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The r-process constitutes one of the major challenges in nuclear astrophysics. Its astrophysical site has not yet been identified but there is observational evidence suggesting that at least two possible sites should contribute to the solar system abundance of r-process elements and that the r-process responsible for the production of elements heavier than Z=56 operates quite robustly producing always the same relative abundances. From the nuclear-physics point of view the r-process requires the knowledge of a large number of reaction rates involving exotic nuclei. These include neutron capture rates, beta-decays and fission rates, the latter for the heavier nuclei produced in the r-process. We have developed for the first time a complete database of reaction rates that in addition to neutron-capture rates and beta-decay half-lives includes all possible reactions that can induce fission (neutron-capture, beta-decay and spontaneous fission) and the corresponding fission yields. In addition, we have implemented these reaction rates in a fully implicit reaction network. We have performed r-process calculations for the neutrino-driven wind scenario to explore whether or not fission can contribute to provide a robust r-process pattern.

We have performed shell-model calculations of the half-lives and neutron-branching probabilities of the r-process waiting point nuclei at the magic neutron number N=82. These new calculations use a larger model space than previous shell model studies and an improved residual interaction which is adjusted to recent spectroscopic data around A=130. Our shell-model results give a good account of all experimentally known half-lives and $Q_\beta$-values for the N=82 r-process waiting point nuclei. Our half-life predictions for the N=82 nuclei with Z=42--46 agree well with recent estimates based in the energy-density functional method.

We study the composition of the outer layers of a protoneutron star and show that light nuclei are present in substantial amounts. The composition is dominated by nucleons, deuterons, tritons and alpha particles; 3He is present in smaller amounts. This composition can be studied in laboratory experiments with new neutron-rich radioactive beams that can reproduce similar densities and temperatures. After including the corresponding neutrino interactions, we demonstrate that light nuclei have a small impact on the average energy of the emitted electron neutrinos, but are significant for the average energy of antineutrinos. During the early post-explosion phase, the average energy of electron antineutrinos is slightly increased, while at later times during the protoneutron star cooling it is reduced by about 1 MeV. The consequences of these changes for nucleosynthesis in neutrino-driven supernova outflows are discussed., Comment: 10 pages, 4 figures, submitted to PRC

Microscopic calculations show a strong parity dependence of the nuclear level density at low excitation energy of a nucleus. Previously, this dependence has either been neglected or only implemented in the initial and final channels of Hauser-Feshbach calculations. We present an indirect way to account for a full parity dependence in all steps of a reaction, including the one of the compound nucleus formed in a reaction. To illustrate the impact on astrophysical reaction rates, we present rates for neutron captures in isotopic chains of Ni and Sn. Comparing with the standard assumption of equipartition of both parities, we find noticeable differences in the energy regime of astrophysical interest caused by the parity dependence of the nuclear level density found in the compound nucleus even at sizeable excitation energies., Comment: 7 figures, 7 pages

Based on the shell model for Gamow-Teller and the Random Phase Approximation for forbidden transitions, we have calculated reaction rates for inelastic neutrino-nucleus scattering (INNS) under supernova (SN) conditions, assuming a matter composition given by Nuclear Statistical Equilibrium. The rates have been incorporated into state-of-the-art stellar core-collapse simulations with detailed energy-dependent neutrino transport. While no significant effect on the SN dynamics is observed, INNS increases the neutrino opacities noticeably and strongly reduces the high-energy tail of the neutrino spectrum emitted in the neutrino burst at shock breakout. Relatedly the expected event rates for the observation of such neutrinos by earthbound detectors are reduced by up to about 60%., Comment: 4 pages, 2 figures, 1 table

A generalized method to calculate the excitation-energy dependent parity ratio in the nuclear level density is presented, using the assumption of Poisson distributed independent quasi particles combined with BCS occupation numbers. It is found that it is crucial to employ a sufficiently large model space to allow excitations both from low-lying shells and to higher shells beyond a single major shell. Parity ratios are only found to equilibrate above at least 5-10 MeV of excitation energy. Furthermore, an overshooting effect close to major shells is found where the parity opposite to the ground state parity may dominate across a range of several MeV before the parity ratio finally equilibrates. The method is suited for large-scale calculations as needed, for example, in astrophysical applications. Parity distributions were computed for all nuclei from the proton dripline to the neutron dripline and from Ne up to Bi. These results were then used to recalculate astrophysical reaction rates in a Hauser-Feshbach statistical model. Although certain transitions can be considerably enhanced or suppressed, the impact on astrophysically relevant reactions remains limited, mainly due to the thermal population of target states in stellar reaction rates., Comment: 15 pages, 17 figures; corrected/updated references in v2; additional material can be found at http://nucastro.org/adndt.html#parity

