**Light Elements and the Origin of the Cosmic Rays**

Reuven Ramaty, Richard E. Lingenfelter, Benzion Kozlovsky

The understanding of the origin of the elements, that is their sites of
origin, the variety of processes involved, and the epochs in the
evolutionary history of the universe when they occurred, is one of the
greatest achievement of modern science. The importance of this endeavor
is not only in its ability to provide answers to questions such as
"where did we come from\", but also in its being the prime tool for
setting the time markers that delineate cosmic evolution. Research over
the past half century has clarified much of the situation. The cosmic
rays, high energy particles that pervade our Galaxy, not only provide a
direct sample of cosmic matter carrying information on the processes
that produce the elements, but also play a major role in the synthesis
of the light elements, lithium, beryllium and boron. As we shall see,
investigations with powerful ground and space based telescopes of the
abundances of these elements have led to entirely new insights into the
origin of the cosmic rays.

**Element Genesis**

We now know that nucleosynthesis, the genesis of the chemical elements
and their isotopes, took place both universally, shortly after the Big
Bang, as well as in stars, much later. Isotopes of the same element have
the same atomic number but different masses. In

the first few minutes after the Big Bang, at temperatures exceeding
10^9^ K, several of the lightest elements and their isotopes were
created. Most of the helium (He), essentially all of the deuterium
(^2^H, the heavy isotope of hydrogen) and some lithium were thus
produced. Lithium has two stable isotopes, ^7^Li and ^6^Li, but the
relevant nuclear processes are such that the Big Bang produced
significant amounts of only the heavier one. Beryllium has one stable
isotope (^9^Be), while boron has two (^10^B and ^11^B). These light
elements were not produced in significant quantities in the Big Bang.
Likewise, because of the rapid expansion of the universe and the
concomitant decrease of the density and temperature, neither were the
heavier elements (C, O , etc.). These, so-called metals, have been and
still are synthesized much later in the interior of stars, as well as in
stellar explosions (supernovae) that are the death throes of the most
massive stars.

The idea that the synthesis of all the elements was associated with the
origin of the Universe came from George Gamow and his co-workers in the
late 1940\'s. A competing theory at that time was that of Fred Hoyle,
which maintained that all the elements are synthesized in stars in
galaxies. The strongest argument against an initial, universal synthesis
of all the elements is the fact that very significant variations of
elemental abundances are observed in stars of different ages, indicating
that nucleosynthesis is an ongoing process. Indeed, the theories of
stellar evolution, supernova dynamics and Galactic chemical evolution
are capable of accounting for many the observed elemental abundances at
a great variety of astronomical sites. These theories are based in large
part on the pioneering work in the 1950\'s of Margaret and Geoff
Burbidge, Willie Fowler and Fred Hoyle, and independently that of Al
Cameron.

On the other hand, there are several isotopes whose abundances cannot be
understood by stellar nucleosynthesis. Even though deuterium is produced
in stellar interiors, it is also very rapidly destroyed. But in the Big
Bang, deuterium produced by the capture of neutrons on protons, can
survive under certain conditions on the universal density which allow
the synthesized ^2^H to escape destruction owing to the rapid expansion
of the universe. In fact, the observed deuterium abundance in the solar
system, in the Galaxy and even in distant extragalactic space, is one of
the best indicators of the overall matter density of the universe.

The other light elements, Li, Be and B, hold a unique place among the
elements. Even though their abundances are exceedingly low, only about
10^-9^ that of H and about 10^-6^ that of the next heavier elements C, N
and O, they play important roles both in cosmology and cosmic-ray
origin. Li, Be and B are very easily destroyed in stellar interiors, and
they are not generated in the normal course of stellar nucleosynthesis,
which proceeds directly from helium to carbon via the fusion of three
alpha particles (helium nuclei). ^7^Li is somewhat of an exception, as
it is produced via the fusion of the two helium isotopes, ^3^He and
^4^He, in giant stars and supernovae. Thus, until about 1970, the origin
of Li, Be and B remained a mystery. At that time, Hubert Reeves, Willie
Fowler and Fred Hoyle suggested that these light elements could be
produced in nuclear interactions of cosmic rays with the atoms of the
gas and dust that pervade interstellar space in galaxies (the
interstellar medium). The cosmic rays, high energy particles most likely
accelerated by shock waves produced by supernovae, also pervade
interstellar space.

