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Nuclear Astrophysics: the Unfinished Quest for the Origin of the Elements

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Nuclear Astrophysics: the Unfinished Quest for the Origin of the Elements Nuclear astrophysics: the unfinished quest for the origin of the elements Jordi Jos´e Departament de F´ısica i Enginyeria Nuclear, EUETIB, Universitat Polit`ecnica de Catalunya, E-08036 Barcelona, Spain; Institut d’Estudis Espacials de Catalunya, E-08034 Barcelona, Spain E-mail: [email protected] Christian Iliadis Department of Physics & Astronomy, University of North Carolina, Chapel Hill, North Carolina, 27599, USA; Triangle Universities Nuclear Laboratory, Durham, North Carolina 27708, USA E-mail: [email protected] Abstract. Half a century has passed since the foundation of nuclear astrophysics. Since then, this discipline has reached its maturity. Today, nuclear astrophysics constitutes a multidisciplinary crucible of knowledge that combines the achievements in theoretical astrophysics, observational astronomy, cosmochemistry and nuclear physics. New tools and developments have revolutionized our understanding of the origin of the elements: supercomputers have provided astrophysicists with the required computational capabilities to study the evolution of stars in a multidimensional framework; the emergence of high-energy astrophysics with space-borne observatories has opened new windows to observe the Universe, from a novel panchromatic perspective; cosmochemists have isolated tiny pieces of stardust embedded in primitive meteorites, giving clues on the processes operating in stars as well as on the way matter condenses to form solids; and nuclear physicists have measured reactions near stellar energies, through the combined efforts using stable and radioactive ion beam facilities. This review provides comprehensive insight into the nuclear history of the Universe arXiv:1107.2234v1 [astro-ph.SR] 12 Jul 2011 and related topics: starting from the Big Bang, when the ashes from the primordial explosion were transformed to hydrogen, helium, and few trace elements, to the rich variety of nucleosynthesis mechanisms and sites in the Universe. Particular attention is paid to the hydrostatic processes governing the evolution of low-mass stars, red giants and asymptotic giant-branch stars, as well as to the explosive nucleosynthesis occurring in core-collapse and thermonuclear supernovae, γ-ray bursts, classical novae, X-ray bursts, superbursts, and stellar mergers. (Some figures in this article are in color only in the electronic version) PACS numbers: 26.00, 20.00, 90.00, 97.10.Cv Submitted to: Reports on Progress in Physics Nuclear astrophysics 2 1. Introduction Most of the ordinary (visible) matter in the Universe, from a tiny terrestrial pebble to a giant star, is composed of protons and neutrons (nucleons). These nucleons can assemble in a suite of different nuclear configurations. The masses of such bound nuclei are actually smaller than the sum of the masses of the free individual nucleons. The difference is a measure of the nuclear binding energy that holds the nucleus together, according to Einstein’s famous equation E = ∆mc2, where c is the speed of light and ∆m denotes the mass difference between the bound nuclei and the free nucleons. For example, the nuclide 56Fe is among the most tightly bound species, with a binding energy near 8.8 MeV per nucleon. There are 82 elements that have stable isotopes, all the way from H to Pb (except for Tc and Pm). Several dozen elements have only unstable isotopes, naturally abundant or artificially produced in nuclear physics laboratories (the super-heaviest nucleus discovered to date has 118 protons and a half-life of 0.89 ms [1]). The distribution of the nuclide abundances in the Solar System, plotted as a function of the number of nucleons (mass number), A, is shown in Fig. 1. It displays a complicated pattern, with H and He being by far the most abundant species. The abundances generally decrease for increasing mass number. Exceptions to this behavior include the light elements Li, Be and B, which are extremely underabundant with respect to their neighbors, a pronounced peak near 56Fe (the so-called iron-peak), and several noticeable maxima in the region of A & 100, for the distributions of both odd- and even-A species. The reasons for favoring the presence of some species and for the specific pattern inferred for the Solar System abundances have spawned large debates. Following earlier attempts, Hoyle proposed in 1946 that all heavy elements are synthesized in stars [3]. A major breakthrough was achieved with the detection of technetium, an element with no stable isotopes, in the spectra of several S stars by Merrill [4] in the early 1950s. Since the technetium isotopes have rather short half-lives on a Galactic time scale, the obvious conclusion was that technetium must have been produced in the observed stars and thus nucleosynthesis must still be going on in the Universe. Shortly after, two seminal papers that provided the theoretical framework for the origin of the chemical species elements, were published (almost exactly a century after