Cosmic ray history encoded in abundances of light elements

June 01, 1999

CHAMPAIGN, Ill. -- By taking a closer look at two of the lightest elements in the universe, a University of Illinois scientist is helping to solve a mystery that lies at the intersection of cosmology, cosmic rays and chemical evolution.

"The origin and history of cosmic rays are encoded in the cosmic abundances of beryllium and boron," said Brian Fields, a visiting professor of astronomy at the U. of I. "These elements, 'orphans' of nucleosynthesis, are made neither in stars nor in the big bang. Instead, they are created by cosmic rays, high-energy particles that fly through space and smash into atoms in interstellar gas, fragmenting them into lighter elements."

While astronomers generally agree that cosmic rays originate in supernova explosions, they differ over the mechanism responsible. "The traditional view is that cosmic rays are particles of the interstellar medium that were ionized and accelerated by a supernova shock wave," Fields said. "An alternate view, however, suggests that cosmic rays are pieces of the star blown off in the explosion."

The cosmic abundances of three elements -- hydrogen, helium and lithium -- were initially set in the big bang. But these amounts -- as well as those of all the other elements -- are continually changing because of the life cycle of stars. In a grand recycling theme, supernova remnants enrich the interstellar medium with heavy elements that eventually condense into new stars.

Because stars are net producers of certain elements -- iron and oxygen, for example -- the amounts of those elements will slowly increase in the interstellar medium. "This provides a kind of 'yardstick' for measuring the cosmic abundances of beryllium and boron over time and space," Fields said. "These two elements should scale with iron in definite, predictable ratios."

When measured, however, these ratios were found to be off significantly, leading many astronomers to conclude that the standard cosmic ray scenario was either incomplete or incorrect. Additional sources and mechanisms for light-element production were then proposed.

Fields and colleague Keith Olive, a professor of physics at the University of Minnesota, decided to re-examine the data. "Instead of comparing how the abundances of beryllium and boron change with iron, we chose to compare them with oxygen," Fields said. "After all, oxygen nuclei are what are being broken up by cosmic rays to make the lighter elements."

By carefully analyzing the abundances of beryllium, boron, iron and oxygen in stars of different ages, Fields and Olive derived new scaling factors that strongly support the traditional view of cosmic ray and light-element production, without requiring additional sources or mechanisms.

"The standard picture of cosmic ray origin may be correct after all," Fields said. "That is, cosmic ray particles originate in interstellar gas, not directly from supernovae. The data, as we read them, support this."

Fields and Olive discussed their findings in the May issue of the Astrophysical Journal.

University of Illinois at Urbana-Champaign

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