THE EVOLUTION OF GALACTIC BORON AND THE PRODUCTION SITE OF THE LIGHT-ELEMENTS

The Goddard High Resolution Spectrograph (GHRS) of the Hubble Space Te lescope (HST) has been used to obtain spectra of the 2500 Angstrom reg ion in eight stars with metallicities ranging from [Fe/H] = -0.4 to -3 .0, including the most metal-poor star ever observed for boron. Spectr um synthesis utilizing latest Kurucz model atmospheres has been used t o determine the B abundance for each star, with particular attention p aid to the errors of each point, to permit judgment of the quality of the fit of models of Galactic chemical evolution. Previous observation s were combined with new ones, bringing the number of stars analyzed t o 11. A straight line of slope approximate to 1 gives an excellent fit to a plot(4) of log epsilon(B-LTE) versus [Fe/H], and if NLTE B abund ances are used, the slope is approximate to 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 elem ents. Our data do not show a change in slope between halo and disk met allicities, but the number of stars near the disk-halo transition is s mall, and a modest change is not precluded. The NLTE B/Be ratio is typ ically approximate to 15 throughout the lifetime of the Galaxy, a rati o 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 alpha-particles, probab ly in the vicinity of massive supernovae in star-forming regions. Unti l recently, most models have emphasized light-element production in th e general ISM from the spallation of high-energy protons and alpha-par ticles onto CNO nuclei. Especially during the Galaxy's early history, when the metallicity of the ISM was low, the spallation of protons and alpha-particles onto CNO nuclei cannot account for as much B as we ob serve, unless the CR flux was sufficiently high for compensation. The observed relation also constrains any direct production of B by the nu -process in supernovae to be at most a small part of total B productio n. It is possible that the gamma rays recently detected from the Orion Nebula region are the signature of spallation by energetic C and O nu clei. Nevertheless, B, Be, and Fe data alone give the strongest eviden ce of the importance of spallation by C and O for producing light elem ents.