**Cosmic Rays**

The existence of cosmic radiation was first suggested in 1900 by C. T.
R. Wilson following his discovery of atmospheric ionization, that is the
dissociation of the atoms into their constituent nuclei and electrons.
Wilson proposed that extraterrestrial gamma rays (very high energy
quanta of light) might be responsible for the ionization. Twelve years
later, with balloon-borne ionization detectors, showing that the
ionization rate increased with altitude and had no diurnal variation,
Victor Hess demonstrated the extraterrestrial and extrasolar origin of
the ionizing radiation, which soon came to be known as \"cosmic rays\".
These rays were generally thought to be gamma rays until the variation
of their flux with geomagnetic latitude was discovered in 1927. This
clearly established that they were not gamma rays, but charged particles
deflected by the magnetic field. Subsequent observations with
instruments flown on balloons and satellites demonstrated that the
cosmic rays are mostly relativistic protons. Heavy elements in the
cosmic rays were discovered in the late 1940\'s by Phyllis Freier and
coworkers, and independently by Hale Bradt and Bernard Peters, both
groups employing balloon-borne detectors. At more recent times, with
detectors on satellites, it has been possible to establish the existence
of the entire periodic table of elements in the cosmic rays.
Nevertheless, the ultimate origin of these high energy particles has
remained a mystery.

Any lingering doubt about whether the bulk of the cosmic rays are
Galactic or extragalactic has been settled in favor of a Galactic origin
by gamma ray observations with the EGRET instrument on the COMPTON Gamma
Ray Observatory, a major orbiting observatory launched in 1991 and still
operational. Even though the cosmic rays are not gamma rays, they
produce gamma rays when they interact with ambient matter in the region
of their confinement. Observations of such gamma rays showed that the
cosmic-ray energy density in a nearby galaxy (Small Magellanic Cloud) is
much lower than that found locally in our own Galaxy and is thus
inconsistent with a uniform extragalactic density of cosmic rays. The
Galactic origin of the cosmic rays has long been connected with
supernovae, as Walter Baade and Fritz Zwicky pointed out in 1934 that
supernova explosions could provide the power necessary to maintain the
cosmic rays in the Galaxy. This idea has received considerable support
in the late 1950\'s from Vitaly Ginzburg and S.I. Syrovatskii who
associated cosmic-ray origin with supernovae, based on the polarization
properties of visible light seen from supernova remnants which strongly
suggested that the radiation is produced by relativistic electrons
moving in the magnetic fields of the remnants (synchrotron radiation).
Synchrotron radiation from very high energy electrons has been recently
observed also as X-rays both with the Advanced Satellite for Cosmology
and Astrophysics (ASCA) and with the very recently launched X-ray
observatory, Chandra.

It is quite generally believed that the cosmic rays are accelerated by
shock waves driven by the rapidly moving ejecta of supernovae. Indeed,
there is ample evidence, from direct spacecraft detections, for particle
acceleration at the variety of shocks that are generated by disturbances
in the interplanetary medium. If 10% of the kinetic energy of the ejecta
of supernovae is imparted to the accelerated particles (a reasonable
value based both on theoretical calculations and on direct
interplanetary observations), then the known ejecta kinetic energy
combined with the average Galactic supernova frequency, yields a power
that is sufficient to maintain the observed cosmic-ray density against
escape from the Galaxy. This escape lifetime, about 10 million years, is
also roughly the age of an average cosmic-ray particle. Cosmic-ray
escape from the Galaxy is controlled by the structure and complexity of
the interstellar magnetic fields, and its rate is derived from the
observations of \`\`radioactive clocks\", unstable cosmic-ray nuclei, in
particular ^10^Be (half life = 1.5x10^6^ years) and ^26^Al (half life =
7.4x10^5^ years).

