Stellar nucleosynthesis processes

In first generation stars, those that consist almost entirely of hydrogen and helium, and in stars of later generations where the core temperature is less than 20 million degrees (M M ), hydrogen bums to helium via the proton-proton chains. The simplest chain, and the one activated at the lowest temperature ( 5 million degrees), is called PPI. Three reactions are involved. In the first step, two protons fuse to form a 2He atom, which immediately emits a positron (P1) to form a deuterium atom and a neutrino [ H(p,P v)2D]. This reaction is very slow and controls the rate of the PPI chain. Deuterium then quickly combines with a proton to form 3He (2D(p,y)3He), and two 3He nuclei combine to form 4He and two hydrogen atoms [3He(3He,2p)4He]. 

Helium-3 can also react with 4He to make 7Be, which then captures an electron to form 7Li and a neutrino [3He(a,y)7Be(P, v)7Li], Lithium-7 then captures a proton and the resulting 8Be nucleus breaks up into two 4He atoms, one more than was present initially [7Li(p,a)4He]. This is the PPII chain, and it becomes more important as the star ages and 4He becomes more abundant, and as temperature increases beyond 15 million degrees inside the star. 

Beryllium-7 also reacts via two branches. Instead of capturing an electron to become 7Li, 7Be can react with a proton to form 8B plus a gamma ray. 8B emits an electron and neutrino to form 8Be, which then breaks up into two 4He atoms [7Be(p,y)8B(P+ v)8Be (,a)4He]. This is the PPIII chain. PPIII becomes important at temperatures above 25 million degrees. 

P+ v) reactions are essentially instantaneous and so are much faster than the other reactions at the temperatures in low- and intermediate-mass stars. The fastest proton reaction in this series is 15N(p,a)12C, while the slowest reaction is 14N(p,y)150. As a result, extensive CN cycling converts much of the 15N, 12C, and 13C into 14N. In the CN cycle, 12C is destroyed more rapidly than 13C by about a factor of three. In the solar composition,12C is 89 times more abundant than 13C, so initially more 13C is produced from the destruction of 12C than is destroyed by proton reactions. The 12C/13C ratio decreases until it reaches an equilibrium value equal to the inverse of the reaction rates, where as much 13C is being destroyed as is being produced. From this point on the 12C/13C ratio remains the same as carbon is gradually converted to 14N. 

Schematic cross-section of a 1.5 M star showing concentrations of various nuclei as a function of depth within the star at the end of its main sequence lifetime. The position within the star is given in units of mass normalized to the solar mass, with zero being the center and 1.5 M being the surface of the star. The outer portion of the star from 1 M to the surface has the initial abundances of the nuclei because this portion of the star was never hot enough for nuclear burning. The region from 

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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. 

Hoyle successfully predicts existence of a 7.6 MeV resonance state of the carbon-12 nucleus on grounds that otherwise little carbon would survive further processing into oxygen during stellar nucleosynthesis by helium burning, whereas in fact the C/O ratio is about 0.5. Discovery of strange particles. 

The excess was first suggested to have a nuclear origin in stars. Almost pure O is produced in He-buming shells in massive stars, and in supemovae. On the other hand it has been shown that non-mass dependent fractionation can be produced in the laboratory by non-nuclear processes (Thiemens and Heidenreich 1983 Thiemens 1988). Similar non-linear effects have been found for O isotopes in atmospheric gases (Schueler et al. 1990 Thiemens et al. 1995). Although stellar nucleosynthesis is indeed at the origin of the O observed in the universe, the link between O isotopic anomalies in inclusions and nucleosynthesis is still under debate (Thiemens 1999 Clayton 2002). 

In summary, the extinct radioactivities which have a limited time of existence in the solar system, constrain the time interval between the late stages of stellar nucleosynthesis and the formation of the solar system. Some production may also occur within the solar system during active periods of the young Sun. There have been numerous studies about how this matter was added into the solar system as a late spike of about 10 solar masses of freshly stellar processed material or from constant production in the galaxy (Wasserburg et al. 1996 Goswami and Vanhala 2000 Russell et al. 2001). These models are refined constantly with the input of new data and will probably continue to evolve in the future. 

Over a period of forty years, the theory of stellar nucleosynthesis has been transformed from an abstract description of the various nuclear processes that fashion matter to a fully evolved discipline, quite able to stand up to confrontation with a vast body of observational data. 

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... 

