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Frontiers of Nuclear Astrophysics
Nuclear Physics A 805 (2008) 303c–312c www.elsevier.com/locate/nuclphysa
Frontiers of Nuclear Astrophysics Grant J. Mathews Center for Astrophysics/JINA, Department of Physics, University of Notre Dame
Abstract The main goals of nuclear astrophysics have been to probe the interiors of stars, stellar explosions, the early moments of cosmic expansion, and the formation and evolution of galaxies and cosmic structure by measurement and application of the relevant nuclear physics. The approach to these goals have generally been from three directions: 1) Careful measurements of the relevant nuclear reactions; 2) Detailed computer models of the relevant astrophysical environments; and 3) Observations of the relevant terrestrial and extra-terrestrial atomic and isotopic abundances. These approaches provide not only insight into the formation and evolution of the elements, but are also pillars upon which a variety of cosmological models as well as models for physics beyond the standard model of particle physics can stand or fall. At present there is a very exciting frontier on all three of these approaches. The development and applications of radioactive-ion-beam and low-background facilities have begun to clarify the input nuclear physics. The development of hydrodynamic stellar-evolution and explosion models in three spatial dimensions, along with detailed radiation and neutrino transport has provided new and unexpected insights into some of the deep mysteries regarding the origin and evolution of the elements. Also, for the first time in history, ground and space-based observations of elemental abundances in stars and gas are being made from the time of the very first stars and cosmic structures to the present. Observation of the cosmic microwave background can now also be used to analyze the development of structure in the universe from before the time of photon decoupling to the present. These observations provide unprecedented views of the history of cosmic evolution. They also provide new questions. In this review we summarize some some of the developments in each of these areas and highlight the exciting frontiers where new breakthroughs are likely to occur.
Key words: Nucleosythesis, Nuclear Reactions, nuclear properties, radioactive ion beams, Stellar interiors, Stellar Evolution, Thermonuclear burning in stars, s-process, r-process, big bang nucleosynthesis PACS: 26.20.+f, 26.30.+k, 26.35.+c, 26.40.+v, 26.50.+x, 26.60.+c, 97.10.-q, 97.10.Cv, 97.10.Gz, 97.10.Th, 97.60.Jd, 98.80.Ft
0375-9474/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2008.02.258
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1. Background It is an honor to have been asked to prepare a review paper on such an exciting topic. Indeed, this is a very special time in the history of nuclear astrophysics for several reasons. For one, this year marks the 50th year since the appearance of the seminal papers by Burbidge, Burbidge, Fowler and Hoyle [1] and Cameron [2]. These papers are often referenced as the beginnings of this field, though the topic of the the synthesis of the elements in stars was not entirely new at that time. Nevertheless, these papers provided us with names for a host of nucleosynthesis processes in stars, most of which endure to the present time. At the same time two other crucial developments have occurred in recent history. A new generation of accelerators for radioactive ion beams are either proposed or coming on line along with low background underground facilities [3]. This is occurring in concert with the development of new astronomical surveys (e.g. SDSS/SEGUE [4]), space based satellite observatories at multiple wavelengths (e.g. HST, Chandra, Spitzer, WMAP) and next-generation large ground based observatories (e.g. the Subaru, Keck, Gemini, VLT, and LBT telescopes). At the same time, computational technology has finally reached the point where fully three dimensional simulations of stars and astrophysical environments is possible with a level of complexity approaching the real world. The convergence of these various developments makes the present time exciting indeed. 2. Nucleosynthesis At the heart of what we do in nuclear astrophysics is the unraveling of the complexities observed in nature in terms of the underlying nuclear processes. One of the great achievements in the B2 FH was to identify and name the nuclear processes within stars. It is useful to summarize the processes as identified then and contrast them with the modern counterparts. For example, what was referred to as the x-process for producing the isotopes of Li, Be and B in B2 FH is now believed to represent a combination of big-bang nucleosynthesis, galactic cosmic ray nucleosynthesis, and neutrino-induced nucleon emission in the ν-process. Although the basic processes of hydrogen and helium burning in the cores or burning shells of main sequence and giant stars remain as outlined in B2 FH, a number of new processes have been added. These include hot hydrogen burning and the rp-process which may occur on accreting compact neutron stars or white dwarfs and also the ν- and νp-processes which occur in supernova shells. What was called the α process is now attributed to helium burning plus C/O/Ne burning. The e-process of B2 FH to form iron group elements is now identified with quasi-equilibrium silicon burning in supernova progenitors and explosive nucleosynthesis. 3. Frontiers in stellar nuclear astrophysics Thermonuclear Reaction Rates Understanding stellar interiors and the associated nuclear processes has been a main focus of nuclear astrophysics for the past fifty years. Indeed, there remain many questions regarding various thermonuclear burning processes [3]. Among the most important outstanding nuclear reaction rate issues discussed in this conference I refer the reader to the interesting presentations given on the 3 He(α, γ)7 Be reaction [5] which plays a key role in