Advances in our understanding and the modeling of stellar core-collapse and supernova explosions over the past 15 years are reviewed, concentrating on the evolution of hydrodynamical simulations, the description of weak interactions and nuclear equation of state effects, and new insights into the nucleosynthesis occurring in the early phases of the explosion, in particular the neutrino-p process. The latter is enabled by the proton-richness of the early ejecta, which was discovered because of significant progress has been made in the treatment of neutrino transport and weak interactions. This progress has led to a new generation of sophisticated Newtonian and relativistic hydrodynamics simulations in spherical symmetry. Based on these, it is now clear that the prompt bounce-shock mechanism is not the driver of supernova explosions, and that the delayed neutrino-heating mechanism can produce explosions without the aid of multi-dimensional processes only if the progenitor star has an ONeMg core inside a very dilute He-core, i.e., has a mass in the 8--10 solar mass range. Hydrodynamic instabilities of various kinds have indeed been recognized to occur in the supernova core and to be of potential importance for the explosion. Neutrino-driven explosions, however, have been seen in two-dimensional simulations with sophisticated neutrino transport so far only when the star has a small iron core and low density in the surrounding shells as being found in stars near 10--11 solar masses. The explosion mechanism of more massive progenitors is still a puzzle. It might involve effects of three-dimensional hydrodynamics or might point to the relevance of rapid rotation and magnetohydrodynamics, or to still incompletely explored properties of neutrinos and the high-density equation of state., Comment: 49 pages, 20 figures; submitted to the Bethe Centennial Volume of Physics Reports

This manuscript reviews recent progress in our understanding of the nucleosynthesis of medium and heavy elements in supernovae. Recent hydrodynamical models of core-collapse supernovae show that a large amount of proton rich matter is ejected under strong neutrino fluxes. This matter constitutes the site of the vp-process where antineutrino absorption reactions catalyze the nucleosynthesis of nuclei with A > 64. Supernovae are also associated with the r-process responsible for the synthesis of the heaviest elements in nature. Fission during the r-process can play a major role in determining the final abundance patter and in explaining the almost universal features seen in metal-poor r-process-rich stars., Comment: 10 pages, 3 figures, invited talk at NIC-IX, International Symposium on Nuclear Astrophysics - Nuclei in the Cosmos - IX, CERN, Geneva, Switzerland, 25-30 June, 2006

Core collapse supernovae are the leading actor in the story of the cosmic origin of the chemical elements. Existing models, which generally assume spherical symmetry and parameterize the explosion, have been able to broadly replicate the observed elemental pattern. However, inclusion of neutrino interactions produces noticeable improvement in the composition of the ejecta when compared to observations. Neutrino interactions may also provide a supernova source for light p-process nuclei., Comment: 7 pages, 2 figures, in proceedings of Astronomy with Radioactivities V, Clemson University, September 5-9, 2005, to appear in New Astronomy Reviews

We present a new nucleosynthesis process, that we denote nu p-process, which occurs in supernovae (and possibly gamma-ray bursts) when strong neutrino fluxes create proton-rich ejecta. In this process, antineutrino absorptions in the proton-rich environment produce neutrons that are immediately captured by neutron-deficient nuclei. This allows for the nucleosynthesis of nuclei with mass numbers A >64. Making this process a possible candidate to explain the origin of the solar abundances of 92,94Mo and 96,98Ru. This process also offers a natural explanation for the large abundance of Sr seen in an hyper-metal-poor star., Comment: 5 pages, 3 figures, submitted to Physical Review Letters

We review the nuclear physics input necessary for the study of the collapse of massive stars as precursor to supernova explosions. Recent theoretical advances for the calculation of the relevant weak-interaction processes and their influence in the collapse and postbounce dynamics are discussed. Future improvements are expected to come with the advent of radioactive ion-beam facilities., Comment: 37 pages, 11 figures, for special issue of Nucl. Phys. A

