322 research outputs found
The Primordial Lithium Problem
Big-bang nucleosynthesis (BBN) theory, together with the precise WMAP cosmic
baryon density, makes tight predictions for the abundances of the lightest
elements. Deuterium and 4He measurements agree well with expectations, but 7Li
observations lie a factor 3-4 below the BBN+WMAP prediction. This 4-5\sigma\
mismatch constitutes the cosmic "lithium problem," with disparate solutions
possible. (1) Astrophysical systematics in the observations could exist but are
increasingly constrained. (2) Nuclear physics experiments provide a wealth of
well-measured cross-section data, but 7Be destruction could be enhanced by
unknown or poorly-measured resonances, such as 7Be + 3He -> 10C^* -> p + 9B.
(3) Physics beyond the Standard Model can alter the 7Li abundance, though D and
4He must remain unperturbed; we discuss such scenarios, highlighting decaying
Supersymmetric particles and time-varying fundamental constants. Present and
planned experiments could reveal which (if any) of these is the solution to the
problem.Comment: 29 pages, 7 figures. Per Annual Reviews policy, this is the original
submitted draft. Posted with permission from the Annual Review of Nuclear and
Particle Science, Volume 61. Annual Reviews, http://www.annualreviews.org .
Final published version at
http://www.annualreviews.org/doi/abs/10.1146/annurev-nucl-102010-13044
Testing Spallation Processes With Beryllium and Boron
The nucleosynthesis of Be and B by spallation processes provides unique
insight into the origin of cosmic rays. Namely, different spallation schemes
predict sharply different trends for the growth of LiBeB abundances with
respect to oxygen. ``Primary'' mechanisms predict BeB O, and are well
motivated by the data if O/Fe is constant at low metallicity. In contrast,
``secondary'' mechanisms predict BeB O and are consistent with
the data if O/Fe increases towards low metallicity as some recent data suggest.
Clearly, any primary mechanism, if operative, will dominate early in the
history of the Galaxy. In this paper, we fit the BeB data to a two-component
scheme which includes both primary and secondary trends. In this way, the data
can be used to probe the period in which primary mechanisms are effective. We
analyze the data using consistent stellar atmospheric parameters based on
Balmer line data and the continuum infrared flux. Results depend sensitively on
Pop II O abundances and, unfortunately, on the choice of stellar parameters.
When using recent results which show O/Fe increasing toward lower metallicity,
a two-component Be-O fits indicates that primary and secondary components
contribute equally at [O/H] = -1.8 for Balmer line data; and
[O/H] = -1.4 to -1.8 for IRFM. We apply these constraints to recent
models for LiBeB origin. The Balmer line data does not show any evidence for
primary production. On the other hand, the IRFM data does indicate a preference
for a two-component model, such as a combination of standard GCR and
metal-enriched particles accelerated in superbubbles. These conclusions rely on
a detailed understanding of the abundance data including systematic effects
which may alter the derived O-Fe and BeB-Fe relations.Comment: 40 pages including 11 ps figures. Written in AASTe
Evolution of Beryllium and Boron in the Inhomogeneous Early Galaxy
A model of supernova-driven chemical evolution of the Galactic halo, recently
proposed by Tsujimoto, Shigeyama, & Yoshii (1999, ApJL, 519, 64), is extended
in order to investigate the evolution of light elements such as Be and B (BeB),
which are produced mainly through spallative reactions with Galactic cosmic
rays. In this model each supernova sweeps up the surrounding interstellar gas
into a dense shell and directly enriches it with ejecta which consist of heavy
elements produced in each Type II supernova with different progenitor masses.
