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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)