Gabriel Martínez Pinedo

We compute the static density and spin structure factors in the long wavelength limit for pure neutron matter at subsaturation densities relevant to core-collapse supernovae within the Brueckner-Hartree-Fock (BHF) approach. The BHF results are reliable at high densities, extending beyond the validity of the virial expansion. Motivated by the similarities between the dilute neutron gas and a unitary gas, we propose a phenomenological approach to derive the static structures with finite momentum transfer as well as the dynamic ones with simple analytical expressions, based on the computed static structures in the long wavelength limit. We also compare the in-medium neutrino-neutron scattering cross sections using different structure factors. Our study emphasizes the importance of accurately computing the static structure factors theoretically and utilizing the full dynamic structure factors in core-collapse supernova simulations.

NUCLEAR REACTIONS 3 He (32 S, α) 31 S, E= 128 MeV; measured reaction products, Eγ, Iγ; deduced γ-ray energies, levels, T 1/2, resonances. Comparison with the shell-model code NuShellX calculations. Modern Markov chain Monte Carlo-based Bayesian statistical techniques. The Doppler Shift Lifetimes (DSL2) facility at the TRIUMF Isotope Separator and Accelerator (ISAC-II) facility.

The aim of the present work is a state-by-state analysis of possible E1 and M1 transitions in Ni with a high-resolution (p,p') experiment at 295 MeV and very forward angles including 0{\deg} and a comparison to results from studies of the dipole strength with the and (e,e') reactions. The E1 and M1 cross sections of individual peaks in the spectra are deduced with a multipole decomposition analysis and converted to reduced E1 and spin-M1 transition strengths using the virtual photon and the unit cross-section method, respectively. Despite the high level density good agreement is obtained for the deduced excitation energies of J = 1 states in the three types of experiments indicating that the same states are excited. The B(E1) and B(M1) strengths from the experiments are systematically smaller than in the present work because of the lack of information on branching ratios to lower-lying excited states and the competition of particle emission. Fair agreement with the B(M1) strengths extracted from the (e,e') data is obtained after removal of E1 transitions uniquely assigned in the present work, which belong to a low-energy toroidal mode with unusual properties mimicking M1 excitations in electron scattering. The experimental M1 strength distribution is compared to large-scale shell-model calculations with the effective GXPF1A and KB3G interactions. They provide a good description of the isospin splitting and the running sum of the M1 strength. A quenching factor 0.74 for the spin-isospin part of the M1 operator is needed to attain quantitative agreement with the data.

The structure of Po 208 populated through the EC/β+ decay of At 208 is investigated using γ-ray spectroscopy at the ISOLDE Decay Station. The presented level scheme contains 27 new excited states and 43 new transitions, as well as a further 50 previously observed γ rays which have been (re) assigned a position. The level scheme is compared to shell model calculations. Through this analysis approximately half of the β-decay strength of At 208 is found to proceed via allowed decay and half via first-forbidden decay. The first-forbidden transitions predominantly populate core excited states at high excitation energies, which is qualitatively understood using shell model considerations. This mass region provides an excellent testing ground for the competition between allowed and first-forbidden β-decay calculations, important for the detailed understanding of the nucleosynthesis of heavy elements.

Neutrinos are known to play important roles in many astrophysical scenarios from the early period of the big bang to current stellar evolution being a unique messenger of the fusion reactions occurring in the center of our sun. In particular, neutrinos are crucial in determining the dynamics and the composition evolution in explosive events such as core-collapse supernovae and the merger of two neutron stars. In this paper, we review the current understanding of supernovae and binary neutron star mergers by focusing on the role of neutrinos therein. Several recent improvements on the theoretical modeling of neutrino interaction rates in nuclear matter as well as their impact on the heavy element nucleosynthesis in the supernova neutrino-driven wind are discussed, including the neutrino-nucleon opacity at the mean field level taking into account the relativistic kinematics of nucleons, the effect due to the nucleon …

Recent analysis of the kilonova, AT2017gfo, has indicated that this event was highly spherical. This may challenge hydrodynamics simulations of binary neutron star mergers, which usually predict a range of asymmetries, and radiative transfer simulations show a strong direction dependence. Here we investigate whether the synthetic spectra from a 3D kilonova simulation of asymmetric ejecta from a hydrodynamical merger simulation can be compatible with the observational constraints suggesting a high degree of sphericity in AT2017gfo. Specifically, we determine whether fitting a simple P-Cygni line profile model leads to a value for the photospheric velocity that is consistent with the value obtained from the expanding photosphere method. We would infer that our kilonova simulation is highly spherical at early times, when the spectra resemble a blackbody distribution. The two independently inferred …

We present a new nucleosynthesis process that may take place on neutron-rich ejecta experiencing an intensive neutrino flux. The nucleosynthesis proceeds similarly to the standard -process, a sequence of neutron-captures and beta-decays, however with charged-current neutrino absorption reactions on nuclei operating much faster than beta-decays. Once neutron capture reactions freeze-out the produced -process neutron-rich nuclei undergo a fast conversion of neutrons into protons and are pushed even beyond the -stability line producing the neutron-deficient -nuclei. This scenario, which we denote as the -process, provides an alternative channel for the production of -nuclei and the short-lived nucleus Nb. We discuss the necessary conditions posed on the astrophysical site for the -process to be realized in nature. While these conditions are not fulfilled by current neutrino-hydrodynamic models of -process sites, future models, including more complex physics and a larger variety of outflow conditions, may achieve the necessary conditions in some regions of the ejecta.

