Facility for Rare Isotope Beams

The Universe emerged from a hot and dense phase (temperatures in excess of 1011 K) constituted only by hydrogen, helium and traces of deuterium and lithium 7. The other complex nuclei have been built up at subsequent times, mainly by stars. The stars of the early Galaxy (i.e. older than 12 Gyr) are the fossil record of the nucleosynthesis at that time and their chemical composition conveys important information on the processes involved. In this talk I shall report on some recent results on this topic obtained by myself and my collaborators. I will start with Li, the lightest "metal". Its primordial abundance allows to derive the baryonic density and is thus of cosmological relevance. I will report on VLT-UVES observation of lithium in extremely metal-poor stars. I will go on to Be, this element is not produced in stars, but in the interstellar medium, through cosmic-ray induced spallation of C, N and O nuclei. The conceptual simplicity of the Be production allows to use it as a chronometer and I will report on progress on this topic and on the observation of Be in the Globular Cluster NGC 6397. I will then report on recent results on the abundances of the nucleosynthetically important nuclei C,N and O. Finally I will report on new results on sulphur abundances. Sulphur is produced through the alpha process, like O, Ne, Mg and Si. It is rather difficult to observe it in stars, however it is relatively easily measured in external galaxies, e.g. through emission lines in Blue Compact Galaxies or through absorption lines in DLAs. When observed in the gaseous phase of the external galaxies S has the advantage of being volatile, i.e. forms no dust, therefore no corrections are required to the observed abundances. For this reason it is important to provide a solid reference for S abundances in our Galaxy, to be able to compare the chemical evolution histories of the different environments. I will report on recent results on stellar S abundances in the Galaxy, and also in the Sgr dSph, which show some unexpected features.

Two major themes and challenges of modern science and in particular, of many-body systems, are those of complexity and simplicity, namely understanding how complex systems can be constructed in terms of simple ingredients and conversely, how these systems can display the astonishingly regular and simple properties that they do. Often, as in atomic nuclei, addressing these themes corresponds to microscopic and macroscopic, or symmetry-based approaches. Nuclei indeed offer an ideal playing ground in which to pursue these challenges. One of the key issues in nuclear structure today is the evolution of structure as a function of the numbers of constituent protons and neutrons in the nucleus. Recent work has shown that rapid changes in nuclear shapes with neutron number can be described in terms of geometric symmetries for quantum phase transitions. This concept of critical point symmetries is intimately linked to the role of the microscopic proton-neutron interaction in the development of collectivity, and a direct empirical correlation has now been found between varying strengths of p-n interactions in different mass regions and differences in rates of structural change. Lastly, an unexpected outgrowth of mapping the evolution of structure has been the discovery of ordered and chaotic regions of nuclei and of an isolated “arc of regularity” lying along a particular locus in N and Z.