RELEASE NO: 99-070

NEW FINDINGS NARROW THEORIES ON COSMIC RAY ORIGIN

Where do those fast-flying atoms that pelt the Earth come from? Scientists catching cosmic rays with a NASA spacecraft have tightened the constraints on the evolving theory of how atoms travelling at nearly the speed of light are produced in stars and are strewn across the Universe through star explosions, or supernovae.

Cosmic rays bombard the Earth's atmosphere constantly. These highly energetic particles are not "rays," however, but rather atoms that were stripped of their electrons when they were accelerated to enormous speeds. While many scientists agree that the energy of supernovae is needed to produce cosmic rays, debates rage over the "seed particles," or the actual atoms that are being accelerated. Are the particles accelerated directly from a supernova, like shrapnel in an explosion? Or are they from dust and gas already present in the region between stars, bumped to high speeds by the blast wave of a supernova explosion?

Results from NASA's Advanced Composition Explorer (ACE) suggest that cosmic rays are not accelerated directly from supernovae, as some current models predict. Rather, it is material that has been sitting around for hundreds of thousands of years that gets accelerated by the shock wave of a supernova explosion.

Dr. Paul Hink of Washington University in St. Louis (WUSL) and a team of scientists from WUSL, California Institute of Technology, NASA's Jet Propulsion Laboratory and NASA's Goddard Space Flight Center present these results from the ACE Cosmic Ray Isotope Spectrometer (CRIS) at the Centennial Meeting of the American Astronomical Society in Chicago on May 31, 1999.

CRIS scientists analyzed cosmic rays from the elements nickel (Ni) and cobalt (Co). The radioactive isotope Ni-59, which is produced in supernova explosions, ultimately decays into the more stable Co-59; the half-life is 75,000 years. Hink said the team found in their cosmic ray collection that this decay process had already run its course. This means that Ni-59, formed in one supernova explosion, sat decaying for over a 100,000 years before another supernova blast wave came along and propelled it into the surrounding galaxy at cosmic ray energies.

If the supernova itself accelerated the freshly-made Ni-59 to cosmic ray energies, Hink said, then the CRIS scientists would have seen more Ni-59 in their cosmic ray collection, and less Co-59. Team members ruled out the possibility of Ni-59 decaying while en-route to Earth, because in order to decay, the Ni-59 nucleus has to absorb one of its electrons. Hink said that once an atom of Ni-59 is accelerated to cosmic ray energies and loses its electrons, it is relatively stable and invulnerable to decay, as long as it keeps moving.

Dr. Mark Wiedenbeck of NASA's Jet Propulsion Laboratory, a member of the CRIS team, said their results rule out the possibility of the cosmic ray seed particles being fresh supernova ejecta. Instead, the results support theories for the origin of cosmic rays to be old stellar or interstellar material. Results are also consistent with the superbubble theory, put forth last year. According to this theory, a succession of supernovae carve out a superbubble, or a cavity within a giant molecular cloud in our galaxy, in which the seed particles of cosmic rays, produced directly by the supernovae, can linger (and decay) for millions of years before breaking out of the bubble and being accelerated to cosmic ray energies.

CRIS was developed by the California Institute of Technology, Washington University in St. Louis, Goddard Space Flight Center, and the Jet Propulsion Laboratory. The instrument is one of nine aboard ACE, launched by NASA in 1997 to collect and analyze matter from the sun, from the space between planets and from the Milky Way galaxy beyond our solar system.

EDITOR'S NOTE: Images to support this story are available on the web at: http://www.gsfc.nasa.gov/ftp/newsmedia/AAS/ACE