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big bang nucleosynthesis as well as hydrogen burning; the 7 Be(p, γ)8 B reaction [6] also of importance in hydrogen burning, the 14 N(p, γ)15 O reaction [7] relevant to the the CNO cycle and globular cluster ages, and the most challenging 12 C(α, γ)16 O reaction whose uncertainty leads to a large ambiguity in the core mass and nucleosynthesis of massive stars [3,8]. Other nuclear uncertainties in thermonuclear reaction rates discussed below. There is, however, the major question of the physics of the stellar interiors themselves. This makes for yet another frontier which I would state as: What is the complex interplay between the nuclear reactions and turbulent convection in stars? For most of the past 50 years we have had to be content with modeling stellar interiors in terms of one dimensional spherical systems with mixing length theory as the description for convection. However, stellar interiors are much more complex environments. For the first time we have the tools to address them based upon the development of fully three-dimensional stellar evolution codes. One code with which I have had a small involvement is the Dejehuty code [9,10] developed at the Lawrence Livermore National Laboratory. This code uses an Adaptive Euler-Lagrange scheme which is second order accurate in space and time to fully model the convection and thermonuclear burning in stellar interiors. The mesh is constructed of multi-block logically rectangular nonorthogonal hexahedrons which can be decomposed into slices for parallel operation. In this way stars have been resolved into up to a billion zones. Such studies have been then used to analyze the underlying convection and radiation transport. 1) What is the role of convection on nucleosynthesis? As an example of the subtleties which could not have been detected in simple in spherical treatments, a study of the formation of 3 He in low mass stars was undertaken in [10]. The problem is that 1D models for low mass stars produce a large amount of 3 He. This 3 He would be transported to the surface when the star becomes deeply convective late in its evolution. The observed abundances of 3 He, however, are little different from the yields of big bang nucleosynthesis discussed below. It is unknown as to how this overproduction of 3 He is avoided, but it is clear from globular cluster data, that the 3 He is already destroyed before the star undergoes its core helium flash. A good candidate environment for 3 He destruction [12] is the deepest penetration of the outer convective zone during the first ascent up the giant branch. A star in this phase was analyzed [10] with the 3D Dejehuty code. It was discovered that clouds of hydrogen depleted material began to appear above the hydrogen burning shell. This material was depleted by the 3 He(3 He,2p)4 He reaction and then slowly transported to the outer convective region by buoyancy forces. This exciting result may finally explain the puzzle of 3 He. In other work this tool has been used to provide insight into convective overshooot and the core helium flash [10]. 2) How are the heavy s-process nuclei formed? One issue of particular interest for nuclear astrophysics has been the astrophysical site for slow neutron capture reactions in the s-process. Even at the time of B2FH it was already apparent from astronomical observations of radioactive 99 Tc in stars that the s-process must occur in red giants. The problem, however, has been to deduce the precise mechanism by which the correct distribution of neutron exposures could be produced within the stars. In the most popular model [13] the s-process occurs in a ”pocket” of 13 C which has been formed by mixing between the outer hydrogen convective shell and the inner degenerate 12 C core. Within
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this pocket it is believed that quiescent neutrons are produced via the 13 C(α, n)16 O reaction. When the 3α →12 C reaction ignites at the base of the helium shell, the temperature rises sufficiently that the 22 Ne(α, n)25 Mg reaction also contributes leading to the desired distribution of neutron exposures and a satisfactory pattern of s-process abundances. The problem with this scenario, however, is that it relies on a poorly understood parameter for the mixing of protons and 12 C into the helium shell. Some very impressive advances in the understanding of this mixing problem have been made in calculations by the Los Alamos group [14]. These results, however, are limited to two dimensions and to a limited region of the star. It remains a frontier to convincingly model this important mixing and nucleosynthesis process. 3) What is the role of convection and new nucleosynthesis processes in the first generation of stars? The study of the peculiar abundances in the first generaqtions of stars is a very active area of curren research [4]. Regarding the s-process for example, there is yet another mystery[15] surrounding the s-process in the first stars formed in the Galaxy. There appears to be good evidence [16] for the occurrence of an sprocess driven by the hydrogen shell flash in a subset of low-metallicity stars. Unraveling this possibility is sure to be a frontier in the coming years. Nuclear Reactions in the s-process Regarding the nuclear reactions of importance to s-process nucleosynthesis, the reader is referred to some very interesting new measurements described in this conference proceedings. These include: the neutron source reactions 13 C(α, n)16 O [17], and also the 22 Ne(α, n)25 Mg reaction. In addition, it is important to better quantify the various neutron poison reactions such as the 12 C(n, γ)13 C and 17 O(n, γ)18 O reactions[18] and some key neutron capture reactions along the s-process path, particularly at the branch points due to radioactive isotopes, e.g 79 Se(n, γ)80 Se [19] and the 93 Zr(n, γ)94 Zr reaction [20] for which interesting experimental techniques have been developed. 4) What are the details of convection, neutrino and nuclear reactions in core collapse supernovae? Another very important frontier in nuclear astrophysics involves the mechanism by which core-collapse supernovae explode and the possible associated rapid neutron capture nucleosynthesis in the r-process and also their shock-induced explosive nucleosynthesis in the outer envelopes. Regarding the explosion mechanism itself, it remains true that even after many decades of effort, detailed models for the core collapse of a massive star to form a Type-II supernova are still being worked out. There is now wide acceptance of the importance of delayed neutrino heating (cf. [21]). Nevertheless, in supernova most models with pure spherical symmetry, the neutrino luminosity from the proto-neutron star is too low to heat the infalling material sufficiently to expel matter from the star (c.f.. [22,23] and references therein). There is, however one model which does produce an explosion with the right characteristics [21,23]. One feature of the successful model is the complex interplay between neutrino-nuclear scattering and convection. For example, one key ingredient is that the proto-neutron star becomes hydrodynamically unstable a few hundred milliseconds after the core bounce due to the so-called ”neutron-finger instability” [21]. This EOS-dependent instability arises from the build up of a high neutron to proton ratio near the surface of the proto-neutron star to a sufficiently high value so as to over come the buoyancy of the higher entropy near the surface. Alternatively, magnetic convection induced by the magnetic-rotationinstability could contribute [23].
