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The Evolution of Galactic Boron and the Production Site of the Light Elements

The Goddard High Resolution Spectrograph (GHRS) of the Hubble Space Telescope (HST) has been used to obtain spectra of the 2500 Å region in eight stars with metallicities ranging from [Fe/H] = -0.4 to -3.0, including the most metal-poor star ever observed for boron. Spectrum synthesis utilizing latest Kurucz model atmospheres has been used to determine the B abundance for each star, with particular attention paid to the errors of each point, to permit judgment of the quality of the fit of models of Galactic chemical evolution. Previous observations were combined with new ones, bringing the number of stars analyzed to 11. A straight line of slope ≈ 1 gives an excellent fit to a plot of log ϵ(BLTE) versus [Fe/H], and if NLTE B abundances are used, the slope is ≈ 0.7. Plotting B versus [O/H] rather than [Fe/H] increases the slope of either plot by about 0.2. The observed relation suggests that the production of light elements such as B and Be is directly related to the production of heavier elements. Our data do not show a change in slope between halo and disk metallicities, but the number of stars near the disk-halo transition is small, and a modest change is not precluded. The NLTE B/Be ratio is typically ≈ 15 throughout the lifetime of the Galaxy, a ratio naturally produced by cosmic-ray (CR) spallation. Our data support a model in which most light-element production comes from low-energy CR spallation of C and O nuclei onto protons and α-particles, probably in the vicinity of massive supernovae in star-forming regions. Until recently, most models have emphasized light-element production in the general ISM from the spallation of high-energy protons and α-particles onto CNO nuclei. Especially during the Galaxy's early history, when the metallicity of the ISM was low, the spallation of protons and α-particles onto CNO nuclei cannot account for as much B as we observe, unless the CR flux was sufficiently high for compensation. The observed relation also constrains any direct production of B by the ν-process in supernovae to be at most a small part of total B production. It is possible that the gamma rays recently detected from the Orion Nebula region are the signature of spallation by energetic C and O nuclei. Nevertheless, B, Be, and Fe data alone give the strongest evidence of the importance of spallation by C and O for producing light elements.