Even though the electromagnetic counterpart AT2017gfo to the binary neutron star merger GW170817 is powered by the radioactive decay of r-process nuclei, only few tentative identifications of light r-process elements have been made so far. One of the major limitations for the identification of heavy nuclei is incomplete or missing atomic data. While substantial progress has been made on lanthanide atomic data over the last few years, for actinides there has been less emphasis, with the first complete set of opacity data only recently published. We perform atomic structure calculations of neodymium (Z = 60) as well as the corresponding actinide uranium (Z = 92). Using two different codes [flexible atomic code (fac) and hartree–fock-relativistic (hfr)] for the calculation of the atomic data, we investigate the accuracy of the calculated data (energy levels and electric dipole transitions) and their effect on kilonova …

Modeling the r-process nucleosynthesis requires a huge amount of nuclear physics input, and in particular, the knowledge of beta-decay rates for thousands of neutron-rich nuclei. Due to their very short lifetimes, most of these nuclei are not accessible experimentally and astrophysical simulations thus heavily rely on theoretical methods, which should be as universal and reliable as possible. In this context, microscopic methods based on the self-consistent mean field, such as the quasiparticle random phase approximation (QRPA), are good candidates as they are applicable to arbitrarily heavy nuclei at relatively low numerical cost. QRPA, however, suffers from a lack of correlations that typically leads to an imprecise description of the nuclear response, in particular the low-energy behavior, which determines beta decay. Extending this approach is thus crucial to improve the precision and reliability of the calculations …

We study the impact of pions in simulations of neutron star mergers and explore the impact on gravitational-wave observables. We model charged and neutral pions as a noninteracting boson gas with a chosen, constant effective mass. We add the contributions of pions, which can occur as a condensate or as a thermal population, to existing temperature-and composition-dependent equations of state. Compared with the models without pions, the presence of a pion condensate decreases the characteristic properties of cold, nonrotating neutron stars such as the maximum mass, the radius and the tidal deformability. We conduct relativistic hydrodynamical simulations of neutron star mergers for these modified equations of state models and compare to the original models, which ignore pions. Generally, the inclusion of pions leads to a softening of the equation of state, which is more pronounced for smaller effective …

We implement a multigroup and discrete-ordinate neutrino transport model in spherical symmetry which allows to simulate collective neutrino oscillations by including realistic collisional rates in a self-consistent way. We utilize this innovative model, based on strategic parameter rescaling, to study a recently proposed collisional flavor instability caused by the asymmetry of emission and absorption rates between ν e and ν e for four different static backgrounds taken from different stages in a core-collapse supernova simulation. Our results confirm that collisional instabilities generally exist around the neutrinosphere during the supernova accretion and postaccretion phase, as suggested by Johns [arXiv: 2104.11369.]. However, the growth and transport of flavor instabilities can only be fully captured by models with global simulations as done in this work. With minimal ingredient to trigger collisional instabilities, we find …

The astrophysical r-process produces about half of the elements heavier than iron in the Universe and all of the transactinides. Recently neutron star mergers have been identified as one site of r-process nucleosynthesis. Simulations of this site and the associated nucleosynthesis requires essential nuclear input, ranging from the Equation of State (EoS) of nuclear matter at extreme densities and temperatures to the properties of very neutron-rich nuclei. Many of these quantities have to be modeled, however, constrained by a steadily increasing amount of experimental data. This manuscript summarizes the knowledge of nuclear input required for r-process studies in neutron star mergers.

We report on precision mass measurements of Ru 113, 115, 117 performed with the JYFLTRAP double Penning trap mass spectrometer at the Accelerator Laboratory of University of Jyväskylä. The phase-imaging ion-cyclotron-resonance technique was used to resolve the ground and isomeric states in Ru 113, 115 and enabled for the first time a measurement of the isomer excitation energies, E x (Ru m 113)= 100.5 (8) keV and E x (Ru m 115)= 129 (5) keV. The ground state of Ru 117 was measured using the time-of-flight ion-cyclotron-resonance technique. The new mass-excess value for Ru 117 is around 36 keV lower and seven times more precise than the previous literature value. With the more precise ground-state mass values, the evolution of the two-neutron separation energies is further constrained and a similar trend as predicted by the BSkG1 model is obtained up to the neutron number N= 71.

