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The Galactic Evolution of Beryllium and Boron Revisited
The largest, highest-quality, and most near-homogeneously treated extantavailable samples of Be, B, Fe, and O abundances are analyzed on fourdifferent stellar parameter scales, considering different O abundanceindicators and deriving uncertainties in their relation with therequired aid of jackknife and bootstrap simulations/resampling. Despitelarge slope and zero-point differences, the various Fe-poor([Fe/H]<~-1) BeB-FeO relations are found to be independent ofparameter scale within the present, sometimes substantial,uncertainties. Variations in the BeB-O relations (as large as 1.12dex/dex and 1.24 dex in slope and zero point) from differing Oindicators do significantly differ; surprisingly, the largestdifferences are within the same parameter scale and not across differentones. The well-defined mean Be-Fe relation isBe~Fe1.16+/-0.04 the B-Fe relation is virtually identical,B~Fe1.17+/-0.08. The BeB-mean O relations show smallerdispersion than BeB-OH or BeB-O I relations alone, because of thesignificant reduction in parameter uncertainties, and are in remarkableagreement, indicating Be~mean O1.51+/-0.05 and B~meanO1.61+/-0.12. The latter is in good agreement with the slope(B~O1.39+/-0.08) derived for metal-rich dwarfs by Smith etal. utilizing enhanced Mg I b-f opacity and presumed reliableλ6300 [O I] and λ6158 O I features. The BeB-FeO slopes arealso all in excellent agreement with the reanalysis of Garcia Lopez, whoutilizes a Hipparcos-based gravity scale. The equivalence of the Be- andB-FeO slopes limits prodigious ν-process 11B production atlow metallicity and suggests little Galactic evolution of the B/Beratio. The BeB-mean O slopes differ significantly from pure ``primary''and ``secondary'' values, requiring a combination of productionmechanisms. The differing behavior of [O/Fe] and [Be/Fe] with [Fe/H]seems to rule out production by accelerated CO-rich grain debris inejecta of Type II supernovae having progenitor masses M>~8Msolar. Instead, the data are in fine accord withnear-primary/intermediate BeB-FeO slopes produced by varioustwo-component models, including standard GCR and superbubble production.Such models with a low-energy cosmic-ray source from supernovaerestricted to very large progenitor mass may be consistent with thelarge Be abundance in the ultra-metal-poor dwarf G64-12 found by Primaset al.; however, they predict unobserved maxima in B/Be evolution near[Fe/H]~-2, produce too much total Li at intermediate metallicity, andhave been suggested to be energetically untenable. Superbubble modelsconsidering a range of supernova progenitor mass and a constantcosmic-ray source composition predict the inferred modest or flat slopesin B/Be evolution. These models face possible difficulties inreproducing any nonprimordial Be plateaus at very low [Fe/H], and notunderproducing 6Li for [Fe/H]<~-2 additional data arerequired to provide firmer observational constraints. The BeB/FeO ratiosdo not show consistent evidence for two metal-poor populations expectedfrom bimodal (isolated supernovae and collective supernovae insuperbubbles) production mechanisms, though these signatures may be lostin the scatter or have drastically different contributing fractions.Finally, comparison of the metal-poor BeB-Fe and BeB-mean O slopessuggests that [O/Fe]~-0.25 [Fe/H]-not constant, but not as steep assuggested in some recent analyses and in agreement with the shallow[O/Fe] increase with declining [Fe/H] suggested by King.
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