Electron capture in stars.

Electron capture on nuclei plays an essential role in the dynamics of several astrophysical objects, including core-collapse and thermonuclear supernovae, the crust of accreting neutron stars in binary systems and the final core evolution of intermediate-mass stars. In these astrophysical objects, the capture occurs at finite temperatures and densities, at which the electrons form a degenerate relativistic electron gas. The capture rates can be derived from perturbation theory, where allowed nuclear transitions [Gamow-Teller (GT) transitions] dominate, except at the higher temperatures achieved in core-collapse supernovae, where forbidden transitions also contribute significantly to the capture rates. There has been decisive progress in recent years in measuring GT strength distributions using novel experimental techniques based on charge-exchange reactions. These measurements not only provide data for the GT distributions of ground states for many relevant nuclei, but also serve as valuable constraints for nuclear models which are needed to derive the capture rates for the many nuclei for which no data yet exist. In particular, models are needed to evaluate stellar capture rates at finite temperatures, where capture can also occur on nuclei in thermally excited states. There has also been significant progress in recent years in the modeling of stellar capture rates. This has been made possible by advances in nuclear many-body models as well as in computer soft- and hardware. Specifically, to derive reliable capture rates for core-collapse supernovae, a dedicated strategy has been developed based on a hierarchy of nuclear models specifically adapted to the abundant nuclei and astrophysical conditions present under various collapse conditions. In particular, for the challenging conditions where the electron chemical potential and the nuclearQvalues are of the same order, large-scale shell-model diagonalization calculations have proved to be an appropriate tool to derive stellar capture rates, often validated by experimental data. Such situations are relevant in the early stage of the core collapse of massive stars, for the nucleosynthesis of thermonuclear supernovae, and for the final evolution of the cores of intermediate-mass stars involving nuclei in the mass rangeA∼ 20-65. This manuscript reviews the experimental and theoretical progress recently achieved in deriving stellar electron capture rates. It also discusses the impact these improved rates have on our understanding of the various astrophysical objects.

Fingerprints of Heavy-Element Nucleosynthesis in the Late-Time Lightcurves of Kilonovae.

The kilonova emission observed following the binary neutron star merger event GW170817 provided the first direct evidence for the synthesis of heavy nuclei through the rapid neutron capture process (r process). The late-time transition in the spectral energy distribution to near-infrared wavelengths was interpreted as indicating the production of lanthanide nuclei, with atomic mass number A≳140. However, compelling evidence for the presence of even heavier third-peak (A≈195) r-process elements (e.g., gold, platinum) or translead nuclei remains elusive. At early times (∼days) most of the r-process heating arises from a large statistical ensemble of ß decays, which thermalize efficiently while the ejecta is still dense, generating a heating rate that is reasonably approximated by a single power law. However, at later times of weeks to months, the decay energy input can also possibly be dominated by a discrete number of α decays, ^{223}Ra (half-life t_{1/2}=11.43 d), ^{225}Ac (t_{1/2}=10.0 d, following the ß decay of ^{225}Ra with t_{1/2}=14.9 d), and the fissioning isotope ^{254}Cf (t_{1/2}=60.5 d), which liberate more energy per decay and thermalize with greater efficiency than ß-decay products. Late-time nebular observations of kilonovae which constrain the radioactive power provide the potential to identify signatures of these individual isotopes, thus confirming the production of heavy nuclei. In order to constrain the bolometric light to the required accuracy, multiepoch and wideband observations are required with sensitive instruments like the James Webb Space Telescope. In addition, by comparing the nuclear heating rate obtained with an abundance distribution that follows the solar r abundance pattern, to the bolometric lightcurve of AT2017gfo, we find that the yet-uncertain r abundance of ^{72}Ge plays a decisive role in powering the lightcurve, if one assumes that GW170817 has produced a full range of the solar r abundances down to mass number A∼70.

