Neutron reactions in astrophysics Reifarth, R; Lederer, C; Käppeler, F
Journal of physics. G, Nuclear and particle physics,
05/2014, Letnik:
41, Številka:
5
Journal Article
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The quest for the origin of matter in the Universe had been the subject of philosophical and theological debates over the history of mankind, but quantitative answers could be found only by the ...scientific achievements of the last century. A first important step on this way was the development of spectral analysis by Kirchhoff and Bunsen in the middle of the 19th century, which provided first insight in the chemical composition of the sun and the stars. The energy source of the stars and the related processes of nucleosynthesis, however, could be revealed only with the discoveries of nuclear physics. A final break-through came eventually with the compilation of elemental and isotopic abundances in the solar system, which reflect the various nucleosynthetic processes in detail. This review focuses on the mass region above iron, where the formation of the elements is dominated by neutron capture, mainly in the slow (s) and rapid (r) processes. Following a brief historic account and a sketch of the relevant astrophysical models, emphasis is put on the nuclear physics input, where status and perspectives of experimental approaches are presented in some detail, complemented by the indispensable role of theory.
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The primary aim of experimental nuclear astrophysics is to determine the rates of nuclear reactions taking place in stars in various astrophysical conditions. These reaction rates are an important ...ingredient for understanding the elemental abundance distribution in our solar system and the galaxy. The reaction rates are determined from the cross sections which need to be measured at energies as close to the astrophysically relevant ones as possible. In many cases the final nucleus of an astrophysically important reaction is radioactive which allows the cross section to be determined based on the off-line measurement of the number of produced isotopes. In general, this technique is referred to as the activation method, which often has substantial advantages over in-beam particle- or
-detection measurements. In this paper the activation method is reviewed from the viewpoint of nuclear astrophysics. Important aspects of the activation method are given through several reaction studies for charged particle, neutron and
-induced reactions. Various techniques for the measurement of the produced activity are detailed. As a special case of activation, the technique of Accelerator Mass Spectrometry in cross section measurements is also reviewed.
The slow neutron capture process in massive stars (weak s process) produces most of the s-process isotopes between iron and strontium. Neutrons are provided by the {sup 22}Ne(alpha,n){sup 25}Mg ...reaction, which is activated at the end of the convective He-burning core and in the subsequent convective C-burning shell. The s-process-rich material in the supernova ejecta carries the signature of these two phases. In the past years, new measurements of neutron capture cross sections of isotopes beyond iron significantly changed the predicted weak s-process distribution. The reason is that the variation of the Maxwellian-averaged cross sections (MACS) is propagated to heavier isotopes along the s path. In the light of these results, we present updated nucleosynthesis calculations for a 25 M{sub sun} star of Population I (solar metallicity) in convective He-burning core and convective C-burning shell conditions. In comparison with previous simulations based on the Bao et al. compilation, the new measurement of neutron capture cross sections leads to an increase of s-process yields from nickel up to selenium. The variation of the cross section of one isotope along the s-process path is propagated to heavier isotopes, where the propagation efficiency is higher for low cross sections. New {sup 74}Ge, {sup 75}As, and {sup 78}Se MACS result in a higher production of germanium, arsenic, and selenium, thereby reducing the s-process yields of heavier elements by propagation. Results are reported for the He core and for the C shell. In shell C-burning, the s-process nucleosynthesis is more uncertain than in the He core, due to higher MACS uncertainties at higher temperatures. We also analyze the impact of using the new lower solar abundances for CNO isotopes on the s-process predictions, where CNO is the source of {sup 22}Ne, and we show that beyond Zn this is affecting the s-process yields more than nuclear or stellar model uncertainties considered in this paper. In particular, using the new updated initial composition, we obtain a high s-process production (overproduction higher than {sup 16}O, {approx}100) for Cu, Ga, Ge, and As. Using the older abundances by Anders and Grevesse, also Se, Br, Kr, and Rb are efficiently produced. Our results have important implications in explaining the origin of copper in the solar abundance distribution, pointing to a prevailing contribution from the weak s-process in agreement with spectroscopic observations and Galactic chemical evolution calculations. Because of the improvement due to the new MACS for nickel and copper isotopes, the nucleosynthesis of copper is less affected by nuclear uncertainties compared to heavier s-process elements. An experimental determination of the {sup 63}Ni MACS is required for a further improvement of the abundance prediction of copper. The available spectroscopic observations of germanium and gallium in stars are also discussed, where most of the cosmic abundances of these elements derives from the s-process in massive stars.
