Scientific Rationale

The light elements (H, He, Li, Be, B and their isotopes) deserve special attention because of their key-role in several astrophysical fields: they provide important clues to understand stellar and ISM structure and evolution, galaxy formation and evolution, Big Bang nucleosynthesis (hereafter BBN) and cosmology. They are one of the few bridges connecting all astrophysical communities.

The previous IAU Symposium on this topic was held in 1999, and other, non-IAU supported, meetings also took place about a decade ago. Since then, there have been many significant developments both on the observational and theoretical sides. Striking progress was achieved thanks to the accurate determination of the baryon density of the Universe by recent cosmic microwave background experiments, most particularly from WMAP, which confirmed the BBN Ω_baryon and led to an unprecedented precision on the determination of the yields of Standard Big Bang Nucleosynthesis (hereafter SBBN) to be compared with the abundance of D, 3He, 4He and 7Li observed in low-metallicity environments.

In parallel, the advent of new generation ground and space based telescopes allowed the observation of light elements in objects previously unreachable, with new intriguing results on their present and past abundances. Thanks to multi-fiber instruments, a wealth of data can be gathered in a consistent way, still awaiting to be fully understood and interpreted.

On the theoretical side, we are entering a golden age for the description of stellar interiors and evolution thanks to improved treatments of stellar rotation, magnetic fields, and internal gravity waves in new generation stellar models. Most of these improvements were achieved thanks to constraints coming from combined observations of light element abundances in various types of stars. Also, 3D time-dependent hydrodynamical model atmospheres have been developed and helped providing more reliable surface abundance determinations. However, stellar yields for light elements are far from being certain. Last but not least, several independent and fully consistent chemical evolution models able to fit the general trends of Galaxies physical and chemical features were developed. These must be developed and tested with respect to the evolution of light elements.

However, despite all these efforts we are far from understanding and reproducing in detail all light element patterns, their local, short-term variations, as well as their global evolution in the Universe. Most of the important issues that are listed below are still highly controversial and will be tackled during this Symposium. This task requires the collective input from observers, stellar and galactic physicists, and cosmologists.

Nucleosynthesis in the early Universe.

The study of the early evolution of the Universe and, in particular, primordial nucleosynthesis, has entered the precision era of cosmology. It is timely to review the results of the recent experiments on cosmic microwave background anisotropies, and to discuss the articulations with cosmology and particle physics as well as the possible alternatives to SBBN.

Deuterium.

The primordial abundance of deuterium derived from WMAP+SBBN is in good agreement with the average value of D/H observations in cosmological clouds along the line of sight of quasars. However, the question of the local ISM value of D is still highly controversial, with very different abundances inferred from similar data by different groups. Results depend on the assumptions on dust absorption, with important consequences on the local evolution of D and of the Galaxy itself.

Helium 3.

The observational data of 3He in Galactic HII regions remain scarce and must be corrected for contamination of the observed gas by ejecta from earlier generations of stars. However, the "best" determination of 3He in a Galactic HII region has yielded a 3He abundance almost identical to the CMB-derived value, and to the proto-solar value. This indicates that the stellar contribution to 3He in the Galaxy is minute, and pauses serious problems to classical stellar models that predict a strong production of this isotope by low-mass stars. This raises the serious issue of mixing processes in stellar interiors that hampers the predictions for reliable theoretical stellar yields.

Helium 4.

The CMB-predicted primordial 4He abundance is consistent with values derived recently from the determinations in complex low-metallicity HII regions (both Galactic and extra-galactic) and the extrapolation to zero oxygen abundance. There is an indication that the primordial 4He abundance could be even slightly higher than that inferred from the CMB observations implying slight deviation of the primordial nucleosynthesis from the SBBN. Currently systematic effects in the abundance analysis are still a hot debated topic that has to be clarified to further improve the determination of the 4He primordial value.

Another important issue concerns the finding of multiple sequences in the colour-magnitude diagrams of globular clusters, which is currently interpreted by strong excesses of helium. This question must be addressed from both the stellar and galactic points of view.

Lithium 7.

The CMB-derived primordial abundance of 7Li is clearly higher (by about a factor of 2 to 4) than its reported abundances in Pop II stars. Although the temperature scale could still be an issue, we have to face the fact that the Li abundance in halo stars is not pristine and that these stars have undergone Li depletion during their evolution, just as Population I stars and the Sun. This reinforces Li as a stellar tomographer that can be used, together with Be and B, to probe non-standard mechanisms in stellar interiors. In particular, the impact of rotation, magnetic fields and internal gravity waves in stars of various masses and types must be stated without delay. Another major issue concerns the stellar sources of 7Li, which are still a matter of debate that hamper a clear vision of the Galactic evolution of this isotope.

Lithium 6, Beryllium 9, Boron 10 and Boron 11.

It is widely admitted that these isotopes are produced through spallation reactions induced by energetic particles interacting with the ISM. However, the debate regarding the observed relationship between these isotopes and metallicity is still widely open, hampering the disentanglement between the various theoretical modes of spallative LiBeB production.

A timely IAU Symposium on Light Elements in the Universe.

Complete understanding of the evolution of the Light Elements in the Universe is a challenging task that requires the exchange of ideas and the collaboration of astrophysicists with observational and theoretical expertise in stellar hydrodynamics and evolution, Galactic and extra-galactic astronomy, and cosmology.

In view of the impressive advances made by the various parties over the past decade, we think it is time for a new meeting to let a wide community share necessary knowledge, discuss about remaining hot issues, and hopefully come to the characterization of the physical processes that support the interpretation of the observations.

We believe the time would be appropriate also in view of the launch of Planck, a satellite designed to image Cosmic Background radiation anisotropies, and the HST refurbishing Servicing Mission 4, and expected to bring new fundamental information to the field. 2009 will be the International Year of Astronomy, the 450th anniversary of the University of Geneva, the 175th anniversary of the University of Bern, the 40th anniversary of the man of the Moon, and the 400th anniversary of the Galilee\u2019s astronomical telescope. We will thus organize several public manifestations (public talks, exhibition at the Museum) during the Symposium in connection with all these events.