Nucleosynthesis explosive

Supernova remnants (SNR) in early stages of expansion display results of advanced (explosive) nucleosynthesis, but in later stages light up the interstellar medium by shock excitation and give information about the ISM in external galaxies complementary to that derived from H n regions. 

More advanced He-burning reactions 160(a, y )20Ne(a, y)24Mg may take place at temperatures 109 K under some conditions, e.g. shell-burning in advanced stages and explosive nucleosynthesis. 

Fig. 5.10. Upper panel chemical profile of a 25 Af0 star immediately before core collapse. (Note change in horizontal scale at 2 Af0.) Lower panel the same, after modification by explosive nucleosynthesis in a supernova outburst. The amount of 56Ni (which later decays to 56Fe) ejected depends on the mass cut, somewhere in the 28Si 56Ni zone, and is uncertain by a factor of 2 or so. Adapted from Woosley and Weaver (1982).

The contributions of different stars to nucleosynthesis depend on their initial mass and chemical composition, their mass loss history in the course of evolution and effects of close binaries (especially SN la). When mass loss is small, as is believed to be the case for low metallicities, the distribution of primary elements (those synthesized directly from hydrogen and helium) in ejecta from massive stars is mainly the result of hydrostatic evolution with some modifications to deeper layers resulting from explosive nucleosynthesis in the final SN outburst, classically... 

Remaining for a moment with the massive stars, a distinction must be made between slow, secular, quasi-static and explosive nucleosynthesis. The time-scale in the latter case is of the order of 1 second and it only affects the innermost layers of stars, rich in silicon, oxygen and carbon. 

Globally, then, oxygen, neon and magnesium originate in hydrostatic shell combustion and the quantity synthesised and ejected increases with the mass of the progenitor star, whilst sulphur, argon, calcium and iron are essentially due to explosive nucleosynthesis and the ejected mass is much less variable from one star to another. 

Explosive nucleosynthesis adds a few last trinkets to the abundance table, in particular, gold, platinum and uranium, through an ultimate nuclear process relating to neutron physics. 

It should be noted that odd elements produced in explosive nucleosynthesis depend less on metalhcity than their counterparts fashioned by slow (hydrostatic) nucleosynthesis, for the n/p ratio is steadily modified by various weak interactions operating in the advanced stages. In other words, the n/p ratio deep down in the star, in regions affected by explosive nucleosynthesis, no longer reflects the initial ratio inherited from the interstellar medium. At least, this is what calculations suggest. However, the cause of all these phenomena remains relatively obscure, given the complex way in which nuclear reactions are interwoven within massive stars in the advanced stages of their evolution. 

Theoretically, nuclear strength is enhanced by internal transmutations of protons into neutrons, under the mandate of the weak interaction, either by positron emission (p — n + e+ + v) or by electron capture (p + e n + v). However, the weak interaction is much slower than the strong interaction. The question remains as to whether it will happen inside the star, or outside, once the matter has been expelled, i.e. after the explosion. This is not just an academic question. The answer we give will determine whether or not we can corroborate explosive nucleosynthesis by observation. 

It also shows just how sensitive are the results of explosive nucleosynthesis to the n/p ratio. A numerical experiment should convince us of this ... 

Furthermore, the fact that iron is s5mthesised in the form of a nickel isotope has important implications from an observational standpoint. Indeed, it provides a check on the foundations of the whole theory of explosive nucleosynthesis. These implications are twofold, as we have seen. They concern supernova light curves and gamma emission from these objects. 

Explosive nucleosynthesis occurs under conditions where temperature and density are changing rapidly with time, either due to the passage of a shock wave or because of a runaway explosion. We highlight some of the more important processes here. 

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. 

Titanium-44 decays by electron capture via 44Sc to 44Ca with a half-life of 60 years. Its half-life is too short for it to have played any role in the solar system. Titanium-44 is produced by explosive nucleosynthesis and it is useful as an indicator of the supernova source for a subset of 44Ca-rich presolar grains. 

Fig. 2 - Composition of the 6 M helium core used in all studies. Interior to about 3 M the composition is a result of explosive nucleosynthesis. Farther out the fossil remnants of previous burning stages are ejected.

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. 

Table 2 Explosive nucleosynthesis in supemovae. Similar to Table 1, but for the explosion of the star. The (A, B) notation means A is on the ingoing channel and B is on the outgoing channel. An a is same as " He, y denotes a photon, i.e., a photodisintegration reaction when on the ingoing channel, n is a neutron, /3 shows 3-decay, and...

Arnett W. D. (1995) Explosive nucleosynthesis revisited yields. Ann. Rev. Astron. Astrophys. 33, 115-132. 

Thielemann F.-K., Nomoto K., and Yokoi K. (1986) Explosive nucleosynthesis in carbon deflagration models of Type I sapenmvae. Astron. Astrophys. 158, 17-33. 

Casse, M., and A. Soutoul. (1978). Time delay between explosive nucleosynthesis and cosmic ray acceleration11, ApJ 200, L75. 

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