Nuclear reaction rates play a critical role in the understanding of stellar evolution and explosions. However, in many cases nuclear reaction rates still carry large uncertainties due to the paucity ...of experimental data and incomplete theoretical understanding of the underlying reaction mechanisms. New experimental methods and techniques, combined with the development of new theoretical tools, have exposed fresh avenues to pursue nuclear reactions of significance for nucleosynthesis at, or near, the actual temperatures of stellar burning. This review provides an overview of the most critical nuclear reactions for a number of nucleosynthesis environments. It also presents the current status of these reactions and provides insight into the specific uncertainties associated with the reaction rates. We identify existing shortcomings in the data and highlight the needs and opportunities for additional future experiments.
High-resolution spectroscopic observations of 100 metal-poor carbon and s-rich stars (CEMP-s) collected from the literature are compared with the theoretical nucleosynthesis models of the asymptotic ...giant branch (AGB) presented in Paper I (M
AGB
ini= 1.3, 1.4, 1.5, 2 M⊙, − 3.6 ≲ Fe/H ≲− 1.5). The s-process enhancement detected in these objects is associated with binary systems: the more massive companion evolved faster through the thermally pulsing AGB phase (TP-AGB), synthesizing s-elements in the inner He intershell, which are partly dredged up to the surface during the third dredge-up (TDU) episode. The secondary observed low-mass companion became CEMP-s by the mass transfer of C- and s-rich material from the primary AGB.
We analyse the light elements C, N, O, Na and Mg, as well as the two s-process indicators, hs/ls (where ls =〈Y, Zr〉 is the the light-s peak at N = 50 and hs =〈La, Nd, Sm〉 the heavy-s peak at N = 82) and Pb/hs. We distinguish between CEMP-s with high s-process enhancement, hs/Fe >rsim 1.5 (CEMP-sII), and mild s-process enhanced stars, hs/Fe < 1.5 (CEMP-sI). To interpret the observations, a range of s-process efficiencies at any given metallicity is necessary. This is confirmed by the high spread observed in Pb/hs (∼2 dex). A degeneration of solutions is found with some exceptions: most main-sequence CEMP-sII stars with low Na/Fe can only be interpreted with M
AGB
ini= 1.3-1.4 M⊙. Giants having suffered the first dredge-up (FDU) need a dilution >rsim1 dex (dil is defined as the mass of the convective envelope of the observed star, M
obs
★, over the material transferred from the AGB to the companion, M
trans
AGB). Then AGB models with higher AGB initial masses (M
AGB
ini= 1.5-2 M⊙) are adopted to interpret CEMP-sII giants. In general, solutions with AGB models in the mass range M
AGB
ini= 1.3-2 M⊙ and different dilution factors are found for CEMP-sI stars.
About half of the CEMP-s stars with europium measurements show a high r-process enhancement (CEMP-s/r). The scenario for the origin of CEMP-s/r stars is a debated issue. We propose that the molecular cloud from which the binary system formed was previously enriched in r-process elements, most likely by local SN II pollution. This initial r-enrichment does not affect the s-process nucleosynthesis. However, for a high r-process enrichment (r/Feini= 2) the r-process contributions to solar La, Nd and Sm (30, 40 and 70 per cent) have to be considered. This increases the maximum hs/ls up to ∼0.3 dex. CEMP-s/r stars reflect this behaviour, showing higher hs/ls than observed in CEMP-s on average.
Detailed analyses for individual stars will be provided in Paper III.
We provide an individual analysis of 94 carbon-enhanced metal-poor stars showing an s-process enrichment (CEMP-s) collected from the literature. The s-process enhancement observed in these stars is ...ascribed to mass transfer by stellar winds in a binary system from a more massive companion evolving faster towards the asymptotic giant branch (AGB) phase. The theoretical AGB nucleosynthesis models have been presented in Bisterzo et al. (Paper I of this series). Several CEMP-s show an enhancement in both s- and r-process elements (CEMP-s/r). In order to explain the peculiar abundances observed in CEMP-s/r, we assume that the molecular cloud from which CEMP-s formed was previously enriched in r-elements by supernova pollution.
