BERYLLIUM AND BORON EVOLUTION IN THE GALAXY

We present a model for the evolution of the light-nuclide abundances i n the Galaxy, aimed especially at interpreting the observed beryllium and boron abundances as a function of that of iron. We present two mod els, one for the Galactic halo and the other for the Galactic disk. Th e main characteristics of the halo model are (1) the relatively rapid change in the physical conditions, on a timescale of less than 2 Gyr, because of the exponentially increasing flow of gas from the halo to f orm the Galactic bulge-after this period, less than 30% of the initial gas remains in the halo, and star formation there is brought to a hal t, (2) the low inferior mass limit for the initial mass function (m(l) = 0.01), implying that similar to 60% of the mass that condenses into massive bodies takes the form of substellar objects (masses less than or equal to 0.1 M.). With these assumptions, we can explain the abrup t increase in the observed metallicity distribution of halo stars near [Fe/H] = -1.7, the evolution of [O/Fe], He-4/H, [N/Fe], and C-12/C-13 versus [Fe/H], and that of [C/O] versus [O/H], and give an account of [Fe/H] as a function of time, during the halo phase. The main charact eristics of the disk model are (1) a timescale of order 15 Gyr and (2) an exponentially increasing infall of gas with very low metallicity. With these assumptions, we can explain the prominent peak in the obser ved metallicity distribution of disk stars near [Fe/H] = -0.4, the evo lution of [O/Fe], He-4/H, [N/Fe], and C-12/C-13 versus [Fe/H], and tha t of [C/O] versus [O/H] and also give a good fit to observed [Fe/H] as a function of time. The production of light elements (D, He-3, Li-6, Li-7, Be-9, B-10, and B-11) occurs principally via Galactic cosmic ray (GCR) reactions for all nuclides except deuterium and He-3. Differenc es between the halo and the disk are (1) a flatter GCR energy flux spe ctrum and (2) more GCR flux at early epochs (halo) than more recently (disk), as a result of better GCR confinement, both conditions first s uggested by Prantzos, Casse, & Vstngioni-Flam. A significant contribut ion of the present paper is to explain the almost linear dependence of Be-9 on Fe (or on O) at very low metallicities: the observations show a more nearly linear than quadratic dependence, without requiring the very high local cosmic-ray fluxes implied by the explanation of Feltz ing & Gustafsson of spallation close to supernovae. The explanation is that exponentially increasing outflow of gas from the star-forming zo ne implies the presence of more star-forming gas at very low metallici ties ([Fe/H] similar to -3.0). The low inferior mass limit taken here in the initial mass function implies a reduction in the predicted rela tive number of high-mass stars formed. These conditions, together with the increasing yields of carbon for stars of intermediate and low mas s at low metallicities, while the metallicity indicators O and Fe were being produced mainly in massive stars, cause the observed Be-9 abund ance at very low metallicities, which is enhanced compared with the pr edictions of models in which Be-9, as a secondary element, depends qua dratically on Fe. The exponentially increasing outflow also explains t he sharp rise in the abundances of O and Fe and the observed peak in t he stellar frequency distribution near [Fe/H] similar to -1.7. A featu re of interest in the disk model, due to the exponentially rising infa ll of nonenriched gas, is the observed loop-back of the Be-9-Fe curve at near-solar metallicity; the Be-9 abundance is rising steadily while that of Fe has fallen back.