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Post Info TOPIC: Cosmological Lithium Discrepancy
Blobrana


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Cosmic Lithium problem
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Title: A resolution of the cosmic Lithium problem
Authors: Rachid Ouyed (Department of Physics and Astronomy, University of Calgary, Calgary, Alberta, Canada)

In 1982, Monique and Francois Spite discovered that the 7Li abundance in the atmosphere of old metal-poor dwarf stars in the galactic halo was independent of metallicity and temperature. Since then, 7Li abundance in the Universe has become a subject of intrigue, because there is less of it in Population II dwarf stars (by a factor of 3) than standard big bang nucleosynthesis predicts; a discrepancy which is still far from being solved. The most challenging features of the plateau is the lack of variability in the -2.8 < [Fe/H] < -2.0 metallicity range and the "meltdown" (a drop and an increase in variability) for [Fe/H] < -2.8. Here we show how quark-novae (QNe) occurring in the wake of Pop III stars, (a few days to a few weeks following the core-collapse SN explosions of 20-40 solar mass progenitors) can elegantly produce an A(Li) ~ 2.2 Lithium plateau in Pop II (low-mass) stars formed in the pristine cloud swept up by the mixed SN+QN ejecta. We also find an increase in the scatter as well as an eventual drop in A(Li) below the Spite plateau values for very low metallicity ([Fe/H] <-3) in excellent agreement with observations. We propose a solution to the discrepancy between the Big Bang Nucleosynthesis 7Li abundance and the Spite plateau and list some implications and predictions of our model.

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Blobrana


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RE: Cosmological Lithium Discrepancy
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Lithium mystery deepened by galaxy gas probe

Earlier research probed the atmospheres of stars near the edge of the Milky Way for lithium-7, the most common isotope. These stars are very old, and the amount of lithium they contain is related in a predictable way to the composition of the early universe. The observations focused on the stars' outer layers because they don't mix with material from the core, where lithium can be produced.
The amount of lithium-7 these studies found is about a third of what big bang models say it should be. One possible reason is that lithium is sinking into the stars' interior and being destroyed by high temperatures.
Christopher Howk of the University of Notre Dame in Indiana and colleagues have expanded the search to gas clouds in a nearby satellite galaxy, the Small Magellanic Cloud. Using the Very Large Telescope (VLT) in northern Chile, they discovered nearly as much lithium in these clouds as big bang models predict.
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Blobrana


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Title: 6Li detection in metal-poor stars: can 3D model atmospheres solve the second lithium problem?
Authors: M. Steffen, R. Cayrel, E. Caffau, P. Bonifacio, H.-G. Ludwig, M. Spite

The presence of 6Li in the atmospheres of metal-poor halo stars is usually inferred from the detection of a subtle extra depression in the red wing of the 7Li doublet line at 670.8 nm. However, the intrinsic line asymmetry caused by convective flows in the photospheres of cool stars is almost indistinguishable from the asymmetry produced by a weak 6Li blend on a (presumed) symmetric 7Li profile. Previous determinations of the 6Li/ 7Li isotopic ratio based on 1D model atmospheres, ignoring the convection-induced line asymmetry, must therefore be considered as upper limits. By comparing synthetic 1D LTE and 3D non-LTE line profiles of the Li 670.8 nm feature, we quantify the differential effect of the convective line asymmetry on the derived 6Li abundance as a function of effective temperature, gravity, and metallicity. As expected, we find that the asymmetry effect systematically reduces the resulting 6Li/7Li ratios. Depending on the stellar parameters, the 3D-1D offset in 6Li/7Li ranges between -0.005 and -0.020. When this purely theoretical correction is taken into account for the Asplund 2006 sample of stars, the number of significant 6Li detections decreases from 9 to 5 (2 sigma criterion), or from 5 to 2 (3 sigma criterion).
We also present preliminary results of a re-analysis of high-resolution, high S/N spectra of individual metal-poor turn-off stars, to see whether the "second Lithium problem" actually disappears when accounting properly for convection and non-LTE line formation in 3D stellar atmospheres. Out of 8 stars, HD84937 seems to be the only significant (2 sigma) detection of 6Li. In view of our results, the existence of a 6Li plateau appears questionable.