We investigate the nucleosynthesis and kilonova properties of binary neutron star (NS) merger models that lead to intermediate remnant lifetimes of∼ 0.1–1 s until black hole (BH) formation and describe all components of the material ejected during the dynamical merger phase, NS remnant evolution, and final viscous disintegration of the BH torus after gravitational collapse. To this end, we employ a combination of hydrodynamics, nucleosynthesis, and radiative transfer tools to achieve a consistent end-to-end modeling of the system and its observables. We adopt a novel version of the Shakura–Sunyaev scheme allowing the approximate turbulent viscosity inside the NS remnant to vary independently of the surrounding disk. We find that asymmetric progenitors lead to shorter remnant lifetimes and enhanced ejecta masses, although the viscosity affects the absolute values of these characteristics. The integrated …

[en] The 2017 observation of the electromagnetic counterpart to the gravitational wave signal GW170817 provided direct evidence that r-process elements are created in neutron-star mergers. The electromagnetic transient, also known as a kilonova, has revealed two distinct ejecta components: one containing heavy r-process material, including lanthanides and potentially actinides, and a second one characterized by low lanthanide abundance. Spectroscopic data can often be used to identify features due to specific elements. However, having accurate atomic data is crucial for the interpretation of observed spectra, as even small relative errors (eg, a few percent) in the transition wavelengths, or larger errors (tens of percent) in the transition strengths can make the task more difficult or even impossible to achieve. The lack of atomic data has led to a number of computations of weakly ionised r-process opacities, primarily focussing on lanthanides, being published in recent years. Nevertheless, if the merging process results in the ejection of material with an electron fraction (Y e) of 0.15 or less, nucleosynthesis should progress to actinides, which are projected to possess photon opacities similar to, or perhaps higher than, lanthanides. Moreover, large discrepancies can still be found in the opacities, which are related to the uncertainty of the atomic data produced using different methodologies.

The accretion disk formed after a neutron star merger is an important contributor to the total ejecta from the merger, and hence to the kilonova and the -process yields of each event. Axisymmetric viscous hydrodynamic simulations of these disks can capture thermal mass ejection due to neutrino absorption and in the advective phase -- after neutrino cooling has subsided -- and are thus likely to provide a lower-limit to the total disk ejecta relative to MHD evolution. Here we present a comparison between two viscous hydrodynamic codes that have been used extensively on this problem over the past decade: ALCAR and FLASH. We choose a representative setup with a black hole at the center, and vary the treatment of viscosity and neutrino transport. We find good overall agreement ( level) in most quantities. The average outflow velocity is sensitive to the treatment of the nuclear binding energy of heavy nuclei, showing a larger variation than other quantities. We post-process trajectories from both codes with the same nuclear network, and explore the effects of code differences on nucleosynthesis yields, heating rates, and kilonova light curves. For the latter, we also assess the effect of varying the number of tracer particles in reconstructing the spatial abundance distribution for kilonova light curve production.

We present the state-of-the-art single-zone nuclear reaction network WinNet, which is capable of calculating the nucleosynthetic yields of a large variety of astrophysical environments and conditions. This ranges from the calculation of the primordial nucleosynthesis, where only a few nuclei are considered, to the ejecta of neutron star mergers with several thousands of involved nuclei. Here we describe the underlying physics and implementation details of the reaction network. We additionally present the numerical implementation of two different integration methods, the implicit Euler method and Gears method, along with their advantages and disadvantages. We furthermore describe basic example cases of thermodynamic conditions that we provide together with the network and demonstrate the reliability of the code by using simple test cases. With this publication, WinNet will be publicly available and open source …

Variation in the proton single-particle energy gaps is investigated within the framework of nuclear shell model. The change is identified to originate mainly from the central force of the proton-neutron interaction. The relationship between the nuclear state and the single-particle energy gap is discussed. The first 3/2− state of 55Sc reveals the fragile character of N = 34 semi-magic shell gap.

The gravitational wave signal GW170817 from a merger of two neutron stars has been complemented by a rich dataset of electromagnetic signals from the associated kilonova, AT2017gfo 1, 2. Simple analysis of the luminosity decline rate of AT2017gfo indicates decaying material that has undergone rapid neutron captures, a regime responsible for producing many of the elements beyond Fe.The time-series spectra of the kilonova in the first few days after the merger in principle provide a high level of detail about the ejecta composition and physical conditions, if we have the tools to interpret them. Detailed interpretation of these spectra requires knowledge of a large amount of relevant atomic and nuclear data, and numerical modelling tools to make the connections between the initial conditions, subsequent evolution, and the emitted spectra for comparison to observations.

We present 3D radiative transfer calculations for the ejecta from a neutron star merger that include line-by-line opacities for tens of millions of bound–bound transitions, composition from an r-process nuclear network, and time-dependent thermalization of decay products from individual α and β− decay reactions. In contrast to expansion opacities and other wavelength-binned treatments, a line-by-line treatment enables us to include fluorescence effects and associate spectral features with the emitting and absorbing lines of individual elements. We find variations in the synthetic observables with both the polar and azimuthal viewing angles. The spectra exhibit blended features with strong interactions by Ce iii, Sr ii, Y ii, and Zr ii that vary with time and viewing direction. We demonstrate the importance of wavelength calibration of atomic data using a model with calibrated Sr, Y, and Zr data, and find major differences in …