With presently known input physics and computer simulations in 1D, a self-consistent treatment of core collapse supernovae does not yet lead to successful explosions, while 2D models show some promise. Thus, there are strong indications that the delayed neutrino mechanism works combined with a multi-D convection treatment for unstable layers. On the other hand there is a need to provide correct nucleosynthesis abundances for the progressing field of galactic evolution and observations of low metallicity stars. The innermost ejecta is directly affected by the explosion mechanism, i.e. most strongly the yields of Fe-group nuclei for which an induced piston or thermal bomb treatment will not provide the correct yields because the effect of neutrino interactions is not included. We apply parameterized variations to the neutrino scattering cross sections and alternatively, parameterized variations are applied to the neutrino absorption cross sections on nucleons in the ``gain region''. We find that both measures lead to similar results, causing explosions and a Ye>0.5 in the innermost ejected layers, due to the combined effect of a short weak interaction time scale and a negligible electron degeneracy, unveiling the proton-neutron mass difference. We include all weak interactions (electron and positron capture, beta-decay, neutrino and antineutrino capture on nuclei, and neutrino and antineutrino capture on nucleons) and present first nucleosynthesis results for these innermost ejected layers to discuss how they improve predictions for Fe-group nuclei. The proton-rich environment results in enhanced abundances of 45Sc, 49Ti, and 64Zn as requested by chemical evolution studies and observations of low metallicity stars as well as appreciable production of nuclei in the mass range up to A=80., Comment: 13 pages, 8 figures. Final version

We ensure successful explosions (of otherwise non-explosive models) by enhancing the neutrino luminosity via reducing the neutrino scattering cross sections or by increasing the heating efficiency via enhancing the neutrino absorption cross sections in the heating region. Our investigations show that the resulting electron fraction Ye in the innermost ejecta is close to 0.5, in some areas even exceeding 0.5. We present the effects of the resulting values for Ye on the nucleosynthesis yields of the innermost zones of core collapse supernovae., Comment: 4pages, 2figures; contribution to Nuclei In The Cosmos VIII, to appear in Nucl. Phys. A

Highly precise data on the magnetic dipole strength distributions from the Darmstadt electron linear accelerator for the nuclei 50Ti, 52Cr and 54Fe are dominated by isovector Gamow-Teller-like contributions and can therefore be translated into inelastic total and differential neutral-current neutrino-nucleus cross sections at supernova neutrino energies. The results agree well with large-scale shell-model calculations, validating this model., Comment: 5 pages, 4 figures, RevTeX 4, version accepted in Phys. Rev. Letters

The most important weak nuclear interaction to the dynamics of stellar core collapse is electron capture, primarily on nuclei with masses larger than 60. In prior simulations of core collapse, electron capture on these nuclei has been treated in a highly parameterized fashion, if not ignored. With realistic treatment of electron capture on heavy nuclei come significant changes in the hydrodynamics of core collapse and bounce. We discuss these as well as the ramifications for the post-bounce evolution in core collapse supernovae., Comment: Accepted by PRL, 5 pages, 2 figures

We study neutrino process nucleosynthesis in massive stars using newly calculated cross sections, an expanded reaction network, and complete and self-consistent models of the progenitor star. We reevaluate the production of light isotopes from abundant progenitors as well as that of rare, heavy, proton-rich isotopes. In particular, new results are given for B11, F19, La138, and Ta180. The production of these isotopes places limits on neutrino spectrum and oscialltions., Comment: submitted to PRL; 5 pages, including 4 figures

Supernova simulations to date have assumed that during core collapse electron captures occur dominantly on free protons, while captures on heavy nuclei are Pauli-blocked and are ignored. We have calculated rates for electron capture on nuclei with mass numbers A=65-112 for the temperatures and densities appropriate for core collapse. We find that these rates are large enough so that, in contrast to previous assumptions, electron capture on nuclei dominates over capture on free protons. This leads to significant changes in core collapse simulations., Comment: 5 pages, 4 figures, REVTeX 4