Discovery of an Exceptionally Strong ß-Decay Transition of

A significant fraction of stars between 7 and 11 solar masses are thought to become supernovae, but the explosion mechanism is unclear. The answer depends critically on the rate of electron capture on ^{20}Ne in the degenerate oxygen-neon stellar core. However, because of the unknown strength of the transition between the ground states of ^{20}Ne and ^{20}F, it has not previously been possible to fully constrain the rate. By measuring the transition, we establish that its strength is exceptionally large and that it enhances the capture rate by several orders of magnitude. This has a decisive impact on the evolution of the core, increasing the likelihood that the star is (partially) disrupted by a thermonuclear explosion rather than collapsing to form a neutron star. Importantly, our measurement resolves the last remaining nuclear physics uncertainty in the final evolution of degenerate oxygen-neon stellar cores, allowing future studies to address the critical role of convection, which at present is poorly understood.

Muon Creation in Supernova Matter Facilitates Neutrino-Driven Explosions.

Muons can be created in nascent neutron stars (NSs) due to the high electron chemical potentials and the high temperatures. Because of their relatively lower abundance compared to electrons, their role has so far been ignored in numerical simulations of stellar core collapse and NS formation. However, the appearance of muons softens the NS equation of state, triggers faster NS contraction, and thus leads to higher luminosities and mean energies of the emitted neutrinos. This strengthens the postshock heating by neutrinos and can facilitate explosions by the neutrino-driven mechanism.

First Measurement of Several ß-Delayed Neutron Emitting Isotopes Beyond N=126.

The ß-delayed neutron emission probabilities of neutron rich Hg and Tl nuclei have been measured together with ß-decay half-lives for 20 isotopes of Au, Hg, Tl, Pb, and Bi in the mass region N≳126. These are the heaviest species where neutron emission has been observed so far. These measurements provide key information to evaluate the performance of nuclear microscopic and phenomenological models in reproducing the high-energy part of the ß-decay strength distribution. This provides important constraints on global theoretical models currently used in r-process nucleosynthesis.

Charged-current weak interaction processes in hot and dense matter and its impact on the spectra of neutrinos emitted from protoneutron star cooling.

We perform three-flavor Boltzmann neutrino transport radiation hydrodynamics simulations covering a period of 3 s after the formation of a protoneutron star in a core-collapse supernova explosion. Our results show that a treatment of charged-current neutrino interactions in hot and dense matter as suggested by Reddy et al. [Phys. Rev. D 58, 013009 (1998)] has a strong impact on the luminosities and spectra of the emitted neutrinos. When compared with simulations that neglect mean-field effects on the neutrino opacities, we find that the luminosities of all neutrino flavors are reduced while the spectral differences between electron neutrinos and antineutrinos are increased. Their magnitude depends on the equation of state and in particular on the symmetry energy at subnuclear densities. These modifications reduce the proton-to-nucleon ratio of the outflow, increasing slightly their entropy. They are expected to have a substantial impact on nucleosynthesis in neutrino-driven winds, even though they do not result in conditions that favor an r process. Contrary to previous findings, our results show that the spectra of electron neutrinos remain substantially different from those of other (anti)neutrino flavors during the entire deleptonization phase of the protoneutron star. The obtained luminosity and spectral changes are also expected to have important consequences for neutrino flavor oscillations and neutrino detection on Earth.

Effects of inelastic neutrino-nucleus scattering on supernova dynamics and radiated neutrino spectra.

Based on the shell model for Gamow-Teller and the random phase approximation for forbidden transitions, we calculate cross sections for inelastic neutrino-nucleus scattering (INNS) under supernova (SN) conditions, assuming a matter composition given by nuclear statistical equilibrium. The cross sections are 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%.

Observation of isomeric decays in the r-process waiting-point nucleus 130Cd82.

The gamma decay of excited states in the waiting-point nucleus (130)Cd(82) has been observed for the first time. An 8(+) two-quasiparticle isomer has been populated both in the fragmentation of a (136)Xe beam as well as in projectile fission of 238U, making (130)Cd the most neutron-rich N = 82 isotone for which information about excited states is available. The results, interpreted using state-of-the-art nuclear shell-model calculations, show no evidence of an N = 82 shell quenching at Z = 48. They allow us to follow nuclear isomerism throughout a full major neutron shell from (98)Cd(50) to (130)Cd(82) and reveal, in comparison with (76)Ni(48) one major proton shell below, an apparently abnormal scaling of nuclear two-body interactions.