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Recently, however, simulations in two and three spatial dimensions have begun to show consistent explosions. The 2D and 3D models of the Max Planck group [24] shows the development of an explosion. More insight is suggested in the work of the Oak Ridge group [25] In whose models there is clear evidence that the explosions in 3D are related to the turbulent mixing of oxygen from the outer layers into the high entropy bubble behind the shock where the small additional input of energy from thermonuclear oxygen burning is enough to push forward the explosion. 5) How are the heavy r-process nuclei formed? The precise details for r-process nucleosynthesis has remained a mystery even though it is responsible for the abundance of about half of the nuclei heavier than iron. Nevertheless, among the many proposals (cf. [26]) for its origin, core-collapse supernovae have remained the most likely site in which the necessary ejected mass and neutrons per seed can be achieved. Ultimately, however, if one is ever to truly identify the site for the r-process it is important to explore r-process nucleosynthesis in the context of detailed supernova simulations (cf. [21]) in three dimensions. More importantly, however, the main reason for the difficulty in unraveling the r-process site has been the fact that the actual nuclear data on and near the r-process path far from stability is still poorly known. There are a variety of nuclear physics issues far from stability that must be resolved in order to understand the r-process. These include: thermonuclear reaction rates, electron capture rates, and photodisintegration rates relevant to the pre-collapse; Pauli blocking of electron captures and νe scattering and absorption relevant to collapse; the high density EOS relevant to core bounce; scattering and absorption of all three neutrino flavors relevant to neutron-star cooling and the formation of the high entropy bubble; for nucleosynthesis we need neutrino nucleus scattering/absorption, light-element reactions, alpha-rich freezeout, (n, γ), (γ, n), nuclear masses and neutron separation energies, Qβ , beta lifetimes, beta-delayed neutron emission, partition functions, spins, parities, energy levels, neutron-induced fission, fission yields, and explosive nuclear reaction rates. It is indeed exciting how much we have heard at this conference of the new developments in the use of radioactive ion beams. The accelerator at RIKEN has produced its first uranium beam and indeed has already produced some new Sn isotopes near the neutron drip line [27]. At the same time we have heard of exciting new experiments [28] at the NSCL of Michigan State University, as well as ISOLDE [29] to measure beta-decay half lives near the A = 130 r-process peak and FRS-GSI [30] to measure near the A = 195 peak. We have also learned of the development of a number of beams for isotopes along the r-process path at ORNL [31] for application to indirect measurements (See also [32]). Hope also remains that a Radioactive Ion Beam Facility will be built in the US. Many sites for the r-process have been proposed [26]. Indeed, it is quite clear that there must be at least 2 r-process sites [33]. Perhaps, the most popular model is that , of [34] in which the r-process occurs the high entropy bubble above a nascent proto-neutron star in a core-collapse supernova. In this region the entropy is so high that the nuclear statistical equilibrium (NSE) favors abundant free neutrons and alpha particles plus a few heavy nuclei which experience rapid neutron capture. This is, therefore, an ideal r-process site which satisfies the requirement from observations (e.g. [35,36]) that the relative yields be metallicity independent and not too much material be produced. Using this model, a fit was obtained [34] to the Solar r-process abundance pattern for heavy A > 100 nuclei. A number of issues remain, regarding the viability of this model. For example, a fast timescale may be necessary [37,38], possible associated with
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the formation of jets [39]. It remains a frontier to identify the r-process site. In addition to the above issues, there a few more which come to mind. For completeness, I list them here: 6) What is the effect of fission yields on r-process nucleosynthesis? This is an area of considerable recent interest [40,41] 7) What is the correct model for nuclear flame propagation and detonation in Type Ia supernova explosions? 8)What are the effects on binary evolution on nucleosynthesis? 9)What are the effects of rotation/ magnetic field driven convection on nucleosynthesis?
4. Frontiers in Compact Stars/EOS The question of the equation of state relevant to neutron stars and supernovae is still actively being studied [42]. The possibility of a phase transition to a QCD plasma at hight temperature and/or density is an active area of nuclear research [43,44]. Such a transition if also of import in nuclear astrophysics. There is, for example a possibility [45] of observable effects on primordial nucleosynthesis from a QCD phase transition in the early universe. There is also renewed interest in the effects of such a transition on compact remnants and supernovae. All of these questions fall under the frontier question: What is the equation of state at high density? Among the related questions are: 1) How does it affect binary neutron-star inspiral/ merger/ gravity waves? 2) Does strange quark matter exist in stars? The possible existence of strange-matter stars has been speculated upon for some time. Most of the work concerning their nature and origin has focused on a number of neutron stars which are too compact or are cooling too rapidly to be comprised of normal nuclear matter [46]. There is also the possibility of white dwarfs with strange-matter cores [47], i.e. strange dwarfs. Strange-matter white dwarfs could gradually form during the progenitor mainsequence by the accretion of a strange-matter nugget. Such nuggets could exist either as a relic of the early universe or as an ejected fragment from the merger/coalescence of strange-matter neutron stars. Once captured by a star, strange-matter nuggets would gravitationally settle to the center and begin to convert normal matter to strange matter. The most distinguishing characteristic of the existence of strange-matter stars is that they must have a smaller radius. This unavoidable consequence simply follows from the fact that strange-matter has more degrees of freedom and can therefore be more compact than ordinary electron-degenerate matter. Indeed, recent work [47] has identified several candidate stars whose compact cores may be identifiable through astrosisemology. Before leaving this topic on compact stars, I would should also mention another important question which is of considerable interest [48,49] regarding compact remnants: 3) What are the effects of the rp-process on accreting neutron stars, X-ray bursts, Novae?
