxies, and possess energies large
enough to split atoms when they collide with the interstellar medium.
That, or they can be split themselves, a subtle, but powerful distinction.
To illustrate this, let's look back at the evolving theory of the origin
of cosmic rays. The previous theory suggested that cosmic rays are
produced when supernova shock waves hit particles already present in the
interstellar medium. The new theory suggests that cosmic ray particles
are accelerated directly from the material ejected by the supernova.
Where did this theory come from? It was, in fact, motivated by one of the mysteries of LiBeB abundances. Old stars born soon after the formation of the galaxy have a high abundance of beryllium.
Examining the old halo stars surrounding the Milky Way, scientists discovered that they are mostly made up of hydrogen and helium, and only have traces of heavier elements such as oxygen and iron. However, there is a surprisingly high abundance of beryllium relative to iron. Beryllium is made by the breakup of carbon and oxygen nuclei in collisions between cosmic ray particles and interstellar matter. The high and nearly constant beryllium abundance found in stars of all ages led to the conclusion that cosmic ray produced beryllium has remained roughly constant over time.
This result seems to rule out the acceleration of cosmic rays out of the interstellar medium. In that case, the composition of the cosmic rays would evolve in proportion to that of the interstellar medium. The beryllium yield per supernova would increase as the interstellar abundances of carbon and oxygen increase. This is contrary to observations.
So, the motivation for the new theory becomes clear.
Cosmic rays also play a role in the other elements of LiBeb. One of Professor Venn's projects on the Hubble Space Telescope examines spectral lines in various stars in a small Magellanic Cloud (SMC) and a large Magellanic Cloud (LMC). These clouds have a different cosmic ray flux than in our galaxy, and so, provided cosmic ray spallation is a producer of B, they should have a lower concentration of B than in other areas of that galaxy. Check back later to see what the results of that experiment are!
Spallation reactions between cosmic rays and the interstellar medium accurately predict the local abundances of 6Li, 9Be, and 10B, but underproduce 7Li and 11B. Theoretical predictions favor neutrino-induced spallation in Type II supernovae as the additional sources. Neutrinos colliding with carbon and helium nuclei can produce 11B and 7Li. In light of this, a wider range of estimates have been made, possibly accounting for the missing matter, but have yet to be shown observationally.
Clearly the jury is still out on LiBeB abundances,
but cosmic rays seem to provide a key piece to the intriguing puzzle.