Cool Stars 14 - Submitted Abstract # 8
This version created on 05 October 2006

Atomic diffusion : helium, lithium and heavy elements


Andreas Korn, Uppsala Astronomical Observatory (UAO), Sweden
Frank Grundahl, University of Aarhus, Denmark
Olivier Richard, GRAAL, University of Montpellier II, France
Paul Barklem, UAO
Lyudmila Mashonkina, Institute of Astronomy, Moscow, Russia
Remo Collet, UAO
Nikolai Piskunov, UAO
Bengt Gustafsson, UAO

The measurement of the cosmic microwave background by experiments like
WMAP has strongly constrained the cosmological parameters of the
Universe.  When the measured density of baryons is combined with
standard Big-Bang nucleosynthesis (BBN) calculations, the amounts of
hydrogen, helium and lithium produced shortly after the Big Bang can
be predicted with unprecedented precision.  The predicted primordial
lithium abundance is a factor of 2 ~ 3 higher than the value measured
in the atmospheres of old stars.  With estimated errors of 10  to 25%,
this cosmological lithium discrepancy seriously challenges our
understanding of stellar physics, BBN or both.  Certain modifications
to BBN have been proposed, but found experimentally not to be viable.
Diffusion theory, however, predicts atmospheric abundances of stars to
vary with time.  We present spectroscopic observations of stars in the
metal-poor globular cluster NGC 6397 which reveal trends of
atmospheric abundance with evolutionary stage for various elements.
These element-specific trends are reproduced by stellar-evolution
models with diffusion and turbulent mixing.  The most important
implication concerns the primordial lithium abundance:  diffusive
processes are predominantly responsible for the low apparent stellar
lithium abundance in the atmospheres of old stars.  

----------------------------------