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Session 58 - Stellar Modelling/Abundances.
Display session, Wednesday, June 12
Great Hall,
Since the 1970s it has been believed that the light elements Li, Be, and B are made by the spallation of cosmic rays (CRs; primarily energetic protons and alpha particles) on nuclei of C, N, and O in the interstellar medium. Abundances of the light elements have in fact been used to infer the existence of a large flux of low energy CRs which are kept from our direct detection by solar modulation but which should efficiently produce Li, Be, and B. Our data contradicts this.
The Hubble Space Telescope has been used to obtain spectra of the boron 2500 Å\ region in eight galactic halo stars ranging from [Fe/H] = -0.3 to -2.96. The sample includes the most metal-poor (and presumably oldest) star ever observed for B. Spectrum synthesis using latest Kurucz model atmospheres has been used to determine B abundances for each star, and particular attention paid to the errors of each point, to permit judgement of the goodness-of-fit of models of galactic chemical evolution.
None of the standard models of galactic chemical evolution fit the data; their predicted increase of light elements with time is too rapid. A straight line of slope 0.98 gives an excellent fit to all available data. There is little indication of a change in slope between halo and disk metallicities, and the B/Be ratio is typically 10. These data strongly suggest that the production of light elements is by CR spallation, but in a "primary" process, not a "secondary" one in the interstellar medium. We favor a model of light element production originally suggested by Duncan, Lambert, and Lemke (1992) and developed by Ramaty et al (1996), which reverses the previously-accepted one. In the new model spallation occurs when high-energy C,N,O nuclei hit protons and He nuclei. This occurs in the vicinity of massive supernovae in star-forming regions, where the flux of energetic O, and to a lesser extent C, and N nuclei feed this process and where the nuclei can be accelerated to the required energies. The new model may also explain the long-standing problem that the 11B/10B ratio observed in CRs was not that predicted by the models of the 1970s.
We agree with Casse et al (1995) that gamma-ray emission recently detected by the CGRO Satellite as coming from the Orion Nebula star-forming region and identified with excited states of O and C may be direct evidence of this process in action.