Gamow-Teller strength in the exotic odd-odd nuclei 138La and 180Ta and its relevance for neutrino nucleosynthesis.

The Gamow-Teller strength distributions below the particle threshold in 138La and 180Ta, deduced from high-resolution measurements of the (3He,t) reaction at 0 degrees, allow us to evaluate the role of charged-current reactions for the production of these extremely rare nuclides in neutrino-nucleosynthesis models. The analysis suggests that essentially all 138La in the Universe can be made that way. Neutrino nucleosynthesis also contributes significantly to the abundance of 180Ta but the magnitude depends on the unknown branching ratio for population of the long-lived isomer.

Spin- and parity-resolved level densities from the fine structure of giant resonances.

Level densities of J pi=2+ and 2- states extracted from high-resolution studies of E2 and M2 giant resonances in 58Ni and 90Zr are used to test recent predictions of a possible parity dependence. The experimental results are compared to a combinatorial approach based on the Hartree-Fock-Bogoliubov model and to shell-model Monte Carlo calculations including both spin and parity projection. No parity dependence is observed experimentally, which is in agreement for 90Zr but in contrast with the model predictions for 58Ni.

Neutrino-induced nucleosynthesis of A>64 nuclei: the nu p process.

We present a new nucleosynthesis process that we denote as the 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 (92,94)Mo and (96,98)Ru. This process also offers a natural explanation for the large abundance of Sr seen in a hyper-metal-poor star.

Supernova inelastic neutrino-nucleus cross sections from high-resolution electron scattering experiments and shell-model calculations.

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.

Consequences of nuclear electron capture in core collapse supernovae.

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 parametrized 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 postbounce evolution in core collapse supernovae.

Electron capture rates on nuclei and implications for stellar core collapse.

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.

Isobaric multiplet yrast energies and isospin nonconserving forces.

The isovector and isotensor energy differences between yrast states of isobaric multiplets in the lower half of the pf region are quantitatively reproduced in a shell model context. The isospin nonconserving nuclear interactions are found to be at least as important as the Coulomb potential. Their isovector and isotensor channels are dominated by J=2 and J=0 pairing terms, respectively. The results are sensitive to the radii of the states, whose evolution along the yrast band can be accurately followed.

Coulomb energy differences in t = 1 mirror rotational bands in (50)Fe and (50)Cr.

Gamma rays from the N = Z-2 nucleus (50)Fe have been observed, establishing the rotational ground state band up to the state J(pi) = 11+ at 6.994 MeV excitation energy. The experimental Coulomb energy differences, obtained by comparison with the isobaric analog states in its mirror (50)Cr, confirm the qualitative interpretation of the backbending patterns in terms of successive alignments of proton and neutron pairs. A quantitative agreement with experiment has been achieved by exact shell model calculations, incorporating the differences in radii along the yrast bands, and properly renormalizing the Coulomb matrix elements in the pf model space.

Nuclear quadrupole moment of 57Fe from microscopic nuclear and atomic calculations.

The nuclear quadrupole moment (NQM) of the Ipi = 3/2(-) excited nuclear state of 57Fe at 14.41 keV, important in Mössbauer spectroscopy, is determined from the large-scale nuclear shell-model calculations for 54Fe, 57Fe, and also from the electronic ab initio and density functional theory calculations including solid state and electron correlation effects for the molecules Fe(CO)(5) and Fe(C5H5)(2). Both independent methods yield very similar results. The recommended value is 0.15(2) e b. The NQM of the isomeric 10+ in 54Fe has also been calculated. The new NQM values for 54Fe and 57Fe are consistent with the perturbed angular distribution data.

Presupernova collapse models with improved weak-interaction rates.

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)M(o). 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 at the onset of collapse and smaller iron core masses. These changes may have important consequences for nucleosynthesis and the supernova explosion mechanism.