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JOINT ICTP-OATS-SISSA-UNI/TS COLLOQUIUM
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Speaker: Piercarlo Bonifacio (Observatoire de Paris-Meudon, F)

Title: Lithium and Cosmology: what are the problems?
Date: Thursday, December 16th, 2010
Time: 15:00
Venue: Villa Bazzoni

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Abstract: The nucleus of 7Li is one of the few nuclei which can be synthesised during the Big Bang. In the standard scenario the amount of Li produced is a sensitive function of the baryon to photon ratio, and therefore of the baryonic density of the Universe.It was quickly realised that if such "primordial" abundance could be measured one of the fundamental cosmolgical parameters (Omega_b) could be determined. At the beginning of the 1980's Monique and Francois Spite discovered that the warm metal-poor stars in our Galaxy all share the same lithium abundance, whatever their temperature or metallicity. This plateau, later called the "Spite plateau", was readily interpreted as evidence that this constant Li abundance was in fact the primordial lithium, and the baryonic density deduced from the Spite plateau and Standard Big Bang Nucleosynthesis (SBBN) turned out to be consistent with what deduced from the primordial abundances of other "primordial" nuclei (D,3He,4He), for over twenty years this was considered one of the pillars supporting Big Bang Cosmology. The accurate measurement of the baryonic density from the fluctuations of the Cosmic Microwave Background, achieved from the WMAP satellite opened up the "Cosmological Lithium Problem", i.e. SBBN with the measured baryonic density implied a lithium abundance a factor of 3 to 4 higher than the Spite Plateau. Almost at the same time some claims emerged of the presence of 6Li in stars of the Spite plateau, such nucleus is not predicted to be produced in SBBN, this is sometimes referred as the "Second Cosmological Lithium Problem". The solution of the first problem can be sought either by seeking non-standard conditions at the Big Bang, or by some process which will uniformly deplete lithium in stars from the primordial value to the Spite plateau. I will show through new observations that the Spite plateau exists at least in one other external galaxy and is therefore likely a universal feature. I will furthermore show that although there are indications that the Li abundance in the atmospheres of metal-poor stars vary during their lifetimes, the observations are at odds with popular models which have been championed as "THE" solution of the cosmological lithium problem. For what concerns the 6Li problem I will show that the measurements are flawed by neglect of the hydrodynamical nature stellar atmospheres and that observations are in fact consistent with no 6Li present in metal-poor stars. I will illustrate modern 3D hydrodynamical models and show how these can provide an accurate measurement of the solar Li abundance. Therefore of the two cosmological lithium problems, one is non-existent (6Li), the other awaits for solution, but solutions which invoke Li depletion, although not ruled out, have to face serious difficulties.
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contact: Andrea Biviano (OATS)