Nuclei in the Cosmos 2000

The Nuclei in the Cosmos 2000 conference was held in Arhus, Denmark from June 27 to July 1. It was jointly organised by the Universities of Arhus and Rostock, NORDITA, the Danish Theoretical Astrophysics Center and the Max-Planck Institute for Chemistry in Mainz. The local organisation was chaired by J. CristensenDalsgaard and K. Langanke. It was the sixth conference of a biennial series, the preceding one being held in Volos, Greece in 1998. The aim of these conferences is to combine the latest efforts in laboratory nuclear physics and in theoretical and observational astrophysics. This is an expanding research field at present and the number of participants have increased to reach now nearly 200. Our understanding of the quite different astrophysical sites where nuclear physics plays a role is increasing both in depth and scope. Progress on many subjects was reported at the meeting. The Asymptotic Giant Brach stars have been identified as the site of the s-process, the slow neutron capture process that produces nuclei close to the line of beta-stability. As discussed by Maria Lugaro, I3C pockets are formed through proton injection into the C-rich and He-rich intershell by convection. The 13C then serves as the major neutron source for the neutron capture nucleosynthesis. Some heavy proton-rich beta-stable nuclei cannot be produced during the s-process and are believed to stem from dissociation reactions on nuclei made in the sor r-process. The production of these p-nuclei in Type 11 supernovae and possible explanations for the underproduction of some of them was discussed by M. Rayet. The lighter proton-rich nuclei are important for the understanding of nova nucleosynthesis. Here S. Starrfield presented a review of the observational material as well as of the theoretical calculations. Supernova explosions was a major topic at the conference. Although the main features are believed to be known there remain important and substantial physics problems. As stressed by D. Arnett we need physics models without free fit parameters. The overview of the present state of models of core-collapse supernovae was given by A. Mezzacappa. He highlighted the improvements in the understanding of neutrino energy transport which is believed to play a major role in energising the explosion, but noted that there remain serious uncertainties in the modelling of the collapse and explosion. Departures from spherical symmetry could turn out to be of decisive importance. Hydrodynamical instabilities were emphasised by S. Woosley in his discussion of the supernovae of Type Ia, resulting from ignition of carbon in carbon-oxygen white dwarfs. The ignition could take place a t several points in the star and the burning front will be turbulent, making the problem computationally extremely difficult. Non-radial motion caused by instabilities was discussed in detail by E. Miiler who also presented two-dimensional simulations. Such effects may have a significant influence on the nucleosynthesis and thereby be seen through observations of the supernova remnants. Threedimensional physically realistic calculations seem required for a more complete understanding. Combining the information from all above mentioned studies allows one to attack the problem of the chemical evolution of our galaxy and thereby the prehistory of our solar system. Recent observations of very “metal-poor” stars (from the earliest generations of star-formation) were reviewed by D. Lambert; they display the same element-pattern above Ba as the sun. The detailed information, derived from meteoritic abundances, on the early history of the solar system was reviewed by P. Hoppe in a plenary talk and by A. W. G. Cameron and G. J. ’Wasserburg in an interesting and entertaining public session which attracted substantial attendance. The combined stellar and meteoritic evidence strongly suggests that the rprocess elements in the solar system result from a t least two different sites. Wasserburg gave a comprehensive overview of the history of the Galaxy and the solar system and concluded that the original iron production took place in a first generation of very massive stars formed from the ‘metal-free’ matter produced in the Big Bang. Nuclear physics input is needed for the detailed understanding of the dynamics of all the different astrophysical sites. The nuclear properties, including cross sections, are determined either in the laboratory or through theoretical investigations. The laboratory work is very challenging due to the extremely small cross sections at the energies typical for at least main-sequence stars. As a result, stellar modelling is typically based on extrapolated values, dominated by the penetration through the Coulomb barrier. This carries the risk that unknown nuclear properties, such as resonances at very low energies, can