We propose a two-component source for GCRs such that both interstellar gas and
fresh SN ejecta engulfed in the shell are accelerated by the shock wave. Our
model results include: (1) a prediction of the intrinsic scatter in BeB and
[Fe/H] abundances within the model, (2) a successful prediction of the observed
linear trend between BeB and [Fe/H], (3) a proposal for using BeB as a cosmic
clock, as an alternative to [Fe/H], and (4) a method for possibly constraining
the BBN model from future observations of metal-poor stars.Comment: 3 color figures in 7 pages, accepted by ApJ Letter
Updated Big Bang Nucleosynthesis confronted to WMAP observations and to the Abundance of Light Elements
We improve Standard Big Bang Nucleosynthesis (SBBN) calculations taking into
account new nuclear physics analyses (Descouvemont et al. 2003). Using a
Monte-Carlo technique, we calculate the abundances of light nuclei versus the
baryon to photon ratio.The results concerning omegab are compared to relevant
astrophysical and cosmological observations. Consistency between WMAP, SBBN
results and D/H data strengthens the deduced baryon density and has interesting
consequences on cosmic chemical evolution. A significant discrepancy between
the calculated Li-7 deduced from WMAP and the Spite plateau is clearly
revealed. To explain this discrepancy three possibilities are invoked :
uncertainties on the Li abundance, surface alteration of Li in the course of
stellar evolution or poor knowledge of the reaction rates related to Be-7
destruction. In particular, the possible role of the up to now neglected
Be-7(d,p)2He-4 and Be-7(d,alpha)Li5 reactions is considered. The impressive
advances in CMB observations provide a strong motivation for more efforts in
experimental nuclear physics and high quality spectroscopy to keep BBN in pace.Comment: accepted in ApJ, 22 pages, 5 figure
On the Possible Sources of D/H Dispersion at High Redshift
Recent observations suggest the existence of a white dwarf population in the
Galactic halo, while others suggest that deuterium has been astrated in systems
at high redshift and low metallicity. We propose that these observations could
be signatures of an early population of intermediate-mass stars. Such a
population requires a Population III initial mass function different from that
of the solar neighborhood, as perhaps also suggested by the observed cosmic
infrared background. Also, to avoid overproduction of C and N, it is required
that the Z=0 yields of these stars have low ( solar) abundances
as suggested by some recent calculations. Under these assumptions, we present a
model which reproduces the observed D vs Si trend, and predicts a high cosmic
Type Ia supernova rate, while producing a white dwarf population that accounts
for only 1.5% of the dark halo. This scenario can be tested by
observations of the cosmic supernova rate, and by confirmation and further
studies of the putative white dwarf halo population.Comment: AASTeX, 17 pages, 3 embedded eps fig
Implications of a new temperature scale for halo dwarfs on LiBeB and chemical evolution
Big bang nucleosynthesis (BBN) and the cosmic baryon density from cosmic
microwave background anisotropies together predict a primordial Li7 abundance a
factor of 2--3 higher than that observed in galactic halo dwarf stars. A recent
analysis of Li7 observations in halo stars, using significantly higher surface
temperature for these stars, found a higher Li plateau abundance. These results
go a long way towards resolving the discrepancy with BBN. Here, we examine the
implications of the higher surface temperatures on the abundances of Be and B
which are thought to have been produced in galactic cosmic-ray nucleosynthesis
by spallation of CNO together with Li (produced in alpha + alpha collisions).
While the Be abundance is not overly sensitive to the surface temperature, the
derived B abundances and more importantly the derived oxygen abundances are
very temperature dependent. If the new temperature scale is correct, the
implied increased abundances of these elements poses a serious challenge to
models of galactic cosmic ray nucleosynthesis and galactic chemical evolution.Comment: 23 pages, 10 eps figure
Low Mass Stars and the He3 Problem
The prediction of standard chemical evolution models of higher abundances of
He3 at the solar and present-day epochs than are observed indicates a possible
problem with the yield of He3 for stars in the range of 1-3 solar masses.
Because He3 is one of the nuclei produced in Big Bang Nucleosynthesis (BBN), it
is noted that galactic and stellar evolution uncertainties necessarily relax
constraints based on He3. We incorporate into chemical evolution models which
include outflow, the new yields for He3 of Boothroyd & Malaney (1995) which
predict that low mass stars are net destroyers of He3. Since these yields do
not account for the high \he3/H ratio observed in some planetary nebulae, we
also consider the possibility that some fraction of stars in the 1 - 3 solar
mass range do not destroy their He3 in theirpost main-sequence phase. We also
consider the possibility that the gas expelled by stars in these mass ranges
does not mix with the ISM instantaneously thus delaying the He3 produced in
these stars, according to standard yields, from reaching the ISM. In general,
we find that the Galactic D and He3 abundances can be fit regardless of whether
the primordial D/H value is high (2 x 10^{-4}) or low (2.5 x 10^{-5}).Comment: 20 pages, latex, 9 ps figure
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