archive: SETI [ASTRO] Cosmic Ray History Encoded In Abundances Of Light

SETI [ASTRO] Cosmic Ray History Encoded In Abundances Of Light

Larry Klaes ( lklaes@bbn.com )
Thu, 27 May 1999 14:34:19 -0400

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>Date: Thu, 27 May 1999 18:15:11 GMT
>From: Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
>To: astro@lists.mindspring.com
>Subject: [ASTRO] Cosmic Ray History Encoded In Abundances Of Light Elements
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>Reply-To: Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
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>Cosmic ray history encoded in abundances of light elements
>
>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.
>
>