Bibcode
DOI
Casuso, E.; Beckman, J. E.
Bibliographical reference
Astrophysical Journal v.475, p.155
Advertised on:
1
1997
Citations
26
Refereed citations
21
Description
We present a model for the evolution of the light-nuclide abundances in
the Galaxy, aimed especially at interpreting the observed beryllium and
boron abundances as a function of that of iron. We present two models,
one for the Galactic halo and the other for the Galactic disk. The 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 form 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 halt; (2)
the low inferior mass limit for the initial mass function (ml = 0.01),
implying that ~60% of the mass that condenses into massive bodies takes
the form of substellar objects (masses <=0.1 M&sun;). With these
assumptions, we can explain the abrupt increase in the observed
metallicity distribution of halo stars near [Fe/H] = -1.7, the evolution
of [O/Fe], 4He/H, [N/Fe], and 12C/13C 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 characteristics 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 observed metallicity distribution of
disk stars near [Fe/H] = -0.4, the evolution of [O/Fe], 4He/H, [N/Fe],
and 12C/13C versus [Fe/H], and that 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, 3He, 6Li, 7Li, 9Be, 10B, and 11B) occurs principally
via Galactic cosmic ray (GCR) reactions for all nuclides except
deuterium and 3He. Differences between the halo and the disk are (1) a
flatter GCR energy flux spectrum and (2) more GCR flux at early epochs
(halo) than more recently (disk), as a result of better GCR confinement,
both conditions first suggested by Prantzos, Casse, & Vangioni-Flam.
A significant contribution of the present paper is to explain the almost
linear dependence of 9Be 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 Feltzing & Gustafsson of spallation close to
supernovae. The explanation is that exponentially increasing outflow of
gas from the star-forming zone implies the presence of more star-forming
gas at very low metallicities ([Fe/H] ~ -3.0). The low inferior mass
limit taken here in the initial mass function implies a reduction in the
predicted relative number of high-mass stars formed. These conditions,
together with the increasing yields of carbon for stars of intermediate
and low mass at low metallicities, while the metallicity indicators O
and Fe were being produced mainly in massive stars, cause the observed
9Be abundance at very low metallicities, which is enhanced compared with
the predictions of models in which 9Be, as a secondary element, depends
quadratically on Fe. The exponentially increasing outflow also explains
the sharp rise in the abundances of O and Fe and the observed peak in
the stellar frequency distribution near [Fe/H] ~ -1.7. A feature of
interest in the disk model, due to the exponentially rising infall of
non-enriched gas, is the observed loop-back of the 9Be-Fe curve at
near-solar metallicity; the 9Be abundance is rising steadily while that
of Fe has fallen back.