A simple description for obtaining the parity distribution of nuclear levels in the pf + g9/2 shell as a function of excitation energy was recently derived. We implement this in a global nuclear level density model. In the framework of the statistical model, cross sections and astrophysical reaction rates are calculated in the Fe region and compared to rates obtained with the common assumption of an equal distribution of parities. We find considerable differences, especially for reactions involving particles in the exit channel., Comment: 4 pages, to appear in the proceedings of CGS11 (Prague), World Scientific

General relativistic multi-group and multi-flavor Boltzmann neutrino transport in spherical symmetry adds a new level of detail to the numerical bridge between microscopic nuclear and weak interaction physics and the macroscopic evolution of the astrophysical object. Although no supernova explosions are obtained, we investigate the neutrino luminosities in various phases of the postbounce evolution for a wide range of progenitor stars between 13 and 40 solar masses. The signal probes the dynamics of material layered in and around the protoneutron star and is, within narrow limits, sensitive to improvements in the weak interaction physics. Only changes that dramatically exceed physical limitations allow experiments with exploding models. We discuss the differences in the neutrino signal and find the electron fraction in the innermost ejecta to exceed 0.5 as a consequence of thermal balance and weak equilibrium at the masscut., Comment: 8 pages, 4 figures. Proceedings of the Nuclear Physics in Astrophysics Conference, Debrecen, Hungary, 2002, to appear in Nuc. Phys. A. Color figures added and reference actualized

Recent experimental data and progress in nuclear structure modeling have lead to improved descriptions of astrophysically important weak-interaction processes. The review discusses these advances and their applications to hydrostatic solar and stellar burning, to the slow and rapid neutron-capture processes, to neutrino nucleosynthesis, and to explosive hydrogen burning. Special emphasis is given to the weak-interaction processes associated with core-collapse supernovae. Despite some significant progress, important improvements are still warranted. Such improvements are expected to come from future radioactive ion-beam facilities., Comment: 45 pages, 33 figures, REVTeX 4, submitted to Rev. Mod. Phys., modified following referee's comments

With the exception of the Big Bang, responsible for 1,2H, 3,4He, and 7Li, stars act as sources for the composition of the interstellar medium. Cosmic rays are related to the latter and very probably due to acceleration of the mixed interstellar medium by shock waves from supernova remnants. Thus, the understanding of the abundance evolution in the interstellar medium and especially the enrichment of heavy elements, as a function of space and time, is essential. It reflects the history of star formation and the lifetimes of the diverse contributing stellar objects. Therefore, the understanding of the endpoints of stellar evolution is essential as well. These are mainly planetary nebulae and type II/Ib/Ic supernovae as evolutionary endpoints of single stars, but also events in binary systems can contribute, like e.g. supernovae of type Ia, novae and possibly X-ray bursts and neutron star or neutron star - black hole mergers. Despite many efforts, a full and self-consistent understanding of supernovae (the main contributors to nucleosynthesis in galaxies) is not existing, yet. Their fingerprints, however, seen either in spectra, lightcurves, radioactivities/decay gamma-rays or in galactic evolution, can help to constrain the composition of their ejecta and related model uncertainties., Comment: Review, 20 pages, uses style icrc.sty, to be published in Proc. ICRC 2001

Except for 1H, 2H, 3He, 4He, and 7Li, originating from the Big Bang, all heavier elements are made in stellar evolution and stellar explosions. Nuclear physics, and in many cases nuclear structure far from stability, enters in a crucial way. Therefore, we examine in this review the role of nuclear physics in astrophysics in general and in particular how it affects stellar events and the resulting nucleosynthesis. Stellar modeling addresses four major aspects: 1. energy generation and nucleosynthesis, 2. energy transport via conduction, radiation or possibly convection, 3. hydrodynamics/hydrostatics, and finally 4. thermodynamic properties of the matter involved. Nuclear Physics enters via nuclear reaction cross sections and nuclear structure (affecting the composition changes and nuclear energy generation), neutrino-nucleon and neutrino-nucleus cross sections (affecting neutrino opacities and transport), and e.g. the equation of state at and beyond nuclear densities which creates a relation between the nuclear many-body problem and the hydrodynamic response like pressure and entropy. In the following we review these four topics by highlighting the role and impact of nuclear physics in each of these aspects of stellar modeling. The main emphasis is put on the connection to element synthesis., Comment: 32 pages, 8 figures, review, to appear in Prog. Nuc. Part. Phys.; layout changed (v2), journal reference corrected (v3)