Thus, the source that powers the cosmic rays appears to be well
understood. But the source of the particles that become cosmic rays is
still a matter of very intense debate. It is not clear whether the bulk
of the cosmic rays are accelerated from average Galactic matter that was
synthesized prior to the acceleration by many generations of stars and
has participated in the complex process of Galactic evolution, or from
relatively fresh nucleosynthetic material produced by the same ensemble
of supernovae whose shocks also accelerate the particles. The resolution
of this issue is important not only for the origin of the cosmic rays,
but also because of its potential impact on our understanding of

nucleosynthesis and Galactic evolution. For if the cosmic rays are
indeed accelerated out of fresh nucleosynthetic material, then they
provide the most direct sample of current epoch nucleosynthesis,
unaffected by the evolutionary processes that determine the elemental
abundances of the Sun, the solar system and stars.

Radioactive clocks can play a role in distinguishing between the
possibilities. There is a pair of cosmic-ray isotopes, ^59^Ni and
^59^Co, whose relative abundances can shed light on the elapsed time
between nucleosynthesis and acceleration. This is because ^59^Ni is
radioactive and decays into ^59^Co with a half life of 7.6x10^4^ years,
by capturing one of its orbital electrons. Consequently, the decay can
happen only if the atom is moving at relatively low velocity because
once accelerated, its electrons are stripped and decay is prevented. The
^59^Ni and ^59^Co abundances observed with the currently active Advanced
Composition Explorer satellite (ACE) show that ^59^Ni has mostly
decayed, implying a delay between nucleosynthesis and acceleration of at
least 10^5^ years. This very interesting new piece of information,
however, is not sufficient to distinguish between the above two
alternatives, because it appears that the cosmic rays could be
accelerated out of fresh material even on time scales on the order of
10^5^ years or longer. This would be achieved if successive supernova
shocks accelerated particles inside superbubbles. These giant
interstellar cavities of hot, rarified plasma are produced by the
supernovae themselves and last for tens of millions of years. During
this time period, the medium within the cavities is enriched in fresh
material from the supernovae that formed them, and thus can serve as a
freshly enriched particle injection source. In fact, this rarefied
medium is propitious for the acceleration, since it allows the supernova
shocks to propagate out to large distances and prevents the accelerating
particle from losing their energy before they can become cosmic rays.
The cosmic rays can thus be accelerated out of relatively fresh material
on time scales ranging from several hundred thousands to tens of million
years, consistent with the ^59^Ni and ^59^Co data. Radioactive clocks
with longer half lives are needed to distinguish between old
interstellar and more freshly enriched superbubble injection. These
could be supplied by detailed cosmic-ray measurements of the unstable
actinides (Th, U, Pu, Cm) which have isotopes with half lives in the
relevant range of tens of millions of years, but such observations are
not yet available. However, as we shall see, the light element
observations, particularly those of Be, are providing entirely new
insights into this problem.

**Li, Be, B and Cosmic-Ray Origin**

The present stars have a very wide range of ages. A key indicator of a
star\'s age is its metallicity, specifically the abundance of iron
relative to hydrogen determined from spectroscopic analyses of the
visible light from the star. This is because the metallicity of the
visible surface layers of a star generally reflects the metallicity of
the interstellar medium at the time the star was formed, and the
interstellar metallicity has grown with time through the accumulation of
Fe and other metals produced in stars and supernovae. Stars have been
observed with metallicities ranging from slightly more than the solar
value to as little as 10^-4^ of the solar value, indicating that those
of the highest metallicities were formed more recently than the Sun,
while those with the lowest in the very earliest stages of the evolution
of our Galaxy. It is quite remarkable that stars as old as 10 billion
years still exist, but this is consistent with stellar evolution theory
that predicts very long lifetimes for low mass stars.