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5. Frontiers in neutrino nuclear astrophysics There are a number of exciting recent developments in neutrino nuclear physics. In nuclear astrophysics one could broadly characterize the frontier as: What are the Neutrino effects on supernovae, nucleosynthesis and cosmology? Regarding supernovae, some of the importance of neutrino effects on core collapse supernovae were described above. There is a developing appreciation for the importance of many higher order effects in neutrino-nuclear scattering. For example we have found the neutrino weak magnetism [50] can have a very important effect at late times in the explosion. Related questions include: 1) What is the detailed relativistic neutrino transport in supernovae? 2) Are there neutrino oscillations in supernovae? 3) Neutrino effects on r-process nucleosynthesis? Regarding these points, there is considerable interest in the possibility of detecting [51] the relic neutrinos from supernovae. There is also the exciting development [40,52] of evidence for a new nucleosynthesis process, called the νp-process. Recent hydrodynamical models for core-collapse supernovae show the presence of a proton-rich region surrounding the freshly born neutron star. Neutrino-induced reactions within this proton-rich environment could produce medium-mass nuclei including the previously unexplained isotopes of 92,94 Mo and 96,98 Ru. There is also continued interest in neutrino-induced nucleon emission in the ν-process [53] and a new possibility [54] that 138 La produced by the ν-process is a new cosmo-chronometer. There is also considerable interest in the question: 4) Are there observable effects from heavy sterile neutrinos? A number of related questions are: Is it absolutely stable? Is there one or several? Does it oscillate? Does it affect supernovae? Does it affect nucleosynthesis? Does it affect dark matter Does it affect dark energy? How and when are they produced? Why does it have its present density? What is its role in structure formation? In this regard, It is of some interest that a decaying heavy sterile neutron might be a candidate for both the universal dark matter and the dark energy responsible for the current cosmic acceleration [55] 6. Frontiers in Big Bang Nucleosynthesis Big-bang nucleosynthesis (BBN) plays a crucial role in constraining our views on the universe [56]. It is essentially the only probe of physics in the early radiation dominated epoch during the interval from ∼ 1 − 104 sec. Thus, it is imperative to have accurate predictions of the light element nuclear reactions which take place during this era as well as reliable determinations of primordial abundances from observation. The power of big bang nucleosynthesis comes in part from the fact that the nuclear physics is so accessible. Once the nuclear reaction rates are specified [57], all light abundances are a simple function of the baryon-to-photon ratio η during the nucleosynthesis epoch [58–60]. Nevertheless, some uncertainties remain. For years this was the chief uncertainty in the use of BBN as a probe of the early
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universe. However, the WMAP measurement [61] of the power spectrum of fluctuations in the cosmic microwave background has determined the baryon to photon ratio to high precision due to the effects of ”baryon drag” on the acoustic peaks in the CMB power spectrum. We now have from WMAP a value of η10 = 6.13 ± 0.25, or (Ωb h2 = 0.0224 ± 0.0009), where η10 is the baryon-to-photon ratio in in units of 10−10 , Ωb is the fraction of the closure density in baryons, and h = 0.71 ± 0.08 is the Hubble parameter in units of 100 km s−1 Mpc−1 . At the same time, the dark energy and dark matter content have been fixed primarily by combining the WMAP results with observations of supernovae at high redshift to be ΩCDM = 0.22±0.01 while the dark energy content is ΩΛ = 0.73±0.04. Is there Evidence for non-standard physics in the early universe? A crucial test of the standard BBN (and/or the existence of new physics in the early) is whether a single value of η10 can be found which reproduces all of the observed primordial abundances. The different light element abundances are determined by different means. This makes each determination an important independent check on BBN. Primordial deuterium is best determined from its absorption line in high redshift Lyman α clouds. The average of measurements of six absorption-line systems towards five −5 QSOs gives [62] D/H = 2.78+0.44 . This would imply an value of η10 = 5.9 ± 0.5. −0.38 × 10 This is also very close to the value Ωb h2 = 0.0224 ± 0.0009 (η10 = 6.13 ± 0.25) deduced from WMAP [61]. Is 4 He overproduced in BBN models? The primordial helium abundance is obtained by measuring extragalactic HII regions in low-metallicity irregular galaxies. Often in the past, the deduced primordial helium abundance Yp resided in one of two possible values (a low value, e.g. Yp = 0.238 ± 0.002 ± 0.005, [63] and a high value Yp = 0.2452±0.0015 [64]). However, the combined deuterium and WMAP constraints on the baryon-to-photon ratio implies a high primordial helium abundance [60] Yp = 0.2479±0.0004. There is also, however, a current dilemma regarding the uncertainty in the observationally determined primordial helium abundance. Many recent evaluations [64] give a rather narrow range of abundance uncertainty. On the other hand, the extent of systematic errors in these analyses is still being debated. In Ref. [65] it was concluded that correlations in various uncertainties could stretch the error to 0.232 ≤ Yp ≤ 0.258. Clearly, it is important to frontier to better quantify the primordial helium abundance. There is also a nuclear physics issue. The primordial helium abundance is sensitive to the neutron lifetime. New measurements of the neutron half-life might explain [66] the overproduction of 4 He in BBN models. Is 7 Li overproduced in BBN models? The primordial lithium abundance is inferred from old low-metallicity halo stars. Such stars exhibit a nearly constant plateau lithium abundance as a function of surface tem−10 perature. The inferred primordial abundance is [67] 7 Li= 1.23+0.68 . There is, −0.32 × 10 7 however, some controversy [68] concerning the depletion of Li on the surface of such halo stars. This implies that the observed 7 Li abundance is a factor of three smaller than that inferred from the BBN model. Is 6 Li produced in the Big Bang? It has recently been pointed out (e.g. [69]) that the abundances of both 6 Li and 7 Li observed in metal poor halo stars are not in agreement with those predicted from standard BBN. Specifically, the 6 Li abundance as a function of metallicity exhibits a plateau similar to that for 7 Li in very metal-poor stars, suggesting a primordial origin for both isotopes. This 6 Li abundance, however, is a factor of ∼ 103 larger than that predicted by BBN.