Darwin’s treatise on the origin of biological species) by Burbidge, Burbidge, Fowler & Hoyle [5], and independently, by Cameron [6]. These works identified stars as Galactic factories that synthesize atomic nuclei via nuclear reactions. The stars return this processed material to the interstellar medium through various hydrostatic or explosive means during their evolution and thereby enrich the interstellar medium in metals, that is, any species heavier than He according to the astronomical jargon. Out of this matter new generations of stars are born and a new cycle of nuclear transmutations will be initiated. In fact, without concourse of the many nuclear processes that take place in the stars and that are discussed in this review, the Universe would have remained a chemically poor place, and probably no life form would have ever emerged. This review summarizes more than 50 years of progress in our understanding of the Nuclear astrophysics 3 Figure 1. Solar System isotopic abundances versus mass number, A. Abundances are based on meteoritic samples as well as on values inferred from the solar photosphere (Sect. 2). Values taken from Lodders [2]. nuclear processes that shaped the current chemical abundance pattern of the Universe. The structure of the paper is as follows. In Section 2, we briefly address the main aspects associated with the determination of the Solar System abundances. This is followed by an overview on Big Bang nucleosynthesis (Sect. 3), and by an account of the nuclear processes governing low-mass stars (Sect. 4). Particular attention is paid to the Standard Solar model and to lessons learned from neutrino physics. In Sect. 5, we outline the basic properties of red giant and AGB stars, with emphasis on the s-process. The series of explosive processes that characterize core-collapse supernovae and γ-ray bursts (e.g., p-, ν-, νp-, α-, and r-processes) are reviewed in Sect. 6. This is supplemented with an overview of different astrophysical scenarios that involve binary stellar systems, such as classical and recurrent novae (Sect. 7), type Ia supernovae (Sect. 8), X-ray bursts and superbursts (Sect. 9), and stellar mergers (Sect. 10). We briefly address as well non-stellar processes, such as spallation reactions (Sect. 11), and conclude with a number of key puzzles and future perspectives in this field (Sect. 12). Nuclear astrophysics 4 2. An insight into Solar System abundances The determination of the chemical composition of a distant star is not a trivial task. It basically relies on the analysis of the electromagnetic radiation emitted at different wavelengths (so-called spectra). For the case of the Solar System abundances, several sources of information, that combine radiation and matter samples, are used to this end: the Sun (mainly photospheric absorption lines), meteorites, planets and moons (mainly Earth and Moon). Recently, several space probes have for the first time carried material from the interplanetary medium to Earth. NASA’s Genesis mission collected about 0.4 mg of solar wind particles for two years, using 250 hexagonal (10 cm wide) wafers made of silicon, sapphire, diamond, and gold. During descent in 2007 the parachutes never opened and the capsule crashed, but fortunately the samples could be retrieved and analyzed. NASA’s Stardust mission, in turn, flew through the tail of comet Wild-2 in 2004 and captured dust particles at a speed of 6.1 km s−1 both in aerogel and in Al foils. Those samples were brought to Earth in 2006. Inferring the elemental composition of the Solar System from this variety of sources becomes a complex enterprise. First, Earth does not show a homogeneous chemical composition. Second, meteorites do not constitute a chemically unique class of objects, but different types exhibit large abundance differences. The usual approach to infer the bulk of Solar System abundances relies on a very rare and primitive type of meteorites: CI carbonaceous chondrites, representing the least modified and chemically unaltered class of meteorites. Named after the Ivuna meteorite, the CI carbonaceous chondrite class includes only four other members: Orgueil, Alais, Tonk and Revelstoke. Because of the extreme similarities of the abundances in CI chondrites (determined through mass spectroscopy, wet chemistry, X-ray fluorescence, and neutron activation) and those inferred from the solar photosphere (for non-volatile elements), it is believed that this class of meteorites represents pristine material from which the Solar System formed about 4.6 Gyr ago. Unfortunately, noble gases and other very volatile elements do not easily condense into solids. Hence, values inferred from solar photospheric spectra are used instead to account for the C, N or O abundances, while other specific techniques are adopted for some special elements, such as Ar, Kr, Xe, or Hg. Moreover, the meteoritic abundances have to be expressed relative to an element other than H. Silicon is the species traditionally chosen, with a defined number abundance of N(Si)=106. This requires renormalization of the meteoritic values, such that the photospheric and meteoritic Si abundances agree.
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