Considering similar abundance measurements for the light elements,
Monique and François Spite found in the early 1980\'s that the lithium
abundance of stars of various metallicities is practically constant for
metallicities up to about 0.1 of the solar value. This metallicity range
corresponds to the first billion years of Galactic evolution, a period
that preceded the formation of the Galactic disk. Indeed, the relevant
stars are mostly in the halo. The constancy of the lithium abundance is
prima facie evidence for the extragalactic origin of the lithium, and it
is generally accepted that the Galactic lithium in this period was
synthesized in the Big Bang, along with the deuterium and helium as we
have already mentioned. In stark contrast, the Be and B abundances are
observed to increase essentially monotonically with metallicity, not
just during the billion year halo phase but all the way to the current
epoch. This is strong evidence that these light elements are produced in
the Galaxy. All of these observations are very challenging because the
spectral lines of these rare elements are very weak and they are usually
blended with interfering lines from other more abundant elements. The
observations therefore require large telescopes (for example the 10
meter KECK instrument in Hawaii)

and very efficient, high resolution detectors coupled to these
telescopes. While the spectral lines of Li and Be are in the visible
range of the electromagnetic spectrum, and thus can be observed from the
ground, the B lines are in the ultra-violet and therefore were
investigated only after space-based observatories became available
(specifically the Hubble Space Telescope).

We focus our attention on the beryllium, first because there is more
data, and second because it is generally believed that this element is
almost entirely cosmic-ray produced, while boron is thought to contain
an additional contribution from supernovae. The fact that stars with
metallicity as low as 10^-3^ of the solar value have significant
beryllium abundances means, first of all, that cosmic rays were already
present in the very young Galaxy, 10 billion years ago. But in addition,
the observed Be abundances are surprisingly high. If the cosmic rays in
the early Galaxy were accelerated out average Galactic matter, which was
very poor in metals, only negligible amounts of Be would have been
produced, because this element results from the breakup of carbon and
oxygen nuclei which would have been practically absent both in the
cosmic rays and in the ambient medium. On the other hand, if the cosmic
rays were accelerated from matter in supernova enriched superbubbles, as
described above, then the cosmic rays themselves would have contained
enough C and O to produce the required amounts of Be as they interacted
with the protons and alpha particles of the ambient medium. This argues
very strongly for cosmic-ray acceleration out of fresh nucleosynthetic
material.

These light element studies, which extend the cosmic-ray investigations
from the current epoch cosmic-ray lifetime of only 10 million years to
the 10 billion year age of the Galaxy, thus go a long way toward
settling the question of cosmic-ray origin. But clearly more
investigations are needed, and the longer lived radioactive clocks
mentioned above may provide the decisive test in the future.

**Supernovae, Superbubbles, High Velocity Dust, Cosmic Rays and
Neutrinos**

The collapse of the iron core of a massive star at the end of its
evolution produces a neutron star or black hole and results in a
supernova explosion that blows off all the overlying matter. These
\`\`core-collapse\" supernovae constitute about 80% of all the supernova
events in a galaxy. The remainder are thermonuclear supernovae which
result from runaway nuclear reactions in white dwarfs. These dwarfs are
also the end products of stellar evolution, but unlike neutron stars and
black holes, they originate from smaller stellar progenitors. A variety
of observations show that the bulk of the core-collapse supernovae occur
in tight associations, because their massive stellar progenitors form
from giant molecular clouds and, owing to their relatively short
lifetimes (only 3 to 30 million years), they do not disperse appreciably
from their birth sites before they expire in supernova explosions. The
combined winds of the massive stars and the ejecta of the core-collapse
supernovae form hot, low density superbubbles, that reach dimensions as
large a thousand light years and last for tens of millions of years.
Even though the thermonuclear supernovae are more evenly distributed
throughout the Galaxy, an important fraction of them will also be in
superbubbles, simply by chance because of the large fraction of
interstellar space occupied by these cavities. Calculations show that
the supernova ejecta should dominate the superbubble metallicity. As
already mentioned, this low density medium is an ideal source of matter
to be accelerated to high energies, and produce cosmic rays, whose
metals will thus consist predominantly of fresh nucleosynthetic matter.
While this is expected to be the case at the current epoch, it will
certainly be true in the early Galaxy when the metallicity of the
average interstellar medium was so low that the supernova ejecta
definitely would have been the main source

of metals in the superbubbles.

