1,007 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
Li isotopes in metal-poor halo dwarfs, a more and more complicated story
The nuclei of the lithium isotopes are fragile, easily destroyed, so that, at
variance with most of the other elements, they cannot be formed in stars
through steady hydrostatic nucleosynthesis. The 7Li isotope is synthesized
during primordial nucleosynthesis in the first minutes after the Big Bang and
later by cosmic rays, by novae and in pulsations of AGB stars (possibly also by
the "nu" process). 6Li is mainly formed by cosmic rays. The oldest (most
metal-deficient) warm galactic stars should retain the signature of these
processes if, (as it had been often expected) lithium is not depleted in these
stars. The existence of a "plateau" of the abundance of 7Li (and of its slope)
in the warm metal-poor stars is discussed. At very low metallicity ([Fe/H]<-2.7
dex the star to star scatter increases significantly towards low Li abundances.
The highest value of the lithium abundance in the early stellar matter of the
Galaxy (A(7Li) = 2.2 dex) is much lower than the the value A(7Li) = 2.72
predicted by the standard Big Bang nucleosynthesis, according to the
specifications found by the satellite WMAP. After gathering a homogeneous
stellar sample, and analysing its behaviour, possible explanations of the
disagreement between Big Bang and stellar abundances are discussed (including
early astration and diffusion). On the other hand, possibilities of lower
productions of 7Li in the standard and/or non-standard Big Bang nucleosyntheses
are briefly evoked. A surprisingly high value (A(6Li)=0.8 dex) of the abundance
of the 6Li isotope has been found in a few warm metal-poor stars. Such a high
abundance of 6Li independent of the mean metallicity in the early Galaxy cannot
be easily explained. But are we really observing 6Li
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
Lithium-6 : Evolution from Big Bang to Present
The primordial abundances of Deuterium, he4, and li7 are crucial to
determination of the baryon density of the Universe in the framework of
standard Big Bang nucleosynthesis (BBN). li6 which is only produced in tiny
quantities and it is generally not considered to be a cosmological probe.
However, recent major observational advances have produced an estimate of the
li6/li7 ratio in a few very old stars in the galactic halo which impacts the
question whether or not the lithium isotopes are depleted in the outer layers
of halo stars, through proton induced reactions at the base of (or below) the
convective zone. li6 is a pure product of spallation through the major
production reactions, fast oxygen and alphas interacting on interstellar H, He
(especially in the early Galaxy). The rapid nuclei are both synthesized and
accelerated by SN II. In this context, the \li6 evolution should go in step
with that of beryllium and boron, recently observed by the Keck and HST
telescopes. Li6 adds a new constraint on the early spallation in the Galaxy. In
particular, if confirmed, the Li6/Be9 ratio observed in two halo stars (HD
84937, BD +263578) gives strong boundary conditions on the composition and the
spectrum of the rapid particles involved. We show that Li6 is essentially
intact in halo stars, and a fortiori \li7. We can define a range of the Li6
abundance in the very early Galaxy consistent with Big Bang nucleosynthesis
(5.6 10(-14) to 3. 10(-13) . Following the evolution at increasing metallicity,
we explain the abundance in the solar system within a factor of about 2.Comment: 16 pages, 4 figure
High resolution study of the abundance pattern of the heavy elements in very metal-poor field stars.
9 pagesThe abundances of heavy elements in EMP stars are not well explained by the simple view of an initial basic "rapid" process. In a careful and homogeneous analysis of the "First stars" sample (eighty per cent of the stars have a metallicity [Fe/H]=-3.1±0.4), it has been shown that at this metallicity [Eu/Ba] is constant, and therefore the Eu-rich stars (generally called "r-rich") are also Ba-rich. The very large variation of [Ba/Fe] (existence of "r-poor" and "r-rich" stars) induces that the early matter was not perfectly mixed. On the other hand, the distribution of the values of [Sr/Ba] vs. [Ba/Fe] appears with well defined upper and lower envelopes. No star was found with [Sr/Ba]<-0.5 and the scatter of [Sr/Ba] increases regularly when [Ba/Fe] decreases. To explain this behavior, we suggest that an early "additional" process forming mainly first peak elements would affect the initial composition of the matter. For a same quantity of accreted matter, this additional Sr production would barely affect the r-rich matter (which already contains an important quantity of Sr) but would change significantly the composition of the r-poor matter. The abundances found in the CEMP-rs stars reflect the transfer of heavy elements from a defunct AGB companion. But the abundances of the heavy elements in CEMP-no stars present the same characteristics as the the abundances in the EMP stars
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
Carbon-enhanced metal-poor stars: the most pristine objects?
Carbon-enhanced metal poor stars (CEMP) form a significant proportion of the
metal-poor stars, their origin is not well understood. Three very metal-poor
C-rich turnoff stars were selected from the SDSS survey, observed with the ESO
VLT (UVES) to precisely determine the element abundances. In turnoff stars
(unlike giants) the carbon abundance has not been affected by mixing with deep
layers and is therefore easier to interpret. The analysis was performed with 1D
LTE static model atmospheres. When available, non-LTE corrections were applied
to the classical LTE abundances. The 3D effects on the CH and CN molecular
bands were computed using hydrodynamical simulations of the stellar atmosphere
(CO5BOLD) and are found to be very important. To facilitate a comparison with
previous results, only 1D abundances are used in the discussion. The abundances
(or upper limits) of the elements enable us to place these stars in different
CEMP classes. The carbon abundances confirm the existence of a plateau at A(C)=
8.25 for [Fe/H] \geq -3.4. The most metal-poor stars ([Fe/H] < -3.4) have
significantly lower carbon abundances, suggesting a lower plateau at A(C)
\approx 6.5. Detailed analyses of a larger sample of very low metallicity
carbon-rich stars are required to confirm (or refute) this possible second
plateau and specify the behavior of the CEMP stars at very low metallicity
Non-LTE abundances of Mg and K in extremely metal-poor stars and the evolution of [O/Mg], [Na/Mg], [Al/Mg] and [K/Mg] in the Milky Way
LTE abundances of light elements in extremely metal-poor (EMP) stars have
been previously derived from high quality spectra. New derivations, free from
the NLTE effects, will better constrain the models of the Galactic chemical
evolution and the yields of the very first supernovae. The NLTE profiles of the
magnesium and potassium lines have been computed in a sample of 53 extremely
metal-poor stars with a modified version of the program MULTI and adjusted to
the observed lines in order to derive the abundances of these elements. The
NLTE corrections for magnesium and potassium are in good agreement with the
works found in the literature. The abundances are slightly changed, reaching a
better precision: the scatter around the mean of the abundance ratios has
decreased. Magnesium may be used with confidence as reference element. Together
with previously determined NLTE abundances of sodium and aluminum, the new
ratios are displayed, for comparison, along the theoretical trends proposed by
some models of the chemical evolution of the Galaxy, using different models of
supernovae
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
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