Nucleosynthesis mechanisms

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. 

The bottom line of this brief review of the r-process is that unanswered questions are by far more numerous than solved problems when one is dealing with this nucleosynthesis mechanism. They concern especially the astrophysics of the process, as no single site has been identified decisively yet. It also raises many nuclear physics questions. In such conditions, the modelling of the evolution of the r-nuclide content of the Galaxy and actinide-based chronometric evaluations cannot be based on solid grounds yet. This is in fact a very pleasing situation, as hope for many exciting discoveries is still ahead of us ... 

AGB stars constitute excellent laboratories to test the theory of stellar evolution and nucleosynthesis. Their particular internal structure allows two important processes to occur in them. First is the so-called 3(,ldredge-up (3DUP), a mixing mechanism in which the convective envelope penetrates the interior of the star after each thermal instability in the He-shell (thermal pulse, TP). The other is the activation of the s-process synthesis from alpha captures on 13C or/and 22Ne nuclei that generate the necessary neutrons which are subsequently captured by iron-peak nuclei. The repeated operation of TPs and the 3DUP episodes enriches the stellar envelope in newly synthesized elements and transforms the star into a carbon star, if the quantity of carbon added into the envelope is sufficient to increase the C/O ratio above unity. In that way, the atmosphere becomes enriched with the ashes of the above nucleosynthesis processes which can then be detected spectroscopically. 

The abundance curve of chemical elements was nsedby Bnrbidge, Burbidge, Fowler and Hoyle in 1957 and by Cameron in the same year to establish the basic process of nucleosynthesis operating in stars. The breakdown of mechanisms responsible for the existence of the varions types of atom in the observed proportions is as follows ... 

The sequence of neutron captures followed by P decay produces heavier and heavier elements. It is the only known mechanism for producing gold, platinum, thorium and uranium. The r process is thus considered to close the cycle of nucleosynthesis. 

We conclude that the process responsible for the production of r-type isotopes beyond barium has operated in the same way since the origin of the Galaxy. The process is therefore unique. There is little risk of error in suggesting that it is related to SNII events. We may then define the conditions of neutron irradiation and thermodynamic parameters relevant to this nucleosynthesis, without necessarily being able to establish the detailed mechanism. 

Astrophysicists are also avid customers for nuclear information and theories. Calculations of elemental abundances resulting from r-process nucleosynthesis and related mechanisms depend on the systematics of... 

We begin with a discussion of the poorly understood mechanisms for heavy-element nucleosynthesis and some of our efforts to understand these environments. Then we turn to a discussion of the exotic environments for hot hydrogen burning and some of our experimental and theoretical efforts to obtain the associated nuclear data. 

Fig. 1 The mechanisms for heavy-element nucleosynthesis drawn as lines representing the dominant isotopes produced during the processes [MAT85].

But the relative strengths ofatomic lines differ from star to star. The confluence of atomic physics, of quantum mechanics, and of statistical mechanics has allowed astronomers to understand these variations in detail. These issues were at the heart of the revolution that was 20th-century physics but today they are understood. The net resultis that other stars have different abundances of the elements than does our own. Perhaps one should say modestly different. The broad comparisons between the elements remain valid - iron is quite abundant, vanadium is rather rare. That remains true but many stars have many fewer of each. A few have more of each. This was a great discovery of 20th-century astronomy, because it established the nucleosynthesis of the elements as an observational science. Astronomers also learned how old the stars are, for there do exist telltale signs of a star s age. The oldest stars are found to have many fewer of all chemical elements (except the three lightest elements) than does the Sun. These came to be called metal-poor stars, because the heavy elements were lumped together under the term metals by astronomers. It may seem paradoxical that the oldest stars have the fewest metals but the key is that the abundances within... 

The consequences of mixing, whatever the mechanism, for stellar evolution calculations is that nucleosynthesis may transform the mixture in part of a convection zone. Alternatively a convective boundary may move to incorporate material of different composition. The new material must then be mixed throughout the convection zone. After a time interval 6t, the new composition (x() in the convection zone (cz) will be given by... 

In such an unsatisfactory state of affairs, the best one can do is to try to understand better the physics of neutrino-driven winds through the development of (semi-)analytical models some aspects of which may be inspired by (failed) explosion simulations, and to try to delineate on such grounds favourable conditions for the development of the r-process. These analytical models confirm that the wind nucleosynthesis depends on Ye, entropy s, and Tdyn, as in the o-process discussed in Sect. 7.2. The wind mass-loss rate M is influential as well. Ultimately, the quantities acting upon the synthesis in the neutrino-driven DCCSN model depend crucially on the details of the interaction of neutrinos with the innermost supernova layers, as well as on the mechanisms that might aid to get a successful DCCSN, and whose relative importance remains to be quantified in detail. 

This process occurs in heavy stars after neon-burning in the core. But it does not give the answer to the abundance of phosphorus on Earth or the other planets. Although oxygen burning does produce phosphorus in heavy stellar cores, further nucleosynthesis uses up almost all of it to produce, in the end, a star composed of 90 percent silicon and sulfur. So another mechanism must be responsible for phosphorus production and distribution throughout the universe. 

Another way to appreciate the situation is to consider the following argument In order for two nuclei to fuse together, they must approach each other to a distance approximately equal to the sum of their radii. However, such an approach is counteracted by a strongly repubive Coulomb force, which would seem to render this process impossible. Only following the advent of quantum mechanics was it realized that such a close encounter between nuclei could still occur by means of the phenomenon of quantum mechanical tunneling, which is now believed to take place in stars. In a paper pubhshed in 1946, Hoyle sketched the essential pathways through which stellar nucleosynthesis takes place. 

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