Nucleosynthesis processes

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. 

Stars of mass greater than 1.4 solar masses have thermonuclear reactions that generate heavier elements (see Table 4.3) and ultimately stars of approximately 20 solar masses are capable of generating the most stable nucleus by fusion processes, Fe. The formation of Fe terminates all fusion processes within the star. Heavier elements must be formed in other processes, usually by neutron capture. The ejection of neutrons during a supernova allows neutron capture events to increase the number of neutrons in an atomic nucleus. Two variations on this process result in the production of all elements above Fe. A summary of nucleosynthesis processes is summarised in Table 4.4. Slow neutron capture - the s-process - occurs during the collapse of the Fe core of heavy stars and produces some higher mass elements, however fast or rapid neutron capture - the r-process - occurs during the supernova event and is responsible for the production of the majority of heavy nuclei. 

There are several bodies of information that feed into our understanding of stellar nucleosynthesis. We will start with a discussion of the classification of stars, their masses and mass distributions, and their lifetimes. From this information we can assess the relative importance of different types of stars to the nucleosynthesis of the elements in our solar system and in the galaxy. We will then discuss the life cycles of stars to give a framework for the discussion of nucleosynthesis processes. Next, we will review the nuclear pathways... 

Recent modeling based on the lifetimes of stars, their IMF, the star formation rate as a function of time, and nucleosynthesis processes have succeeded in matching reasonably well the abundances of the elements in the solar system and in the galaxy as a whole (e.g. Timmes et al., 1995). These models are still very primitive and do not include nucleosynthesis in low and intermediate-mass stars. But the general agreement between model predictions and observations indicates that we understand the basic principles of galactic chemical evolution. 

Copper is not a rare element. It ranks 25th in abundance among the 81 stable elements. Both isotopes are produced by a wide variety of nucleosynthesis processes and settings. 

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). 

In this chapter, we will review the characteristics of thermonuclear processing in the three environments we have identified (i) intermediate-mass stars (ii) massive stars and type II supemovae and (iii) type la supemovae. This will be followed by a brief discussion of galactic chemical evolution, which illustrates how the contributions from each of these environments are first introduced into the interstellar media of galaxies. Reviews of nucleosynthesis processes include those by Arnett (1995), Trimble (1975), Truran (1984), Wallerstein et al. (1997), and Woosley et al. (2002). An overview of galactic chemical evolution is presented by Tinsley (1980). 

Figure 9 The history of the [Ba/Eu] ratio is shown as a function of metallicity [Fe/H]. This ratio reflects to a good approximation the ratio of s-process to r-process elemental abundances, and thus measures the histories of the contributions from these two nucleosynthesis processes to galactic matter (o Burris et al. (2000) X Woolf et al. (1995) and Edvardsson et al. (1993) Gratton and Sneden (1994) McWilliam (1997) ...

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 PG1159 class of post-AGB stars are hot, helium-rich objects that show intershell matter of the preceding AGB phase on their surface. For this reason, their chemical analyses allow for a direct insight into nucleosynthesis processes during the AGB phase [123,124], PG1159 stars are quite rare, with only about... 

This chapter briefly introduces the chemistry in circumstellar envelopes (CSE) around old, mass-losing stars. The focus is on stars with initial masses of one to eight solar masses that evolve into red giant stars with a few hundred times the solar radius, and which develop circumstellar shells several hundred times their stellar radii. The chemistry in the innermost circumstellar shell adjacent to the photosphere is dominated by thermochemistry, whereas photochemistry driven by interstellar UV radiation dominates in the outer shell. The conditions in the CSE allow mineral condensation within a few stellar radii, and these grains are important sources of interstellar dust. Micron-sized dust grains that formed in the CSE of red giant stars have been isolated from certain meteorites and their elemental and isotopic chemistry provides detailed insights into nucleosynthesis processes and dust formation conditions of their parent stars, which died before the solar system was bom 4.56 Ga ago. 

The major application of TIMS is to the geochronology and tracer studies using terrestrial radiometric systems, for example, U-Th-Pb, Rb-Sr, Sm-Nd, and Lu-Hf. Geochronology exploits the radioactive decay in closed systems to obtain the date of a specific geological event. Tracer studies use the growth of daughter nuclides from radioactive decay to evaluate the interaction between geochemical systems and/or reservoirs. The application of TIMS in cosmochemical analysis is limited however, it is used to measure the isotopic compositions as tracers of nucleosynthesis processes. This includes the measurement of radionuclides observed mainly in meteorites, for example, Mn-Cr, Al-Mg, Fe-Ni, and Ca-K system in addition to the above-mentioned systems. 

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