Early Nucleosynthesis(Proceedings of Japan-France Seminar on Chemical Evolution of Galaxies with Active Star Formation)

After a brief summary of our present knowledge of the primordial abundances of the very light elements (D, ^3He, ^4He and ^7Li), the simplest model of primordial nucleosynthesis is presented. Its implications on the baryonic density of the Universe and the number of neutrino (lepton) families are outlined. Recent models dealing with specific scenarios of chemical evolution of our Galaxy and with the possible presence of non baryonic particles (massive neutrinos, gravitinos, photinos, quark nuggets...) are also discussed

The first generation of stars: first steps towards chemical evolution of galaxies

We argue that extreme metal-poor stars show a high dispersion in metallicity, because their abundances are the outcome of very few supernova events. Abundance anomalies should appear because of the discrete range of progenitor masses. There is a natural metallicity threshold of Z/Z_\odot\sim 10^{-4} below which one would expect to find very few, if any, halo stars. Similar reasoning is applied to lower mass systems, such as metal-poor compact blue galaxies and Lyman alpha absorption line clouds seen towards high redshift quasars, where a somewhat higher threshold is inferred

Lithium-6 : Evolution from Big Bang to Present

The primordial abundances of Deuterium, he4, and li7 are crucial to determination of the baryon density of the Universe in the framework of standard Big Bang nucleosynthesis (BBN). li6 which is only produced in tiny quantities and it is generally not considered to be a cosmological probe. However, recent major observational advances have produced an estimate of the li6/li7 ratio in a few very old stars in the galactic halo which impacts the question whether or not the lithium isotopes are depleted in the outer layers of halo stars, through proton induced reactions at the base of (or below) the convective zone. li6 is a pure product of spallation through the major production reactions, fast oxygen and alphas interacting on interstellar H, He (especially in the early Galaxy). The rapid nuclei are both synthesized and accelerated by SN II. In this context, the \li6 evolution should go in step with that of beryllium and boron, recently observed by the Keck and HST telescopes. Li6 adds a new constraint on the early spallation in the Galaxy. In particular, if confirmed, the Li6/Be9 ratio observed in two halo stars (HD 84937, BD +263578) gives strong boundary conditions on the composition and the spectrum of the rapid particles involved. We show that Li6 is essentially intact in halo stars, and a fortiori \li7. We can define a range of the Li6 abundance in the very early Galaxy consistent with Big Bang nucleosynthesis (5.6 10(-14) to 3. 10(-13) . Following the evolution at increasing metallicity, we explain the abundance in the solar system within a factor of about 2.Comment: 16 pages, 4 figure

Propositions pour l’enseignement et la diffusion de la science

Ces propositions s’articulent autour de trois thèmes. En ce qui concerne les nouveaux contenus, il convient d’insister sur les approches pluridisciplinaires, parallèlement à l’énoncé des paradigmes fondateurs propres à chaque discipline et à l’histoire de leur apparition. Les nouvelles pédagogies doivent, quant à elles, intégrer une part de techniques du théâtre, de connaissances psychopédagogiques et conduire à la valorisation de l’imagination et de l’effort. Enfin, la formation des maîtres gagnerait à développer un langage commun des enseignants, par-delà leurs spécificités disciplinaires. Plus généralement, deux mots-clés doivent guider la réflexion sur les évolutions nécessaires : invention et enthousiasme.Jean Audouze’s proposals for the teaching of sciences cover three areas. As regards new contents, he calls for the introduction of a multi-disciplinary approach alongside the teaching of the fundamental paradigms proper to each discipline and the chronology of their development. Secondly, new pedagogical approaches should borrow techniques from the world of theatre as well as psycho-pedagogical discoveries to promote creativity and endeavour. Thirdly, teacher training should foster a common vocabulary that cuts across the traditional boundaries between scientific disciplines. More generally, he suggests that invention and enthusiasm should be the bywords of this change process.Estas propuestas se reparten en tres temas. Ante todo, los nuevos contenidos deben insistir sobre enfoques pluridisciplinarios así como sobre la enunciación de los paradigmas fundadores propios a cada asignatura y a la historia de su aparición. Luego, las nuevas pedagogías tienen que integrar algunas técnicas del teatro y conocimientos sicopedagógicos y conllevar a la valorización de la imaginación y del esfuerzo. Finalmente, la formación de maestros podría beneficiarse del desarrollo de un lenguaje común de los docentes, allende de sus especifidades disciplinarias. En resumen, es menester que dos palabras claves orienten la reflexión en cuanto a las evoluciones necesarias : invento y entusiasmo