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A long-standing effort in cosmology has been the search for evidence of unstable particles that might have existed in the early universe. It is thus natural to ask whether the two lithium abundance anomalies might be a manifestation of the existence of such a particle. In this context a number of possible solutions to the 6 Li problem have been proposed which relate the Li anomalies to the possible existence of unstable particles in the early universe ([70] and Refs. therein). This has been extended in several recent studies [71,72] to consider heavy negatively charged decaying particles that modify BBN. This hypothesis is motivated by identifying the X − as the supersymmetric partner of the tau lepton. In this paradigm, the heavy X − particles bind to the nuclei produced in BBN to form X-nuclei. The massive X − particles would be bound in orbits with radii comparable to those of normal nuclei. Hence, they would reduce the reaction Coulomb barriers thereby enhancing the thermonuclear reaction rates and extending the duration of BBN to lower temperatures. Particular nuclear reactions of X − particles can produce a is that a large enhancement of the 6 Li abundance [72]. One example is the transfer reaction involving an X − bound to 4 He, i.e. 4 HeX (d,X − )6 Li. It has been shown [72] that this approach solves both the 6 Li and 7 Li problems simultaneously. Acknowledgments Work at the University of Notre Dame supported by the US Department of Energy under Nuclear Theory grant DE-FG02-95ER40934. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
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Frontiers of Nuclear Astrophysics Grant J. Mathews Center for Astrophysics/JINA, Department of Physics, University of Notre Dame
Abstract The main goals of nuclear astrophysics have been to probe the interiors of stars, stellar explosions, the early moments of cosmic expansion, and the formation and evolution of galaxies and cosmic structure by measurement and application of the relevant nuclear physics. The approach to these goals have generally been from three directions: 1) Careful measurements of the relevant nuclear reactions; 2) Detailed computer models of the relevant astrophysical environments; and 3) Observations of the relevant terrestrial and extra-terrestrial atomic and isotopic abundances. These approaches provide not only insight into the formation and evolution of the elements, but are also pillars upon which a variety of cosmological models as well as models for physics beyond the standard model of particle physics can stand or fall. At present there is a very exciting frontier on all three of these approaches. The development and applications of radioactive-ion-beam and low-background facilities have begun to clarify the input nuclear physics. The development of hydrodynamic stellar-evolution and explosion models in three spatial dimensions, along with detailed radiation and neutrino transport has provided new and unexpected insights into some of the deep mysteries regarding the origin and evolution of the elements. Also, for the first time in history, ground and space-based observations of elemental abundances in stars and gas are being made from the time of the very first stars and cosmic structures to the present. Observation of the cosmic microwave background can now also be used to analyze the development of structure in the universe from before the time of photon decoupling to the present. These observations provide unprecedented views of the history of cosmic evolution. They also provide new questions. In this review we summarize some some of the developments in each of these areas and highlight the exciting frontiers where new breakthroughs are likely to occur.
Key words: Nucleosythesis, Nuclear Reactions, nuclear properties, radioactive ion beams, Stellar interiors, Stellar Evolution, Thermonuclear burning in stars, s-process, r-process, big bang nucleosynthesis PACS: 26.20.+f, 26.30.+k, 26.35.+c, 26.40.+v, 26.50.+x, 26.60.+c, 97.10.-q, 97.10.Cv, 97.10.Gz, 97.10.Th, 97.60.Jd, 98.80.Ft
0375-9474/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2008.02.258
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1. Background It is an honor to have been asked to prepare a review paper on such an exciting topic. Indeed, this is a very special time in the history of nuclear astrophysics for several reasons. For one, this year marks the 50th year since the appearance of the seminal papers by Burbidge, Burbidge, Fowler and Hoyle [1] and Cameron [2]. These papers are often referenced as the beginnings of this field, though the topic of the the synthesis of the elements in stars was not entirely new at that time. Nevertheless, these papers provided us with names for a host of nucleosynthesis processes in stars, most of which endure to the present time. At the same time two other crucial developments have occurred in recent history. A new generation of accelerators for radioactive ion beams are either proposed or coming on line along with low background underground facilities [3]. This is occurring in concert with the development of new astronomical surveys (e.g. SDSS/SEGUE [4]), space based satellite observatories at multiple wavelengths (e.g. HST, Chandra, Spitzer, WMAP) and next-generation large ground based observatories (e.g. the Subaru, Keck, Gemini, VLT, and LBT telescopes). At the same time, computational technology has finally reached the point where fully three dimensional simulations of stars and astrophysical environments is possible with a level of complexity approaching the real world. The convergence of these various developments makes the present time exciting indeed. 2. Nucleosynthesis At the heart of what we do in nuclear astrophysics is the unraveling of the complexities observed in nature in terms of the underlying nuclear processes. One of the great achievements in the B2 FH was to identify and name the nuclear processes within stars. It is useful to summarize the processes as identified then and contrast them with the modern counterparts. For example, what was referred to as the x-process for producing the isotopes of Li, Be and B in B2 FH is now believed to represent a combination of big-bang nucleosynthesis, galactic cosmic ray nucleosynthesis, and neutrino-induced nucleon emission in the ν-process. Although the basic processes of hydrogen and helium burning in the cores or burning shells of main sequence and giant stars remain as outlined in B2 FH, a number of new processes have been added. These include hot hydrogen burning and the rp-process which may occur on accreting compact neutron stars or white dwarfs and also the ν- and νp-processes which occur in supernova shells. What was called the α process is now attributed to helium burning plus C/O/Ne burning. The e-process of B2 FH to form iron group elements is now identified with quasi-equilibrium silicon burning in supernova progenitors and explosive nucleosynthesis. 3. Frontiers in stellar nuclear astrophysics Thermonuclear Reaction Rates Understanding stellar interiors and the associated nuclear processes has been a main focus of nuclear astrophysics for the past fifty years. Indeed, there remain many questions regarding various thermonuclear burning processes [3]. Among the most important outstanding nuclear reaction rate issues discussed in this conference I refer the reader to the interesting presentations given on the 3 He(α, γ)7 Be reaction [5] which plays a key role in
