ACE News Archives

ACE News #37
October 1, 1999

ACE News Archives

Abundances of Iron, Cobalt, and Nickel Isotopes Suggest Cosmic Rays Originate from Interstellar Matter

The elements iron, cobalt, and nickel are produced in the interiors of stars late in their evolution, shortly before they explode as supernovae. Comparisons of the composition of solar system matter with models for the production of heavy nuclei in stars have led to the conclusion that solar-system material contains contributions from two different kinds of supernovae. One type (SN II) occurs when relatively young, massive stars exhaust their nuclear fuel and their cores collapse. The other type (SN Ia) results from mass accretion onto old, low-mass white dwarf stars in binary star systems, which can lead to run-away heating and detonation. The Fe, Co, and Ni in solar system matter are believed to represent an approximately equal mix of material from the two types of supernovae. The abundances of the isotopes of these elements are strongly affected by characteristics of the parent star including its initial mass and heavy element content (termed ``metallicity'' by astronomers), and they can also be altered during ejection from the exploding star.

The Cosmic Ray Isotope Spectrometer (CRIS) on ACE has measured the elemental and isotopic composition of a different sample of matter, the source material from which galactic cosmic rays are accelerated in the galaxy. The plot on the left shows the nickel mass spectrum obtained by CRIS. Besides the major isotopes 58Ni and 60Ni (filled histogram), CRIS also resolves very rare isotopes (unfilled histogram) including 62Ni and 64Ni. The plot on the right compares the galactic cosmic-ray (GCR) source abundances derived from the CRIS data with solar system abundances. The most notable feature of this comparison is the fact that the compositions of these two samples differ by at most a few tens of percent, even though the abundances of the various isotopes range over more than a factor of 1000. This great similarity suggests that a similar mix of star types was involved in producing both populations. This would be the case if cosmic rays are accelerated out of the general mix of interstellar gas and dust from which the solar system condensed and if the composition of this material has not been greatly altered by galactic chemical evolution over the past 4.5 billion years. This interpretation is consistent with a previous CRIS result (ACE News #17) that at least 100,000 years must have elapsed between supernova nucleosynthesis and the acceleration of cosmic rays from the ejecta.

Contributed by Mark Wiedenbeck of the Jet Propulsion Laboratory.

See The CRIS Home Page for more information on ACE CRIS.

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Last modified September 29, 1999. Andrew Davis