THE ASTROPHYSICAL JOURNAL, 447:L53-L57, 1995 July 1
© 1995. The American Astronomical Society. All rights reserved.
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Cosmic rays are usually
considered to be accelerated by magnetic fields in
hydrodynamically active regions, in which kinetic energy of mass
motions can be transferred magnetically into particle energies.
Thus it is understandable that the Orion phenomena of gamma rays
and an excess carbon and oxygen low-energy cosmic-ray flux should
be interpreted as a common property of star formation regions.
However, our analysis suggests an attractive alternative that may
provide a more consistent picture. These phenomena may be direct
consequences of the supernova explosions themselves, and thus
must be limited in time, but may also be episodic when new
explosions occur.
The lifetime of interstellar
molecular clouds, not counting formation times, is 1-3 × 10
yr according
to the ages of the oldest T Tauri stars projected on clouds with
visual extinction greater than 1 mag (e.g., Walter
et al. 1988), or according to ages of OB associations with
associated molecular gas (Blaauw 1991), and so the massive stars formed
within such a cloud only reach the supernova stage for masses
possibly as low as 9 M
during the lifetime of the cloud. Only a
subset of these explosions can eject very energetic carbon and
oxygen ions as Type Ib supernovae, so the distinctive pattern of
the deexcitation gamma rays from these ions must also be quite
rare, and the lifetime of the gamma-ray emission is likely to be
only a few thousand years. The Compton Gamma Ray Observatory
was apparently fortunately launched at a time when the nearest
major star-forming region was host to such an event.
Our analysis of the injection of
short-lived radioactivities into molecular cloud cores situated
close (a few parsecs) to supernova explosions indicates that such
injection should occur in a significant fraction of the cores in
a molecular cloud and the actual level of the radioactivities
found in a resulting planetary system should be quite variable.
Already for 
I in the solar system, with a mean life of 2.3 × 10 yr, the source
appears to have been distant r-process-producing
supernovae (Cameron
1993; Cameron, Thielemann, & Cowan 1993) near 10 M, and thus on
its timescale a significant general background abundance level of
such radioactivities appears to exist in molecular clouds and
will become part of the cores that are formed.
The supernova ejecta may become
well mixed within the core that it has impacted, but
nevertheless, this can leave the grains in the core with a new
set of isotopic anomalies. The condensible atoms within the core
prior to the arrival of the shock wave are already condensed.
After compression and admixture of the swept-up interstellar
medium and diluted supernova ejecta, that admixed material will
also chemically attach, but in this case to the surfaces of the
preexisting grains. In the face of a size spectrum for that dust,
the smallest dust will generally become most enriched (per unit
mass) in the diluted supernova ejecta. These differing isotopic
patterns fingerprint the subsequent chemical rearrangements as a
form of chemical memory (Clayton 1982).
We have concluded that the core
giving rise to the solar system was situated approximately in the
range 2-10 pc from the site of a massive supernova explosion in
the parent molecular cloud. A typical core in a cloud containing
OB associations would lie somewhat farther away from the site of
the association, but within the cloud lifetime the O stars can
travel significant distances into the cloud before exploding; so
the event we have described may be somewhat unusual, but it
should not be rare. Even at larger distances the more gentle
injection of 
Al into a core should be common. However, a majority
of cores that form stars are not subjected to quite such violent
hydrodynamics resulting from this deformation and injection as
appears to have been the case for the Sun, and it is important
that the special character of this type of event should receive
further study.
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