A large fraction of the refractory metals in the ejecta of supernovae
are in dust grains. Indications for such grain formation came from
supernova observations in the infrared, and from the analysis of
meteorites. Additional strong evidence was obtained by the sudden
brightening of the infrared emission due to the formation of dust in the
spectacular supernova that exploded on February 23, 1987 in a nearby
galaxy, the Large Magellanic Cloud. The observations indicate the
formation of refractory grains of graphite, oxides and silicates, with
mean velocities of about 2000 to 3000 km/sec. As these grains collide
with gas atoms in the superbubble, grain atoms are sputtered, or knocked
loose, producing refractory element nuclei that are also moving at high
velocity. It is a well known property of acceleration processes,
including shock acceleration, that particles whose velocities exceed the
thermal speed of the bulk of the ambient gas are preferentially
accelerated. This leads to the observed cosmic-ray enrichment of the
refractory metals (Mg, Si, Fe, Ni) relative to the volatiles (H, He, Ne,
S, Ar). The progenitors of the light elements, carbon, residing mostly
in graphite, and oxygen, incorporated into the oxides, are also
enriched. In addition to being sputtered, the moving grains also scatter
atoms of the ambient gas, which also acquire velocities comparable to
those of the grains. This process can thus account for the acceleration
of volatiles, as well, and provide a comprehensive theory of cosmic-ray
origin. Our preliminary calculations indicate that the expected ratio of
scattered and sputtered atoms is consistent with volatile-to-refractory
particle ratio in the cosmic rays, a strong boost for the theory.

Supernova explosions release most of their energy in a flux of
neutrinos, particles of vanishingly small or zero mass, whose existence
at a nuclear reactor on Earth was first confirmed by Frederick Reines
and Clyde Cowan in the 1950\'s. The supernova in the Large Magellanic
Cloud was close enough that the neutrinos that it produced were detected
with under-ground instruments in Japan and the US. Neutrinos form
supernovae play a role in the origin of boron, as they are responsible
for the production of about 40% of this element. The key observable is
the boron isotopic ratio, since the neutrinos have enough energy to
break off only one proton or neutron from carbon in the supernova to
form ^11^B, but not enough energy to make ^10^B. Consequently, the
neutrino-produced boron enhances the ^11^B-to-^10^B ratio to a value
consistent with that measured in meteorites. This value represents the
ratio that existed at the formation of the solar system, but similar
isotopic data are also needed for earlier times, near the birth of the
Galaxy. These, however, must await observations with still more powerful
space-based telescopes. A similar situation exist with the isotopes of
lithium. We pointed out that the Big Bang produces mostly ^7^Li, so that
^6^Li production requires cosmic rays. Preliminary telescopic data
indicate an excess of ^6^Li in the early Galaxy, suggesting the presence
of pre-Galactic cosmic rays. This very exciting finding needs
confirmation, as it would add important new information to the processes
that govern galaxy formation.

**Outlook**

It is quite remarkable that the study of the composition of the
atmospheres of stars born soon after the formation of our Galaxy has led
to a new understanding of the origin of the cosmic rays. But clearly
much more needs to be done. We mentioned the various radioactive clocks
that will be studied with current and future cosmic-ray detectors and
thus determine the age of the source material that was accelerated to
cosmic-ray energies. These cosmic-ray investigations, coupled with
spectroscopic analyses of the oldest stars that probe, with powerful
ground and space based telescopes, the dawn of Galactic evolution, are
an integral part of the quest for the full understanding of cosmic
evolution, which certainly is one of the noblest undertakings of
humanity.