Nucleosynthesis heavy elements

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].

This area of speculation is controversial. Victor J. Stenger, Emeritus Professor of Physics at the University of Hawaii, has published numerous books and articles that suggest that the conditions for the appearance of a universe with life (and heavy element nucleosynthesis) are not quite as improbable as other physicists have suggested. For more information, see his web site http //spot.colorado.edu/ vstenger. Also see his various books listed... 

Richter S., Ott U., and Begemann F. (1994) s-Process isotope abundance anomalies in meteoritic silicon carbide data for Dy. In Proc. European Workshop on Heavy Element Nucleosynthesis (eds. E. Somorjai and Z. Fiilop). Inst. Nucl. Res. Hungarian Acad. Sci., Debrecen, pp. 44-46. 

There are two basic timescales in this scenario of heavy-element nucleosynthesis by neutron captures (1) the P-decay lifetimes, and (2) the time intervals between successive captures that are inversely proportional to the neutron capture reaction rates and the neutron flux. If the rate of neutron capture is slow compared to the relevant P decays, the synthesis path will follow the bottom of the stability valley very closely. On the other hand, if the rate of neutron capture is faster than the relevant P decays, highly neutron-rich nuclei will be formed. After the neutron flux has ceased, those nuclei will be transformed to stable nuclei by a series of P decays. The above two processes are called s- and r-process, respectively, according to their slow or rapid rate of neutron capture. The observed abundances of nuclei in the solar system, especially in the regions of closed-shell nuclei, suggest that the s- and r-processes contributed more or less equally to the formation of the elements above the iron region (see Fig. 12.12). 

The overall abundance of helium and heavy elements in the Universe today. This reflects the total effect of fuel consumption and nucleosynthesis by all the stars that ever existed. Roughly speaking, one may consider this in two... 

Refractory materials in primitive meteorites were investigated first as they have the best chance of escaping homogenization in the early solar system. Inclusions in C3 carbonaceous chondrites exhibit widespread anomalies for oxygen and the iron group elements. Only a few members, dubbed FUN (for Fractionated and Unknown Nuclear effects), also display anomalous compositions for the heavy elements. Anomalies in inclusions have generally been connected with explosive or supernova nucleosynthesis. 

Nicolussi GK, Pellin MJ, Lewis RS, Davis AM, Clayton RN, Amari S (1998b) Strontium isotopic composition in individual silicon carbide grains a record of s-process nucleosynthesis. Phys Rev Lett 81 3583-3586 Nicolussi GK, Pellin MJ, Lewis RS, Davis AM, Clayton RN, Amari S (1998c) Zirconium and molybdenum in individual circumstellar graphite grains new data on the nucleosynthesis of the heavy elements. Astrophys J 504 492-499... 

These elements are scattered throughout the universe when massive stars end their lives. When there is no fuel left to burn, the core collapses once again, and there is nothing to stop it. A shock wave from this collapse causes a rebound that fuels an enormous explosion a supernova. The outer layers of the star are blown out into space, and the energy that is released triggers new nucleosynthesis reactions, which make the heavy elements beyond bismuth - up to uranium, and at least a little beyond. 

There are several lines of evidence that nucleosynthesis takes place in stars. The compositions of the outer envelopes of evolved low- and intermediate-mass stars show enhancements of the products of nuclear reactions (hydrogen and helium burning and s-process nucleosynthesis, as defined below). The ejecta of supemovae (stellar explosions) are highly enriched in short-lived radioactive nuclides that can only have been produced either just before or during the explosion. At the other extreme, low-mass stars in globular clusters, which apparently formed shortly after the universe formed, are deficient in metals (elements heavier than hydrogen and helium) because they formed before heavy elements were synthesized. 

In this review we wish to discuss how observations of AGB stars can be used to determine the manner in which heavy elements are created during a thermal pulse, and how these heavy elements and carbon are transported to the stellar surface. In particular we wish to study how the periodic hydrogen and helium shell burning above a degenerate carbon-oxygen (C-0) core forms a neutron capture nucleosynthesis site that may eventually account for the observed abundance enhancements at the surfaces of AGB stars. In section II we discuss the nucleosynthesis provided by stellar evolution models (for a general review see [1]). In section III we discuss the isotopic abundances provided by nucleosynthesis reaction network calculations (see [2, 3]). In section IV we discuss how observations of AGB stars can be used to discriminate between the neutron capture nucleosynthesis sources (see [4]). And in section V we note some of the current uncertainty in this work. 