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big bang nucleosynthesis as well as hydrogen burning; the 7 Be(p, γ)8 B reaction [6] also of importance in hydrogen burning, the 14 N(p, γ)15 O reaction [7] relevant to the the CNO cycle and globular cluster ages, and the most challenging 12 C(α, γ)16 O reaction whose uncertainty leads to a large ambiguity in the core mass and nucleosynthesis of massive stars [3,8]. Other nuclear uncertainties in thermonuclear reaction rates discussed below. There is, however, the major question of the physics of the stellar interiors themselves. This makes for yet another frontier which I would state as: What is the complex interplay between the nuclear reactions and turbulent convection in stars? For most of the past 50 years we have had to be content with modeling stellar interiors in terms of one dimensional spherical systems with mixing length theory as the description for convection. However, stellar interiors are much more complex environments. For the first time we have the tools to address them based upon the development of fully three-dimensional stellar evolution codes. One code with which I have had a small involvement is the Dejehuty code [9,10] developed at the Lawrence Livermore National Laboratory. This code uses an Adaptive Euler-Lagrange scheme which is second order accurate in space and time to fully model the convection and thermonuclear burning in stellar interiors. The mesh is constructed of multi-block logically rectangular nonorthogonal hexahedrons which can be decomposed into slices for parallel operation. In this way stars have been resolved into up to a billion zones. Such studies have been then used to analyze the underlying convection and radiation transport. 1) What is the role of convection on nucleosynthesis? As an example of the subtleties which could not have been detected in simple in spherical treatments, a study of the formation of 3 He in low mass stars was undertaken in [10]. The problem is that 1D models for low mass stars produce a large amount of 3 He. This 3 He would be transported to the surface when the star becomes deeply convective late in its evolution. The observed abundances of 3 He, however, are little different from the yields of big bang nucleosynthesis discussed below. It is unknown as to how this overproduction of 3 He is avoided, but it is clear from globular cluster data, that the 3 He is already destroyed before the star undergoes its core helium flash. A good candidate environment for 3 He destruction [12] is the deepest penetration of the outer convective zone during the first ascent up the giant branch. A star in this phase was analyzed [10] with the 3D Dejehuty code. It was discovered that clouds of hydrogen depleted material began to appear above the hydrogen burning shell. This material was depleted by the 3 He(3 He,2p)4 He reaction and then slowly transported to the outer convective region by buoyancy forces. This exciting result may finally explain the puzzle of 3 He. In other work this tool has been used to provide insight into convective overshooot and the core helium flash [10]. 2) How are the heavy s-process nuclei formed? One issue of particular interest for nuclear astrophysics has been the astrophysical site for slow neutron capture reactions in the s-process. Even at the time of B2FH it was already apparent from astronomical observations of radioactive 99 Tc in stars that the s-process must occur in red giants. The problem, however, has been to deduce the precise mechanism by which the correct distribution of neutron exposures could be produced within the stars. In the most popular model [13] the s-process occurs in a ”pocket” of 13 C which has been formed by mixing between the outer hydrogen convective shell and the inner degenerate 12 C core. Within
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this pocket it is believed that quiescent neutrons are produced via the 13 C(α, n)16 O reaction. When the 3α →12 C reaction ignites at the base of the helium shell, the temperature rises sufficiently that the 22 Ne(α, n)25 Mg reaction also contributes leading to the desired distribution of neutron exposures and a satisfactory pattern of s-process abundances. The problem with this scenario, however, is that it relies on a poorly understood parameter for the mixing of protons and 12 C into the helium shell. Some very impressive advances in the understanding of this mixing problem have been made in calculations by the Los Alamos group [14]. These results, however, are limited to two dimensions and to a limited region of the star. It remains a frontier to convincingly model this important mixing and nucleosynthesis process. 3) What is the role of convection and new nucleosynthesis processes in the first generation of stars? The study of the peculiar abundances in the first generaqtions of stars is a very active area of curren research [4]. Regarding the s-process for example, there is yet another mystery[15] surrounding the s-process in the first stars formed in the Galaxy. There appears to be good evidence [16] for the occurrence of an sprocess driven by the hydrogen shell flash in a subset of low-metallicity stars. Unraveling this possibility is sure to be a frontier in the coming years. Nuclear Reactions in the s-process Regarding the nuclear reactions of importance to s-process nucleosynthesis, the reader is referred to some very interesting new measurements described in this conference proceedings. These include: the neutron source reactions 13 C(α, n)16 O [17], and also the 22 Ne(α, n)25 Mg reaction. In addition, it is important to better quantify the various neutron poison reactions such as the 12 C(n, γ)13 C and 17 O(n, γ)18 O reactions[18] and some key neutron capture reactions along the s-process path, particularly at the branch points due to radioactive isotopes, e.g 79 Se(n, γ)80 Se [19] and the 93 Zr(n, γ)94 Zr reaction [20] for which interesting experimental techniques have been developed. 4) What are the details of convection, neutrino and nuclear reactions in core collapse supernovae? Another very important frontier in nuclear astrophysics involves the mechanism by which core-collapse supernovae explode and the possible associated rapid neutron capture nucleosynthesis in the r-process and also their shock-induced explosive nucleosynthesis in the outer envelopes. Regarding the explosion mechanism itself, it remains true that even after many decades of effort, detailed models for the core collapse of a massive star to form a Type-II supernova are still being worked out. There is now wide acceptance of the importance of delayed neutrino heating (cf. [21]). Nevertheless, in supernova most models with pure spherical symmetry, the neutrino luminosity from the proto-neutron star is too low to heat the infalling material sufficiently to expel matter from the star (c.f.. [22,23] and references therein). There is, however one model which does produce an explosion with the right characteristics [21,23]. One feature of the successful model is the complex interplay between neutrino-nuclear scattering and convection. For example, one key ingredient is that the proto-neutron star becomes hydrodynamically unstable a few hundred milliseconds after the core bounce due to the so-called ”neutron-finger instability” [21]. This EOS-dependent instability arises from the build up of a high neutron to proton ratio near the surface of the proto-neutron star to a sufficiently high value so as to over come the buoyancy of the higher entropy near the surface. Alternatively, magnetic convection induced by the magnetic-rotationinstability could contribute [23].
