Astrophysics, stellar nucleosynthesis

A few informative properties of life come from easy category distinctions, such as the fact that all known life makes essential use of carbon and carbon-oxygen-nitrogen molecules in liquid water solution. The seemingly trivial observation that such carbaquist chemistry is ruled out if astrophysical carbon abundance lies below a certain threshold enabled Hoyle [1] to predict the 7.6 MeV carbon-12 ( C) nuclear resonance with remarkable precision because the discovery of the triple-alpha reaction synthesis of in stars happens to be a bottleneck for stellar nucleosynthesis of all the heavy elements. The pragmatic information in this prediction is easy to measure because it guided experimental characterization of nuclear structure where the existing computational capabilities could not. Similar sensitive dependence of the physical state of water has been used to define a habitable zone in planetary physics [10], which is not predictive in the same sense as carbon abundance (we already knew where the earth s orbit lies), but which creates a useful filter in the search for extraterrestrial life. 

BBN theory predicts the universal abundances of (D), He, " He, and Li, which are essentially determined by / 180 s. Abundances are however observed at much later epochs, after stellar nucleosynthesis has commenced. The ejected remains of this stellar processing can alter the light element abundances from their primordial values, but also produce heavy elements such as C, N, O, and Fe ( metals ) Thus one seeks astrophysical sites with low metal abundances, in order to measure light element abundances which are closer to primordial. For all of the light elements, systematic errors are an important and often dominant limitation to the precision of the primordial abundances. 

D. D. Clayton, Principles of Stellar Evolution and Nucleosynthesis, McGraw-Hill 1968, University of Chicago Press 1984. This classic text is particularly well written and gives more accurate and rigorous arguments for results derived here often in a somewhat heuristic manner. A more up-to-date (though in some ways less complete) description of nuclear astrophysics is given in... 

Explosive astrophysical environments invariably lead to the production of nuclei away from stability. An understanding of the dynamics and nucleosynthesis in such environments is inextricably coupled to an understanding of the properties of the synthesized nuclei. In this talk a review is presented of the basic explosive nucleosynthesis mechanisms (s-process, r-process, n-process, p-process, and rp-process). Specific stellar model calculations are discussed and a summary of the pertinent nuclear data is presented. Possible experiments and nuclear-model calculations are suggested that could facilitate a better understanding of the astrophysical scenarios. 

Before we start approaching the nucleosynthesis problem from the astrophysical side, let us briefly sketch the basic picture as we have it today. The nucleosynthesis during the Big Bang was constrained to the production of the four stable isotopes of hydrogen and helium, and to 7Li (see Steigman, this volume). Since then, the abundances of these isotopes have not changed much, except for rLi which apparently was enriched by about one order of magnitude due to stellar production (Casuso Beckman 2000). 

The distribution of elements in the cosmos is the result of many different physical processes in the history of the Universe, from Big Bang to present times. Its study provides us with a powerful tool for understanding the physical conditions of the primordial cosmos, the physics of nucleosynthesis processes that occur in different objects and places, and the formation and evolution of stars and galaxies. Cosmochemistry is a fundamental topic for many different branches of Astrophysics as Cosmology, Stellar Structure and Evolution, Interstellar Medium, and Galaxy Formation and Evolution. 

The reactions involved in the He-burning stage have been discussed in many places (e.g. [5], NACRE [6]). The main ones are displayed in Fig 4. They develop at temperatures in excess of 108 K, and mainly transform 4He into 12C and leO, with some limited contribution to the abundance of some heavier a-particle nuclei (esp. 20Ne,.), at least in massive enough stars. Of very special and dramatic importance for the theories of stellar evolution and of nucleosynthesis is the famed 12C (a, 7) 160 reaction, which has been the subject of a flurry of experimental investigations, as well as of theoretical efforts (for a review, see [11] see also [12] for a recent re-analysis of the case). In spite of that, uncertainties remain, and preclude certain nuclear astrophysics predictions to be made at a satisfactory level (see e.g. [13]). 

In astrophysics much interest has in recent years been focussed on boron. Although its cosmic abundance is extremely low, it plays an important role in testing models of Big Bang Nucleosynthesis [10]. Optical spectroscopy is the only method for establishing B abundances in stellar objects, and thus a good knowledge of energy structure, transition probabilities, hyperfine structure and isotope shifts is needed [11]. 

Abstract This chapter provides the necessary background from astrophysics, nuclear, and particle physics to understand the cosmic origin of the chemical elements. It reflects the year 2008 state of the art in this extremely quickly developing interdisciplinary research direction. The discussion summarizes the nucleosynthetic processes in the course of the evolution of the Universe and the galaxies contained within, including primordial nucleosynthesis, stellar evolution, and explosive nucleosynthesis in single and binary systems. 

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