In addition to the processes of stellar nucleosynthesis, there are two other ways in which isotopes are produced. One is radioactive decay. Many of the nuclides produced by explosive nucleosynthesis are unstable and decay to stable nuclei with timescales ranging from a fraction of a second to billions of years. Those with very short half-lives decayed completely into their stable daughter isotopes before any evidence of their existence was recorded in objects from our solar system. However, radioactive nuclei from stellar nucleosynthesis that have half-lives of >100 000 years left a record in solar system materials. For those with half-lives of more than 50 million years some of the original nuclei from the earliest epoch are still present in the solar system today. The ultimate fate of all radioactive nuclides is to decay to their stable daughter nuclides. Thus, the only real distinction between isotopes produced by stellar nucleosynthesis and those produced by decay of radioactive nuclides produced by stellar nucleosynthesis is the time scale of their decay. We choose to make a distinction, however, because radioactive nuclides are extremely useful to cosmo-chemists. They provide us with chronometers with which to constmct the sequence of events that led to the solar system we live in, and they provide us with probes of stellar nucleosynthesis and the environment in which our solar system formed. These topics appear throughout this book and will be discussed in detail in Chapters 8, 9, and 14. 

Our current level of understanding about the original composition of the solar system is the result of a boot-strap effort involving more than a century of gathering compositional data about the Sun, stars, other astronomical obj ects, the Earth, other planets, and meteorites establishing relationships between objects and developing increasingly sophisticated models of stellar nucleosynthesis and chemical and physical processes that operated in the early solar system. 

The solar system abundances of the elements are the result of the Big Bang, which produced hydrogen and helium, 7.5 billion years of stellar nucleosynthesis, which produced most of the rest of the elements, and the physical processes that mixed the materials together to form the Sun s parent molecular cloud. The unique features of the solar system composition may also reflect the stochastic events that occurred in the region where the Sun formed just prior to solar system formation. 

In the next few chapters, we will investigate these topics further. For example, in Chapter 5, we will introduce presolar grains and show how they can be used to investigate stellar nucleosynthesis and processes operating in interstellar space and in the early solar system, and in Chapter 7, we will discuss chemical and isotopic fractionation processes that operate in the solar system. 

Presolar grains a record of stellar nucleosynthesis and processes in interstellar space... 

In recent years, a new source of information about stellar nucleosynthesis and the history of the elements between their ejection from stars and their incorporation into the solar system has become available. This source is the tiny dust grains that condensed from gas ejected from stars at the end of their lives and that survived unaltered to be incorporated into solar system materials. These presolar grains (Fig. 5.1) originated before the solar system formed and were part of the raw materials for the Sun, the planets, and other solar-system objects. They survived the collapse of the Sun s parent molecular cloud and the formation of the accretion disk and were incorporated essentially unchanged into the parent bodies of the chondritic meteorites. They are found in the fine-grained matrix of the least metamorphosed chondrites and in interplanetary dust particles (IDPs), materials that were not processed by high-temperature events in the solar system. 

The discussion in this paper shows that we in the future can expect the understanding of the stellar nucleosynthesis from observations of supernovae to be considerably improved. Most of the physics in connection with the thermalization of the y-rays is well understood, as well as most of the atomic data going into the calculations. There are, however, in this area some uncertain processes, most importantly the charge transfer reactions between the various ions, like O II + Na I O I + Na II. Also the ionization of the trace elements, Na I, Mg I and Si I, may be sensitive to the treatment of the UV radiation field. However, these problems are likely to be solved in the near future. Therefore, from a given explosion model of the density and abundance structure one can predict what the late spectrum should be, and compare this with the observations. Especially the line profiles are important, since they provide a test of the probably most uncertain part of the explosion calculations. 

The molecules found to date are composed of the elements H, C, N, O, Si, S, and Cl with the bulk of the molecules containing H, C, N, and O. The light elements H, D, and He are of cosmological origin and are therefore tracers of the early universe. On the other hand the heavier elements C, N, O,... are produced in stars by the processes of stellar nucleosynthesis. In addition to the most abundant isotopic forms many stable isotopes such as D, 13C, 170, lsO, 15N, 30Si, 33S, and 34S have been detected (see Appendix 1). The detailed determination of isotopic ratios — though often beset with formidable difficulties — has become a useful indicator of the chemical evolution of molecular clouds and the past chemical history of the galaxy. 

In the mix of interstellar atoms from which the solar system formed, 17O exists primarily owing to the hydrogen-burning process of stellar nucleosynthesis. 170 is made by converting a fraction of the 16 O to 170 by the nuclear reaction... 

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