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The R-Process of Stellar Nucleosynthesis: Astrophysics and Nuclear Physics Achievements and Mysteries

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The R-Process of Stellar Nucleosynthesis: Astrophysics and Nuclear Physics Achievements and Mysteries The r-process of stellar nucleosynthesis: Astrophysics and nuclear physics achievements and mysteries M. Arnould, S. Goriely, and K. Takahashi Institut d'Astronomie et d'Astrophysique, Universit´eLibre de Bruxelles, CP226, B-1050 Brussels, Belgium Abstract The r-process, or the rapid neutron-capture process, of stellar nucleosynthesis is called for to explain the production of the stable (and some long-lived radioac- tive) neutron-rich nuclides heavier than iron that are observed in stars of various metallicities, as well as in the solar system. A very large amount of nuclear information is necessary in order to model the r-process. This concerns the static characteristics of a large variety of light to heavy nuclei between the valley of stability and the vicinity of the neutron-drip line, as well as their beta-decay branches or their reactivity. Fission probabilities of very neutron-rich actinides have also to be known in order to determine the most massive nuclei that have a chance to be involved in the r-process. Even the properties of asymmetric nuclear matter may enter the problem. The enormously challenging experimental and theoretical task imposed by all these requirements is reviewed, and the state-of-the-art development in the field is presented. Nuclear-physics-based and astrophysics-free r-process models of different levels of sophistication have been constructed over the years. We review their merits and their shortcomings. The ultimate goal of r-process studies is clearly to identify realistic sites for the development of the r-process. Here too, the challenge is enormous, and the solution still eludes us. For long, the core collapse supernova of massive stars has arXiv:0705.4512v1 [astro-ph] 31 May 2007 been envisioned as the privileged r-process location. We present a brief summary of the one- or multidimensional spherical or non-spherical explosion simulations available to-date. Their predictions are confronted with the requirements imposed to obtain an r-process. The possibility of r-nuclide synthesis during the decompression of the matter of neutron stars following their merging is also discussed. Given the uncertainties remaining on the astrophysical r-process site and on the involved nuclear physics, any confrontation between predicted r-process yields and observed abundances is clearly risky. A comparison dealing with observed r-nuclide abundances in very metal-poor stars and in the solar system is attempted on grounds of r-process models based on parametrised astrophysics conditions. The virtues of the r-process product actinides for dating old stars or the solar system are also Preprint submitted to Elsevier 4 February 2008 critically reviewed. 1 Introduction A myriad of observations provide a picture of the composition of the various constituents of the Universe that is getting quickly more and more complete, and concomitantly more and more complex. Despite this spectacular progress, the solar system (hereafter SoS) continues to provide a body of abundance data whose quantity, quality and coherence remain unmatched. This concerns especially the heavy elements (defined here as those with atomic numbers in excess of the value Z = 26 corresponding to iron), and in particular their isotopic compositions, which are the prime fingerprints of astrophysical nuclear processes. Except in a few instances, these isotopic patterns indeed remain out of reach even of the most-advanced stellar spectroscopic techniques available today. No wonder then that, from the early days of its development, the theory of nucleosynthesis has been deeply rooted in the SoS composition, especially in the heavy element domain. Since [1], it has proved operationally most rewarding to introduce three categories of heavy nuclides referred to as s-, p-, and r-nuclides. This splitting is not a mere game. It corre- sponds instead to the `topology' of the chart of the nuclides, which exhibits three categories of stable heavy nuclides: those located at the bottom of the valley of nuclear stability, called the s-nuclides, and those situated on the neutron-deficient or neutron-rich side of the valley, named the p- or r-nuclides, respectively. Three different mechanisms are called for to ac- count for the production of these three types of stable nuclides. They are naturally referred to as the s-, r-, and p-processes. An extensive survey of the p-process has been published recently [2], and an overview of the s-process in low- and intermediate mass stars has been prepared by [3]. The main aim of this review is to expand on the limited surveys of the r-process of [4,5] both in its astrophysics and nuclear physics components. It is of course impossible to do justice to the very abundant literature on the subject. It may not be desirable either to try to come close to exhaustiveness in this matter, as it would bring more confusion than otherwise. Also note that no work published or made available after 30 September 2006 is referred to in this review. 