It is this hot convective shell that can be an active site of neutron capture nucleosynthesis in a thermally pulsing AGB star [6], Convection mixes the nucleosynthesis raw materials for a capture and for neutron producing reactions to the hot base of the convective shell. The material mixed to the base contains heavy elements that originally were in the stellar envelope (perhaps in a solar system distribution), as well as heavy elements from previous thermal pulses (perhaps in a neutron-rich distribution). Processed material is simultaneously mixed away from the shell base to cooler outer regions of the shell, and this material contains a-burning byproducts and a rearranged heavy element distribution (if neutron capture nucleosynthesis has occurred). ... 

While stellar evolution models describe neutron production, the calculations must be supplemented with nucleosynthesis calculations to determine what abundances of heavy elements the 13C or 22Ne neutron sources will produce. While analytic theory can approximate the production of heavy elements [3], numerical modelling of - 500 isotopes is used for detailed comparison of stellar evolution theory and observation. In general, the destruction of an isotope between Fe and Bi occurs due to that isotope capturing a neutron or f) decaying, while the creation of an isotope will be due to the neutron capture or f decay of lighter elements. 

While significant uncertainty exists in determining the absolute stellar abundances of the heavy elements, the relative isotopic abundance of an atomic species can be determined somewhat independently of the atmospheric modelling uncertainties [4, 29]. Observations of different ZrO bands in a star do produce slightly different surface abundance values, but the uncertainty this introduces into our analysis of stellar nucleosynthesis is small compared to other uncertainties we have already discussed. In addition, the heavy element... 

It should be emphasized that solar abundance ratios are used here only as a convenient referenoe point. The LMC is known to have a total heavy element abundance that is approximately two to three times less than solar (van Genderen, van Driel, and Greidanus 1986 Dufour 1984). The abundances of Sc, Sr, and Ba in the LMC are not known because of the difficulty in detecting lines of these elements in objects. They are probably not solar however, unless the history of nucleosynthesis in the Large Cloud is completely different from that in our Galaxy, the relative abundances of the s-process elements with respect to each other and to Fe should not differ greatly from those of the sun. 

If the time scale of neutron capture reactions is very much less than 3 -decay lifetimes, then rapid neutron capture or the r process occurs. For r-process nucleosynthesis, one needs large neutron densities, 1028/m3, which lead to capture times of the order of fractions of a second. The astrophysical environment where such processes can occur is now thought to be in supernovas. In the r process, a large number of sequential captures will occur until the process is terminated by neutron emission or, in the case of the heavy elements, fission or (3-delayed fission. The lighter seed nuclei capture neutrons until they reach the point where (3 -decay lifetimes have... 

Figure 12.16 Typical portion of the heavy element chart of the nuclides showing the relative importance of s, r, and p processes in nucleosynthesis. [From J. W. Truran, Nucleosynthesis in Ann. Rev. Nucl. Part. Set, 34, 53 (1984). Copyright 1984 by Annual Reviews, Inc. Reprinted by permission of Annual Reviews, Inc.]...

It was the formation of stars and then second-generation stars (such as our sun) with rocky planets that made life as we know it possible. Entropy still applied, yet the Universe became habitable in the period (which continues today) succeeding that first eon when it would have seemed entropic death was already gripping the Universe. Life was made possible by the nucleosynthesis of heavy elements and the condensation of solids and formation of planets where aqueous fluids could exist. 

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 neutron-capture cross section of any isotope represents the probability with which it is able to capture free neutrons passing by it. This quantity is important for the s process of nucleosynthesis. This process was named s owing to the need to patiently make the s isotopes of the heavy elements by the slow capture of free neutrons, neutrons liberated by other nuclear reactions within the gas in stellar interiors. It was one of the first nucleosynthesis processes identified historically. The capture of a free neutron by a nucleus increases its mass number A by one unit. As the captures continue, each nucleus in the gas is rendered heavier, little by little, capture by capture. When an isotope of mass number A of an element with atomic number Z captures a neutron, the compound nucleus formed from their union becomes an isotope of the same element but having mass number greater by one unit (i.e. A + l). 

---