A general discussion and the method adopted in order to interpret the observations have been provided in Bisterzo et al. (Paper II of this series). We present in this paper a detailed study of spectroscopic observations of individual stars. We consider all elements from carbon to bismuth, with particular attention to the three s-process peaks, ls (Y, Zr), hs (La, Nd, Sm) and Pb, and their ratios hs/ls and Pb/hs. The presence of an initial r-process contribution may be typically evaluated by La/Eu. We found possible agreements between theoretical predictions and spectroscopic data. In general, the observed Na/Fe (and Mg/Fe) provides information on the AGB initial mass, while hs/ls and Pb/hs are mainly indicators of the s-process efficiency. A range of 13C-pocket strengths are required to interpret the observations. However, major discrepancies between models and observations exist. We highlight star by star the agreements and the main problems encountered and, when possible, we suggest potential indications for further studies. These discrepancies provide starting points of debate for unsolved problems in which spectroscopic and theoretical studies may intervene.
A large sample of carbon-enhanced metal-poor stars enriched in s-process elements (CEMP-s) have been observed in the Galactic halo. These stars of low mass (M∼ 0.9 M⊙) are located on the ...main-sequence or the red-giant phase, and do not undergo third dredge-up (TDU) episodes. The s-process enhancement is most plausibly due to accretion in a binary system from a more massive companion when on the asymptotic giant branch (AGB) phase (now a white dwarf). In order to interpret the spectroscopic observations, updated AGB models are needed to follow in detail the s-process nucleosynthesis. We present nucleosynthesis calculations based on AGB stellar models obtained with Frascati Raphson-Newton Evolutionary Code (franec) for low initial stellar masses and low metallicities. For a given metallicity, a wide spread in the abundance of the s-process elements is obtained by varying the amount of 13C and its profile in the pocket, where the 13C(α, n)16O reaction is the major neutron source, releasing neutrons in radiative conditions during the interpulse phase. We also account for the second neutron source 22Ne(α, n)25Mg, partially activated during convective thermal pulses. We discuss the surface abundance of elements from carbon to bismuth, for AGB models of initial masses M= 1.3–2 M⊙, low metallicities (Fe/H from −1 down to −3.6) and for different 13C-pocket efficiencies. In particular, we analyse the relative behaviour of the three s-process peaks: light-s (ls at magic neutron number N= 50), heavy-s (hs at N= 82) and lead (N= 126). Two s-process indicators, hs/ls and Pb/hs, are needed in order to characterize the s-process distribution. In the on-line material, we provide a set of data tables with surface predictions. Our final objective is to provide a full set of theoretical models of low-mass low-metallicity s-process-enhanced stars. In a forthcoming paper, we will test our results through a comparison with observations of CEMP-s stars.
The astrophysical p-process, which is responsible for the origin of the proton-rich stable nuclei heavier than iron, was investigated using a full nuclear reaction network for a Type II supernova ...explosion when the shock front passes through the O/Ne layer. Calculations were performed with a multilayer model adopting the seed of a preexplosion evolution of a 25 M( star. The reaction flux was calculated to determine the main reaction path and branching points responsible for synthesizing the proton-rich nuclei. In order to investigate the impact of nuclear reaction rates on the predicted p-process abundances, extensive simulations with different sets of collectively and individually modified neutron-, proton-, and a-capture and photodisintegration rates have been performed. These results are not only relevant to explore the nuclear-physics-related uncertainties in p-process calculations but are also important for identifying the strategy and planning of future experiments.
ABSTRACT The solar s-process abundances have been analyzed in the framework of a Galactic Chemical Evolution (GCE) model. The aim of this work is to implement the study by Bisterzo et al., who ...investigated the effect of one of the major uncertainties of asymptotic giant branch (AGB) yields, the internal structure of the 13C pocket. We present GCE predictions of s-process elements computed with additional tests in the light of suggestions provided in recent publications. The analysis is extended to different metallicities, by comparing GCE results and updated spectroscopic observations of unevolved field stars. We verify that the GCE predictions obtained with different tests may represent, on average, the evolution of selected neutron-capture elements in the Galaxy. The impact of an additional weak s-process contribution from fast-rotating massive stars is also explored.
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