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Recently, however, simulations in two and three spatial dimensions have begun to show consistent explosions. The 2D and 3D models of the Max Planck group [24] shows the development of an explosion. More insight is suggested in the work of the Oak Ridge group [25] In whose models there is clear evidence that the explosions in 3D are related to the turbulent mixing of oxygen from the outer layers into the high entropy bubble behind the shock where the small additional input of energy from thermonuclear oxygen burning is enough to push forward the explosion. 5) How are the heavy r-process nuclei formed? The precise details for r-process nucleosynthesis has remained a mystery even though it is responsible for the abundance of about half of the nuclei heavier than iron. Nevertheless, among the many proposals (cf. [26]) for its origin, core-collapse supernovae have remained the most likely site in which the necessary ejected mass and neutrons per seed can be achieved. Ultimately, however, if one is ever to truly identify the site for the r-process it is important to explore r-process nucleosynthesis in the context of detailed supernova simulations (cf. [21]) in three dimensions. More importantly, however, the main reason for the difficulty in unraveling the r-process site has been the fact that the actual nuclear data on and near the r-process path far from stability is still poorly known. There are a variety of nuclear physics issues far from stability that must be resolved in order to understand the r-process. These include: thermonuclear reaction rates, electron capture rates, and photodisintegration rates relevant to the pre-collapse; Pauli blocking of electron captures and νe scattering and absorption relevant to collapse; the high density EOS relevant to core bounce; scattering and absorption of all three neutrino flavors relevant to neutron-star cooling and the formation of the high entropy bubble; for nucleosynthesis we need neutrino nucleus scattering/absorption, light-element reactions, alpha-rich freezeout, (n, γ), (γ, n), nuclear masses and neutron separation energies, Qβ , beta lifetimes, beta-delayed neutron emission, partition functions, spins, parities, energy levels, neutron-induced fission, fission yields, and explosive nuclear reaction rates. It is indeed exciting how much we have heard at this conference of the new developments in the use of radioactive ion beams. The accelerator at RIKEN has produced its first uranium beam and indeed has already produced some new Sn isotopes near the neutron drip line [27]. At the same time we have heard of exciting new experiments [28] at the NSCL of Michigan State University, as well as ISOLDE [29] to measure beta-decay half lives near the A = 130 r-process peak and FRS-GSI [30] to measure near the A = 195 peak. We have also learned of the development of a number of beams for isotopes along the r-process path at ORNL [31] for application to indirect measurements (See also [32]). Hope also remains that a Radioactive Ion Beam Facility will be built in the US. Many sites for the r-process have been proposed [26]. Indeed, it is quite clear that there must be at least 2 r-process sites [33]. Perhaps, the most popular model is that , of [34] in which the r-process occurs the high entropy bubble above a nascent proto-neutron star in a core-collapse supernova. In this region the entropy is so high that the nuclear statistical equilibrium (NSE) favors abundant free neutrons and alpha particles plus a few heavy nuclei which experience rapid neutron capture. This is, therefore, an ideal r-process site which satisfies the requirement from observations (e.g. [35,36]) that the relative yields be metallicity independent and not too much material be produced. Using this model, a fit was obtained [34] to the Solar r-process abundance pattern for heavy A > 100 nuclei. A number of issues remain, regarding the viability of this model. For example, a fast timescale may be necessary [37,38], possible associated with
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the formation of jets [39]. It remains a frontier to identify the r-process site. In addition to the above issues, there a few more which come to mind. For completeness, I list them here: 6) What is the effect of fission yields on r-process nucleosynthesis? This is an area of considerable recent interest [40,41] 7) What is the correct model for nuclear flame propagation and detonation in Type Ia supernova explosions? 8)What are the effects on binary evolution on nucleosynthesis? 9)What are the effects of rotation/ magnetic field driven convection on nucleosynthesis?
4. Frontiers in Compact Stars/EOS The question of the equation of state relevant to neutron stars and supernovae is still actively being studied [42]. The possibility of a phase transition to a QCD plasma at hight temperature and/or density is an active area of nuclear research [43,44]. Such a transition if also of import in nuclear astrophysics. There is, for example a possibility [45] of observable effects on primordial nucleosynthesis from a QCD phase transition in the early universe. There is also renewed interest in the effects of such a transition on compact remnants and supernovae. All of these questions fall under the frontier question: What is the equation of state at high density? Among the related questions are: 1) How does it affect binary neutron-star inspiral/ merger/ gravity waves? 2) Does strange quark matter exist in stars? The possible existence of strange-matter stars has been speculated upon for some time. Most of the work concerning their nature and origin has focused on a number of neutron stars which are too compact or are cooling too rapidly to be comprised of normal nuclear matter [46]. There is also the possibility of white dwarfs with strange-matter cores [47], i.e. strange dwarfs. Strange-matter white dwarfs could gradually form during the progenitor mainsequence by the accretion of a strange-matter nugget. Such nuggets could exist either as a relic of the early universe or as an ejected fragment from the merger/coalescence of strange-matter neutron stars. Once captured by a star, strange-matter nuggets would gravitationally settle to the center and begin to convert normal matter to strange matter. The most distinguishing characteristic of the existence of strange-matter stars is that they must have a smaller radius. This unavoidable consequence simply follows from the fact that strange-matter has more degrees of freedom and can therefore be more compact than ordinary electron-degenerate matter. Indeed, recent work [47] has identified several candidate stars whose compact cores may be identifiable through astrosisemology. Before leaving this topic on compact stars, I would should also mention another important question which is of considerable interest [48,49] regarding compact remnants: 3) What are the effects of the rp-process on accreting neutron stars, X-ray bursts, Novae?