2 2 Observed abundances of the r-nuclides 2.1 The bulk solar system composition Much effort has been devoted over the years to the derivation of a meaningful set of elemental abundances representative of the composition of the bulk material from which the SoS formed some 4.6 Gy ago. This bulk material is made of a well-mixed blend of many nucleosynthesis agents that have contributed to its composition over the approximate 10 Gy that have elapsed between the formations of the Galaxy and of the SoS. The latest detailed analysis of the SoS is found in [6]. As in previous compilations, the selected abundances are largely based on the analysis of a special class of rare meteorites, the CI1 carbonaceous chondrites, which are considered as the least-altered samples of primitive solar matter available at present. Materials from other origins may exhibit substantial deviations from the CI1 in their elemental compositions. This results from the physio-chemical processes that may have operated at different levels in different phases of the solar system material. Solar spectroscopic data for some elements up to Fe have been reanalysed in the framework of time-dependent three-dimensional hydrodynamical atmosphere models, which has led to spectacular revisions of the solar photospheric abundances of some major elements lighter than Ne [7]. In general, the solar abundances now come in quite good agreement with the CI1 data for a large variety of elements. Some notable differences result from depletion in the Sun (Li) or in meteorites (H, C, N, O). The newly-determined solar Na abundance does not quite overlap with the meteoritic value. A marginal agreement (within quite large uncertainties) is found for Cl and Au, while the results for Ga, Rb, Ag, In and W are discordant. At least some of these discrepancies may be attributed to spectroscopic or atomic data problems. The abundances of the noble gases Ar, Kr and Xe, as well as of some specific elements like Hg, have still to rely on theoretical considerations. The isotopic composition of the elements in the SoS is mostly based on the terrestrial data, except for H and the noble gases [6], where some adjustments are also applied for Sr, Nd, Hf, Os, and Pb. The practice of using terrestrial isotopic data is justified by the fact that, in contrast to the elemental abundances, the isotopic patterns are not affected to any significant level by geological processes. Only some minor mass-dependent fractionation may operate. A notable exception to the high bulk isotopic homogeneity comes from the decay of relatively short-lived radio-nuclides that existed in the early SoS and decayed in early-formed solids in the solar nebula. Also interplanetary dust particles contain isotopic signatures apparently caused by chemical processes. Additional isotopic `anomalies' are observed in some meteoritic inclusions or grains. Isotopic anomalies in the SoS are discussed further in Sect. 2.4. The SoS nuclidic abundance distribution exhibits a high `iron peak' centred around 56Fe followed by a broad peak in the mass number A ≈ 80 − 90 region, whereas double peaks show up at A = 130 ∼ 138 and 195 ∼ 208. These peaks are superimposed on a curve 3 102 101 ] s 6 r s 100 r 10-1 10-2 10-3 p 10-4 -5 Abundances [Si=10 10 10-6 80 100 120 140 160 180 200 A Fig. 1. Decomposition of the solar abundances of heavy nuclides into s-process (solid line), r-pro- cess (dots) and p-process (squares) contributions. The uncertainties on the abundances of some p-nuclides that come from a possible s-process contamination are represented by vertical bars (from [2]). See Figs. 3 - 5 for the uncertainties on the s- and r-nuclide data decreasing rapidly with increasing A. It has been realised very early that these peaks provide a clear demonstration that a tight correlation exists between SoS abundances and nuclear neutron shell closures. 2.2 The s-, r- and p-nuclides in the solar system: generalities It is very useful to split the abundance distribution of the nuclides heavier than iron into three separate distributions giving the image of the SoS content of the p-, s- and r-nuclides. A rough representation of this splitting is displayed in Fig. 1. In its details, the procedure of decomposition is not as obvious as it might be thought from the very definition of the different types of nuclides, and is to some extent dependent on the models for the synthesis of the heavy nuclides. These models predict in particular that the stable nuclides located on the neutron-rich/neutron-deficient side of the valley of nuclear stability are produced, to a first good approximation, by the r-/p-process only.
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