1,100 research outputs found
The cosmic lithium problem: an observer's perspective
Using the cosmological constants derived from WMAP, the standard big bang
nucleosynthesis (SBBN) predicts the light elements primordial abundances for
4He, 3He, D, 6Li and 7Li. These predictions are in satisfactory agreement with
the observations, except for lithium which displays in old warm dwarfs an
abundance depleted by a factor of about 3. Depletions of this fragile element
may be produced by several physical processes, in different stellar
evolutionary phases, they will be briefly reviewed here, none of them seeming
yet to reproduce the observed depletion pattern in a fully convincing way.Comment: Invited review at the conference Lithium in the cosmos, Paris 27-29
Feb 2012, one reference adde
The low Sr/Ba ratio on some extremely metal-poor stars
It has been noted that, in classical extremely metal-poor (EMP) stars, the
abundance ratio of Sr and Ba, is always higher than [Sr/Ba] = -0.5, the value
of the solar r-only process; however, a handful of EMP stars have recently been
found with a very low Sr/Ba ratio. We try to understand the origin of this
anomaly by comparing the abundance pattern of the elements in these stars and
in the classical EMP stars. Four stars with very low Sr/Ba ratios were observed
and analyzed within LTE approximation through 1D (hydrostatic) model
atmosphere, providing homogeneous abundances of nine neutron-capture elements.
In CS 22950-173, the only turnoff star of the sample, the Sr/Ba ratio is, in
fact, found to be higher than the r-only solar ratio, so the star is discarded.
The remaining stars (CS 29493-090, CS 30322-023, HE 305-4520) are cool evolved
giants. They do not present a clear carbon enrichment. The abundance patterns
of the neutron-capture elements in the three stars are strikingly similar to a
theoretical s-process pattern. This pattern could at first be attributed to
pollution by a nearby AGB, but none of the stars presents a clear variation in
the radial velocity indicating the presence of a companion. The stellar
parameters seem to exclude any internal pollution in a TP-AGB phase for at
least two of these stars. The possibility that the stars are early-AGB stars
polluted during the core He flash does not seem compatible with the theory.Comment: Accepted for publication in Astronomy and Astrophysic
Effects of atomic diffraction on the Collective Atomic Recoil Laser
We formulate a wave atom optics theory of the Collective Atomic Recoil Laser,
where the atomic center-of-mass motion is treated quantum mechanically. By
comparing the predictions of this theory with those of the ray atom optics
theory, which treats the center-of-mass motion classically, we show that for
the case of a far off-resonant pump laser the ray optics model fails to predict
the linear response of the CARL when the temperature is of the order of the
recoil temperature or less. This is due to the fact that in theis temperature
regime one can no longer ignore the effects of matter-wave diffraction on the
atomic center-of-mass motion.Comment: plain tex, 10 pages, 10 figure
Gravitino Dark Matter and the Cosmic Lithium Abundances
Supersymmetric extensions of the standard model of particle physics assuming
the gravitino to be the lightest supersymmetric particle (LSP), and with the
next-to-LSP decaying to the gravitino during Big Bang nucleosynthesis, are
analyzed. Particular emphasis is laid on their potential to solve the "Li7
problem", an apparent factor 2-4 overproduction of Li7 in standard Big Bang
nucleosynthesis (BBN), their production of cosmologically important amounts of
Li6, as well as the resulting gravitino dark matter densities in these models.
The study includes several improvements compared to prior studies. Heavy
gravitinos in the constrained minimal supersymmetric standard model (CMMSM) are
reanalyzed, whereas light gravitinos in gauge-mediated supersymmetry breaking
scenarios (GMSB) are studied for the first time. It is confirmed that decays of
NLSP staus to heavy gravitinos, while producing all the dark matter, may at the
same time resolve the Li7 problem. For NLSP decay times ~ 1000 sec, such
scenarios also lead to cosmologically important Li6 (and possibly Be9)
abundances. However, as such scenarios require heavy > 1 TeV staus they are
likely not testable at the LHC. It is found that decays of NLSP staus to light
gravitinos may lead to significant Li6 (and Be9) abundances, whereas NLSP
neutralinos decaying into light gravitinos may solve the Li7 problem. Though
both scenarios are testable at the LHC they may not lead to the production of
the bulk of the dark matter. A section of the paper outlines particle
properties required to significantly reduce the Li7 abundance, and/or enhance
the Li6 (and possibly Be9) abundances, by the decay of an arbitrary relic
particle.Comment: 13 pages (revtex), 9 figures, minor changes, submitted to PR
Beryllium in Ultra-Lithium-Deficient Halo Stars - The Blue Straggler Connection
There are nine metal-deficient stars that have Li abundances well below the
Li plateau that is defined by over 100 unevolved stars with temperatures above
5800 K and values of [Fe/H] 1.0. Abundances of Be have been determined
for most of these ultra-Li-deficient stars in order to investigate the cause of
the Li deficiencies. High-resolution and high signal-to-noise spectra have been
obtained in the Be II spectral region near 3130 \AA for six ultra-Li-deficient
stars with the Keck I telescope and its new uv-sensitive CCD on the upgraded
HIRES. The spectrum synthesis technique has been used to determine Be
abundances. All six stars are found to have Be deficiencies also. Two have
measurable - but reduced - Be and four have only upper limits on Be. These
results are consistent with the idea that these Li- and Be-deficient stars are
analogous to blue stragglers. The stars have undergone mass transfer events (or
mergers) which destroy or dilute both Li and Be. The findings cannot be matched
by the models that predict that the deficiencies are due to extra-mixing in a
subset of halo stars that were initially rapid rotators, with the possible
exception of one star, G 139-8. Because the ultra-Li-deficient stars are also
Be-deficient, they appear to be genuine outliers in population of halo stars
used to determine the value of primordial Li; they no longer have the Li in
their atmospheres that was produced in the Big Bang.Comment: 17 pages of text, 12 figures, 3 tables Submitted to Ap
Reappraising the Spite Lithium Plateau: Extremely Thin and Marginally Consistent with WMAP
The lithium abundance in 62 halo dwarfs is determined from accurate
equivalent widths reported in the literature and an improved infrared flux
method (IRFM) temperature scale. The Li abundance of 41 plateau stars (those
with Teff > 6000 K) is found to be independent of temperature and metallicity,
with a star-to-star scatter of only 0.06 dex over a broad range of temperatures
(6000 K < Teff < 6800 K) and metallicities (-3.4 < [Fe/H] < -1), thus imposing
stringent constraints on depletion by mixing and production by Galactic
chemical evolution. We find a mean Li plateau abundance of A(Li) = 2.37 dex
(7Li/H = 2.34 X 10^{-10}), which, considering errors of the order of 0.1 dex in
the absolute abundance scale, is just in borderline agreement with the
constraints imposed by the theory of primordial nucleosynthesis and WMAP data
(2.51 < A(Li)[WMAP] < 2.66 dex).Comment: ApJ Letters, in pres
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