Cosmic Star Formation, Reionization, and Constraints on Global Chemical Evolution

Motivated by the WMAP results indicating an early epoch of reionization, we consider alternative cosmic star formation models which are capable of reionizing the early intergalactic medium. We develop models which include an early burst of massive stars (with several possible mass ranges) combined with standard star formation. We compute the stellar ionizing flux of photons and we track the nucleosynthetic yields for several elements: D, He4, C, N, O, Si, S, Fe, Zn. We compute the subsequent chemical evolution as a function of redshift, both in the intergalactic medium and in the interstellar medium of forming galaxies, starting with the primordial objects which are responsible for the reionization. We apply constraints from the observed abundances in the Lyman alpha forest and in Damped Lyman alpha clouds in conjunction with the ability of the models to produce the required degree of reionization. We also consider possible constraints associated with the observations of the two extremely metal-poor stars HE 0107-5240 and CS22949-037. We confirm that an early top-heavy stellar component is required, as a standard star formation model is unable to reionize the early Universe and reproduce the abundances of the very metal-poor halo stars. A bimodal (or top-heavy) IMF (40 - 100 M_\odot) is our preferred scenario compared to the extreme mass range (\ga 100 M_\odot) often assumed to be responsible for the early stages of reionization. A mode of even more extreme stellar masses in the range (\ge 270 M_\odot) has also been considered. All massive stars in this mode collapse entirely into black holes, and as a consequence, chemical evolution and reionization are de-correlated. [Abstract abbreviated.]Comment: 45 pages, 18 eps figures, as accepted in Ap

Lithium-Beryllium-Boron : Origin and Evolution

The origin and evolution of Lithium-Beryllium-Boron is a crossing point between different astrophysical fields : optical and gamma spectroscopy, non thermal nucleosynthesis, Big Bang and stellar nucleosynthesis and finally galactic evolution. We describe the production and the evolution of Lithium-Beryllium-Boron from Big Bang up to now through the interaction of the Standard Galactic Cosmic Rays with the interstellar medium, supernova neutrino spallation and a low energy component related to supernova explosions in galactic superbubbles.Comment: 28 pages, 7 figures, to be published in a special memorial volume of Physics Reports in honor of David Schram

Star Formation Histories versus Redshift : Consequences for Overall Metallicity and Deuterium Destruction

The flood of new data on deep surveys, and above all the CFRS (Canada-France-Redshift-Survey), has had a great impact on studies of galactic evolution. On the basis of cosmological models consistent with the improved values of the Hubble parameter, different star formation histories are tested against the observed UV, B and IR broad band comoving luminosity densities. Using these spectrophotometric results, we analyze the global metal enrichment with the help of chemical evolutionary models and we discuss the pertinence of different metallicity tracers (quasar absorption systems and clusters of galaxies) as representative of the bulk chemical evolution of the Universe. Moreover, as deuterium is very fragile, this isotope is destroyed in all stars and its evolution is particularly sensitive to the history of star formation. Relying on models constrained to fit the solar vicinity, it is shown that models with high D destruction corresponding to a large decrease of the star formation rate (SFR) from $z=1.5$ to 0 are in good agreement with spectrophotometric data. In contrast, low D destruction models which require only a moderate variation of the SFR in the same redshift range seem to encounter difficulties in matching the evolution of the luminosity densities (UV, B and IR) versus redshift. The sensitivity of the results with the cosmological models of the universe is discussed.Comment: 28 pages, latex, 8 ps figures (revised for clarity), as accepted for publication in New Astronom