AAS 198th Meeting, June 2001
Session 59. Galactic ISM
Display, Wednesday, June 6, 2001, 10:00am-7:00pm, Exhibit Hall

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[59.02] Cosmic-Ray Source Abundances: Indicators of the Composition of the Contemporary Interstellar Medium?

M.E. Wiedenbeck (Jet Propulsion Laboratory), A.C. Cummings, A.J. Davis, J.S. George, R.A. Leske, R.A. Mewaldt, E.C. Stone, N.E. Yanasak (California Institute of Technology), W.R. Binns, M.H. Israel (Washington University), E.R. Christian, T.T. von Rosenvinge (NASA/Goddard Space Flight Center)

The abundances of essentially all stable nuclides from He (Z=2) to Zn (Z=30) in the galactic cosmic rays have been measured with the Cosmic Ray Isotope Spectrometer (CRIS) on the Advanced Composition Explorer (ACE) spacecraft. Using these data, source abundances have been derived for the isotopes of a number of refractory elements including Ca, Fe, Co, and Ni. The resulting cosmic-ray source composition bears a strong resemblance to that found for solar-system material. From comparisons of the observed composition with the results of supernova nucleosynthesis models one can conclude that the cosmic-ray source must contain contributions from a wide range of stellar sources. Furthermore, studies of radioactive clock isotopes in the cosmic rays show that at least 105 years elapses between nucleosynthesis and cosmic-ray acceleration. A reasonable hypothesis for explaining these observations is that cosmic-rays are being derived from a sizable, well-mixed volume of the contemporary interstellar medium (ISM). Thus they may provide a sensitive probe of the composition of the present-day ISM, including information not available from other sources about the abundances of refractory elements and the abundances of individual isotopes. Such data can enable tests of the extent to which galactic chemical evolution has changed the composition of the ISM over the 4.6 Gyr since the formation of the solar system.

This work was supported by NASA at Caltech (under grant NAG5-6912), JPL, and GSFC.

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