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Blobrana


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Primordial Lithium Problem
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Title: The Primordial Lithium Problem
Authors: Brian D. Fields

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.

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Blobrana


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RE: Cosmological Lithium Discrepancy
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Title: A Bitter Pill: The Primordial Lithium Problem Worsens
Authors: Richard H. Cyburt, Brian D. Fields, Keith A. Olive

The lithium problem arises from the significant discrepancy between the primordial 7Li abundance as predicted by BBN theory and the WMAP baryon density, and the pre-Galactic lithium abundance inferred from observations of metal-poor (Population II) stars. This problem has loomed for the past decade, with a persistent discrepancy of a factor of 2--3 in 7Li/H. Recent developments have sharpened all aspects of the Li problem. Namely: (1) BBN theory predictions have sharpened due to new nuclear data, particularly the uncertainty on 3He(alpha,gamma)7Be, has reduced to 7.4%, and with a central value shift of ~ +0.04 keV barn. (2) The WMAP 5-year data now yields a cosmic baryon density with an uncertainty reduced to 2.7%. (3) Observations of metal-poor stars have tested for systematic effects, and have reaped new lithium isotopic data. With these, we now find that the BBN+WMAP predicts 7Li/H = (5.24+0.71-0.67) 10^{-10}. The Li problem remains and indeed is exacerbated; the discrepancy is now a factor 2.4--4.3 or 4.2sigma (from globular cluster stars) to 5.3sigma (from halo field stars). Possible resolutions to the lithium problem are briefly reviewed, and key nuclear, particle, and astronomical measurements highlighted.

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Blobrana


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Pop out the battery in your mobile phone and it will contain something that has likely been around for almost 13.7 billion years - lithium. Along with other light elements, including hydrogen and helium, much of the lithium in the universe is thought to have been produced in a primordial fusion factory that powered up when the universe was barely a second old. In little more than 5 minutes, it produced the raw ingredients for all the ordinary matter in the universe, which billions of years later would clump into galaxies and stars.
This early forging of light nuclei is known as big bang nucleosynthesis (BBN) and, for the most part, our theoretical understanding of it is spot on. In fact, the amounts of hydrogen and helium tally so well with predictions that cosmologists claim this is the best evidence we have for the big bang.
Yet there is a problem...

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Blobrana


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Title: The X^- Solution to the ^6Li and ^7Li Big Bang Nucleosynthesis Problems
Authors: Motohiko Kusakabe, To****aka Kajino, Richard N. Boyd, Takashi Yoshida, Grant J. Mathews
(Version v2)

The ^6Li abundance observed in metal poor halo stars appears to exhibit a plateau as a function of metallicity similar to that for ^7Li, suggesting a big bang origin. However, the inferred primordial abundance of ^6Li is \sim1000 times larger than that predicted by standard big bang nucleosynthesis for the baryon-to-photon ratio inferred from the WMAP data. Also, the inferred ^7Li primordial abundance is 3 times smaller than the big bang prediction. We here describe in detail a possible simultaneous solution to both the problems of underproduction of ^6Li and overproduction of ^7Li in big bang nucleosynthesis. This solution involves a hypothetical massive, negatively-charged leptonic particle that would bind to the light nuclei produced in big bang nucleosynthesis, but would decay long before it could be detected. We consider only the X-nuclear reactions and assume that the effect of decay products is negligible, as would be the case if lifetime were large or the mass difference between the charged particle and its daughter were small. An interesting feature of this paradigm is that, because the particle remains bound to the existing nuclei after the cessation of the usual big bang nuclear reactions, a second longer epoch of nucleosynthesis can occur among X-nuclei. We confirm that reactions in which the hypothetical particle is transferred can occur that greatly enhance the production of ^6Li while depleting ^7Li. We also identify a new reaction that destroys large amounts of ^7Be, and hence reduces the ultimate ^7Li abundance. Thus, big-bang nucleosynthesis in the presence of these hypothetical particles, together with or without an event of stellar processing, can simultaneously solve the two Li abundance problems.

