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Session 72 - Cosmic Rays and Abundances.
Oral session, Thursday, June 11
Atomic spectroscopic measurements of Be abundances in old halo stars formed in the early Galaxy can provide information on the origin of the cosmic rays. A major source of all the light elements (Li, Be and B) is cosmic ray spallation, with Be almost certainly being produced solely by cosmic rays. The measured abundance ratio, Be/Fe, as a function of the metallicity (i.e. Fe/H) of the stars in which the abundances are measured is essentially constant, independent of metallicity for Fe/H from 10^-3 to about 10^-1 of the solar value. Fe production in this epoch is dominated by core-collapse supernovae, with the IMF averaged Fe yield per supernova essentially independent of metallicity. This constancy, coupled with the observed constant Be/Fe, requires a constant Be production per core-collapse supernova (within a factor of \sim2). If the cosmic rays were accelerated only out of the ISM, then their composition would evolve proportionally to that of the ISM, and the Be yield per supernova would increase with metallicity because of the increase in the C and O abundances of both the ISM and the cosmic rays. Moreover, Be production by cosmic rays accelerated out of the ISM would also be energetically untenable, requiring about 10^53 erg in cosmic rays per supernova at the lowest metallicities. Thus, the acceleration of all the cosmic rays in the early Galaxy out of the ISM is inconsistent with the Be data, implying that the cosmic rays (or at least the C and O) are accelerated out of freshly synthesized matter before it mixes into the ISM. We shall discuss the limitations of this conclusion and its implication on the origin of the current epoch cosmic rays.
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