Beryllium abundances in halo stars from Keck/HIRES observations

We have determined the abundance of Be in stars with an array of metal abun dances in order to enhance our understanding of the chemical evolution of t he Galaxy, cosmic-ray theory, and cosmology. Observations of the Be II reso nance lines at lambda 3130 and lambda 3131 were made at the Keck telescope with the HIRES spectrometer at a resolution of 46,000 and signal-to-noise r atios of 60-110 (per pixel) typically. Our sample includes 22 halo dwarfs a nd five disk stars (including the Sun). We have taken special care in deter mining the stellar parameters for these stars in a consistent manner. The B e abundances were found (1) from the measured equivalent width of the relat ively unblended Be II line at 3131.065 Angstrom with an analysis that inclu ded 11 weak atomic and molecular lines near that wavelength and (2) from sp ectrum synthesis that included newly derived enhanced O (relative to Fe) in the synthesis calculations. The two methods are in excellent agreement. We find straight-line fits between Be and Fe: log N(Be/H) = 0.96(+/-0.04)[Fe/H] - 10.59(+/-0.03); and between Be and O: log N(Be/H) = 1.45(+/-0.04)[O/H] - 10.69(+/-0.04). It seems that Be and Fe increase at the same rate during the course of the evolution of the Galaxy. But as O increases by a factor of 100, Be increase s more rapidly, by a factor of 800. Traditional models in which energetic c osmic rays interact with ambient CNO nuclei in the interstellar medium to p roduce Be are consistent with this finding, as long as certain chemical evo lution effects (such as mass outflow from the halo) are taken into account. However, models predicting a linear relationship between Be and O, such as those producing Be in the vicinity of Type II supernovae, are less consist ent with our result. There is some evidence for an intrinsic spread in Be a t a given [Fe/H] or [O/H]. There is currently no evidence of a primordial p lateau level of Be down to log N(Be/H) = -13.5.