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5. Frontiers in neutrino nuclear astrophysics There are a number of exciting recent developments in neutrino nuclear physics. In nuclear astrophysics one could broadly characterize the frontier as: What are the Neutrino effects on supernovae, nucleosynthesis and cosmology? Regarding supernovae, some of the importance of neutrino effects on core collapse supernovae were described above. There is a developing appreciation for the importance of many higher order effects in neutrino-nuclear scattering. For example we have found the neutrino weak magnetism [50] can have a very important effect at late times in the explosion. Related questions include: 1) What is the detailed relativistic neutrino transport in supernovae? 2) Are there neutrino oscillations in supernovae? 3) Neutrino effects on r-process nucleosynthesis? Regarding these points, there is considerable interest in the possibility of detecting [51] the relic neutrinos from supernovae. There is also the exciting development [40,52] of evidence for a new nucleosynthesis process, called the νp-process. Recent hydrodynamical models for core-collapse supernovae show the presence of a proton-rich region surrounding the freshly born neutron star. Neutrino-induced reactions within this proton-rich environment could produce medium-mass nuclei including the previously unexplained isotopes of 92,94 Mo and 96,98 Ru. There is also continued interest in neutrino-induced nucleon emission in the ν-process [53] and a new possibility [54] that 138 La produced by the ν-process is a new cosmo-chronometer. There is also considerable interest in the question: 4) Are there observable effects from heavy sterile neutrinos? A number of related questions are: Is it absolutely stable? Is there one or several? Does it oscillate? Does it affect supernovae? Does it affect nucleosynthesis? Does it affect dark matter Does it affect dark energy? How and when are they produced? Why does it have its present density? What is its role in structure formation? In this regard, It is of some interest that a decaying heavy sterile neutron might be a candidate for both the universal dark matter and the dark energy responsible for the current cosmic acceleration [55] 6. Frontiers in Big Bang Nucleosynthesis Big-bang nucleosynthesis (BBN) plays a crucial role in constraining our views on the universe [56]. It is essentially the only probe of physics in the early radiation dominated epoch during the interval from ∼ 1 − 104 sec. Thus, it is imperative to have accurate predictions of the light element nuclear reactions which take place during this era as well as reliable determinations of primordial abundances from observation. The power of big bang nucleosynthesis comes in part from the fact that the nuclear physics is so accessible. Once the nuclear reaction rates are specified [57], all light abundances are a simple function of the baryon-to-photon ratio η during the nucleosynthesis epoch [58–60]. Nevertheless, some uncertainties remain. For years this was the chief uncertainty in the use of BBN as a probe of the early
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universe. However, the WMAP measurement [61] of the power spectrum of fluctuations in the cosmic microwave background has determined the baryon to photon ratio to high precision due to the effects of ”baryon drag” on the acoustic peaks in the CMB power spectrum. We now have from WMAP a value of η10 = 6.13 ± 0.25, or (Ωb h2 = 0.0224 ± 0.0009), where η10 is the baryon-to-photon ratio in in units of 10−10 , Ωb is the fraction of the closure density in baryons, and h = 0.71 ± 0.08 is the Hubble parameter in units of 100 km s−1 Mpc−1 . At the same time, the dark energy and dark matter content have been fixed primarily by combining the WMAP results with observations of supernovae at high redshift to be ΩCDM = 0.22±0.01 while the dark energy content is ΩΛ = 0.73±0.04. Is there Evidence for non-standard physics in the early universe? A crucial test of the standard BBN (and/or the existence of new physics in the early) is whether a single value of η10 can be found which reproduces all of the observed primordial abundances. The different light element abundances are determined by different means. This makes each determination an important independent check on BBN. Primordial deuterium is best determined from its absorption line in high redshift Lyman α clouds. The average of measurements of six absorption-line systems towards five −5 QSOs gives [62] D/H = 2.78+0.44 . This would imply an value of η10 = 5.9 ± 0.5. −0.38 × 10 This is also very close to the value Ωb h2 = 0.0224 ± 0.0009 (η10 = 6.13 ± 0.25) deduced from WMAP [61]. Is 4 He overproduced in BBN models? The primordial helium abundance is obtained by measuring extragalactic HII regions in low-metallicity irregular galaxies. Often in the past, the deduced primordial helium abundance Yp resided in one of two possible values (a low value, e.g. Yp = 0.238 ± 0.002 ± 0.005, [63] and a high value Yp = 0.2452±0.0015 [64]). However, the combined deuterium and WMAP constraints on the baryon-to-photon ratio implies a high primordial helium abundance [60] Yp = 0.2479±0.0004. There is also, however, a current dilemma regarding the uncertainty in the observationally determined primordial helium abundance. Many recent evaluations [64] give a rather narrow range of abundance uncertainty. On the other hand, the extent of systematic errors in these analyses is still being debated. In Ref. [65] it was concluded that correlations in various uncertainties could stretch the error to 0.232 ≤ Yp ≤ 0.258. Clearly, it is important to frontier to better quantify the primordial helium abundance. There is also a nuclear physics issue. The primordial helium abundance is sensitive to the neutron lifetime. New measurements of the neutron half-life might explain [66] the overproduction of 4 He in BBN models. Is 7 Li overproduced in BBN models? The primordial lithium abundance is inferred from old low-metallicity halo stars. Such stars exhibit a nearly constant plateau lithium abundance as a function of surface tem−10 perature. The inferred primordial abundance is [67] 7 Li= 1.23+0.68 . There is, −0.32 × 10 7 however, some controversy [68] concerning the depletion of Li on the surface of such halo stars. This implies that the observed 7 Li abundance is a factor of three smaller than that inferred from the BBN model. Is 6 Li produced in the Big Bang? It has recently been pointed out (e.g. [69]) that the abundances of both 6 Li and 7 Li observed in metal poor halo stars are not in agreement with those predicted from standard BBN. Specifically, the 6 Li abundance as a function of metallicity exhibits a plateau similar to that for 7 Li in very metal-poor stars, suggesting a primordial origin for both isotopes. This 6 Li abundance, however, is a factor of ∼ 103 larger than that predicted by BBN.
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A long-standing effort in cosmology has been the search for evidence of unstable particles that might have existed in the early universe. It is thus natural to ask whether the two lithium abundance anomalies might be a manifestation of the existence of such a particle. In this context a number of possible solutions to the 6 Li problem have been proposed which relate the Li anomalies to the possible existence of unstable particles in the early universe ([70] and Refs. therein). This has been extended in several recent studies [71,72] to consider heavy negatively charged decaying particles that modify BBN. This hypothesis is motivated by identifying the X − as the supersymmetric partner of the tau lepton. In this paradigm, the heavy X − particles bind to the nuclei produced in BBN to form X-nuclei. The massive X − particles would be bound in orbits with radii comparable to those of normal nuclei. Hence, they would reduce the reaction Coulomb barriers thereby enhancing the thermonuclear reaction rates and extending the duration of BBN to lower temperatures. Particular nuclear reactions of X − particles can produce a is that a large enhancement of the 6 Li abundance [72]. One example is the transfer reaction involving an X − bound to 4 He, i.e. 4 HeX (d,X − )6 Li. It has been shown [72] that this approach solves both the 6 Li and 7 Li problems simultaneously. Acknowledgments Work at the University of Notre Dame supported by the US Department of Energy under Nuclear Theory grant DE-FG02-95ER40934. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
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