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Blobrana


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Posts: 131433
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Isotopic 6Li/7Li ratios
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Title: The isotopic 6Li/7Li ratio in Cen X-4 and the origin of Li in X-ray binaries
Authors: J. Casares, P. Bonifacio, J. I. Gonzalez Hernandez, P. Molaro, M. Zoccali

Context: Cool stars, companions to compact objects, are known to show Li abundances which are high compared to field stars of the same spectral type, which are heavily Li depleted. This may be due either to Li production or Li preservation in these systems.
Aims: To measure the lithium isotopic ratio in the companion star of the neutron star X-ray binary Cen X-4.
Method: We use UVES spectra obtained in years 2000 and 2004 around the orbital quadratures. The spectra are analysed with spectrum synthesis techniques and the errors estimated with Monte Carlo simulations.
Results: We measure A(Li)=2.87 ±0.20 and 6Li/7Li = 0.12+0.08-0.05 at 68% confidence level. We also present updated system parameters with a refined determination of the orbital period and component masses i.e. 1.14 ±0.45 Msun and 0.23 ±0.10 Msun for the neutron star and companion, respectively.
Conclusions: In our view the low level of 6Li favours Li preservation scenarios, although Li production mechanisms cannot be ruled out. In the case of preservation, no Li is freshly created in the binary, but the tidally-locked companion has preserved its original Li by some mechanism, possibly inhibited destruction due to its fast rotation.

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Blobrana


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Ultra-Lithium-Deficient Halo Stars
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Title: Beryllium in Ultra-Lithium-Deficient Halo Stars - The Blue Straggler Connection
Authors: Ann Merchant Boesgaard

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.

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Blobrana


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6-Li in metal-poor halo stars
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Title: Is 6-Li in metal-poor halo stars produced in situ by solar-like flares ?
Authors: V. Tatischeff, J.-P. Thibaud

The high 6-Li abundances recently measured in metal-poor halo stars are far above the value predicted by Big Bang nucleosynthesis. They cannot be explained by galactic cosmic-ray interactions in the interstellar medium either. Various pre-galactic sources of 6-Li have been proposed in the literature. We study the possibility that the observed 6-Li was produced by repeated solar-like flares on the main sequence of these low-metallicity stars. The time-dependent flaring activity of these objects is estimated from the observed evolution of rotation-induced activity in Pop I dwarf stars. As in solar flares, 6-Li could be mainly created in interactions of flare-accelerated 3-He with stellar atmospheric 4-He, via the reaction 4-He(3-He,p)6-Li. Stellar dilution and destruction of flare-produced 6-Li are evaluated from the evolutionary models of metal-poor stars developed by Richard and co-workers. Stellar depletion should be less important for 6-Li atoms synthesized in flares than for those of protostellar origin. Theoretical frequency distributions of 6-Li/7-Li ratios are calculated using a Monte-Carlo method and compared with the observations. Excellent agreement is found with the measured 6-Li/7-Li distribution, when taking into account the contribution of protostellar 6-Li originating from galactic cosmic-ray nucleosynthesis. We propose as an observational test of the model to seek for a positive correlation between 6-Li/7-Li and stellar rotation velocity. We also show that the amounts of 7-Li, Be and B produced in flares of metal-poor halo stars are negligible as compared with the measured abundances of these species. 6-Li in low-metallicity stars may be a unique evidence of the nuclear processes occurring in stellar flares.

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