Using the recent shell model evaluation of stellar weak interaction rates we have calculated the neutrino spectra arising from electron capture on pf-shell nuclei under presupernova conditions. We present a simple parametrization of the spectra which allows for an easy implementation into collapse simulations. We discuss that the explicit consideration of thermal ensembles in the parent nucleus broadens the neutrino spectra and results in larger average neutrino energies. The capture rates and neutrino spectra can be easily modified to account for phase space blocking by neutrinos which becomes increasingly important during the final stellar collapse., Comment: 6 pages, 6 figures

Recent shell-model calculations of weak-interaction rates for nuclei in the mass range A = 45 - 65 have resulted in substantial revisions to the hitherto standard set of Fuller, Fowler, & Newman (FFN). In particular, key electron-capture rates, such as that for Co60 are much smaller. We consider here the effects of these revised rates on the presupernova (post-oxygen burning) evolution of massive stars in the mass range 11 to 40 M_sun. Moreover, we include, for the first time in models by our group, the effects of modern rates for beta-decay in addition to electron capture and positron emission. Values for the central electron mole number at the time of iron core collapse in the new models are typically larger, by delta Y_e = 0.005 to 0.015, than those of Woosley & Weaver 1995, with a tendency for the more massive models to display larger differences. About half of this change is a consequence of including beta-decay; the other half, result of the smaller rates for electron capture. Unlike what might be expected solely on basis of the larger Y_e's, the new iron core masses are systematically smaller owing to a decrease in the entropy in the outer iron core. The changes in iron core mass range from zero to 0.1 M_sun. We also observe, as predicted by Aufderheide et al. (1994), a tendency towards beta-equilibrium just prior to the collapse of the core, and the subsequent loss of that equilibrium as core collapse proceeds. We discuss the key weak reaction rates, both beta-decay and electron-capture, responsible for the evolution of Y_e and make suggestions for future measurements., Comment: 22 pages including 17 PostScript figures, uses emulateapj.sty, submitted to ApJ

Improved values for stellar weak interaction rates have been recently calculated based upon a large shell model diagonalization. Using these new rates (for both beta decay and electron capture), we have examined the presupernova evolution of massive stars in the range 15-40 Msun. Comparing our new models with a standard set of presupernova models by Woosley and Weaver, we find significantly larger values for the electron-to-baryon ratio Ye at the onset of collapse and iron core masses reduced by approximately 0.1 Msun. The inclusion of beta-decay accounts for roughly half of the revisions, while the other half is a consequence of the improved nuclear physics. These changes will have important consequences for nucleosynthesis and the supernova explosion mechanism., Comment: 4 pages, 2 figures

The Chandrasekhar mass model for Type Ia Supernovae (SNe Ia) has received increasing support from recent comparisons of observations with light curve predictions and modeling of synthetic spectra. It explains SN Ia events via thermonuclear explosions of accreting white dwarfs in binary stellar systems, being caused by central carbon ignition when the white dwarf approaches the Chandrasekhar mass. As the electron gas in white dwarfs is degenerate, characterized by high Fermi energies for the high density regions in the center, electron capture on intermediate mass and Fe-group nuclei plays an important role in explosive burning. Electron capture affects the central electron fraction Y_e, which determines the composition of the ejecta from such explosions. Up to the present, astrophysical tabulations based on shell model matrix elements were only available for light nuclei in the sd-shell. Recently new Shell Model Monte Carlo (SMMC) and large-scale shell model diagonalization calculations have also been performed for pf-shell nuclei. These lead in general to a reduction of electron capture rates in comparison with previous, more phenomenological, approaches. Making use of these new shell model based rates, we present the first results for the composition of Fe-group nuclei produced in the central regions of SNe Ia and possible changes in the constraints on model parameters like ignition densities and burning front speeds., Comment: 26 pages, 8 figures, submitted to ApJ

Based on large-scale shell model calculations we have determined the electron capture, positron capture and beta-decay rates on more than 100 nuclei in the mass range A=45-65. The rates are given for densities \rho Y_e =10^7-10^{10} mol/cm^3 and temperatures T=10^9-10^{10} K and hence are relevant for both types of supernovae (Type Ia and Type II). The shell model electron capture rates are significantly smaller than currently assumed. For proton-to-baryon ratios Y_e=0.42-0.46 mol/g, the beta-decay rates are faster than the electron capture rates during the core collapse of a massive star., Comment: 16 pages, 10 figures

We have performed large-scale shell-model calculations of the half-lives and neutron-branching probabilties of the r-process waiting point nuclei at the magic neutron number N=82. We find good agreement with the measured half-lives of 129Ag and 130Cd. Our shell-model half-lives are noticeably shorter than those currently adopted in r-process simulations. Our calculation suggests that 130Cd is not produced in beta-flow equilibrium with the other N=82 isotones on the r-process path., Comment: 4 pages, 3 figures

A measurement of the 56Ni cosmic ray abundance has been discussed as a possible tool to determine the acceleration time scale of relativistic particles in cosmic rays. This conjecture will depend on the halflife of totally ionized 56Ni which can only decay by higher-order forbidden transitions. We have calculated this halflife within large-scale shell model calculations and find t_{1/2} \approx 4 \times 10^4 years, only slightly larger than the currently available experimental lower limit, but too short for 56Ni to serve as a cosmic ray chronometer., Comment: 3 pages, 1 figure

We calculate supernova electron capture and beta decay rates for various pf-shell nuclei using large-scale shell model techniques. We show that the centroid of the Gamow-Teller strength distribution has been systematically misplaced in previous rate estimates. Our total electron capture rates are significantly smaller than currently adopted in core collapse calculations, while the total beta decay rates change less. Our calculation shows that for electron-to-baryon ratios Ye=0.42-0.46 beta decay rates are larger than the competing electron capture rates., Comment: 9 pages, 3 figures, RevTeX

At densities between 10^8 and 10^10} g/cm^3 electron capture in a presupernova collapse is believed to mainly occur on odd-odd nuclei. We have derived the rates for six of the most important electron capturing nuclei, {54,56,58}Mn and {56,58,60}Co, based on calculations of the Gamow-Teller strength distributions for the ground states and first excited states. These calculations have been performed by shell model diagonalization in the pf shell using a recently modified version of the KB3 interaction. The shell model rates are noticeably smaller than the presently adopted rates as the latter have been derived by placing the Gamow-Teller (GT) resonance at too low excitation energies., Comment: 5 pages, 3 figures, RevTeX 3.1, submitted to Phys. Lett. B

We have calculated the Gamow-Teller strength distributions for the ground states and first excited states in 55Co and 56Ni. These calculations have been performed by shell model diagonalization in the pf shell using the KB3 interaction. The Gamow-Teller distributions are used to calculate the electron capture rates for typical presupernova conditions. Our 55Co rate is noticeably smaller than the presently adopted rate as it is dominated by weak low-lying transitions rather than the strong Gamow-Teller (GT) resonance which is located at a higher excitation energy in the daughter than usually parametrized. Although our 56Ni rate agrees with the presently adopted rate, we do not confirm the conventional parametrization of the GT centroid. Our results support general trends suggested on the basis of shell model Monte Carlo calculations., Comment: 4 pages, 4 figures, RevTeX 3.1, to appear in Phys. Lett. B

The nucleus 54Mn has been observed in cosmic rays. In astrophysical environments it is fully stripped of its atomic electrons and its decay is dominated by the beta- branch to the 54Fe ground state. Application of 54Mn based chronometer to study the confinement of the iron group cosmic rays requires knowledge of the corresponding halflife, but its measurement is impossible at the present time. However, the branching ratio for the related beta+ decay of 54Mn was determined recently. We use the shell model with only a minimal truncation and calculate both beta+ and beta- decay rates of 54Mn. Good agreement for the beta+ branch suggests that the calculated partial halflife of the beta- decay, (4.94 \pm 0.06) x 10^5 years, should be reliable. However, this halflife is noticeably shorter than the range 1-2 x 10^6 y indicated by the fit based on the 54Mn abundance in cosmic rays. We also evaluate other known unique second forbidden beta decays from the nuclear p and sd shells (10Be, 22Na, and two decay branches of 26Al) and show that the shell model can describe them with reasonable accuracy as well., Comment: 4 pages, RevTeX, 2 figures