arXiv subjects

A. Arcones

Publications and source records attributed to A. Arcones.

At least 19 recordsLinked to original sources

arXiv2025

Constraining r-process nucleosynthesis with multi-objective Galactic chemical evolution models

The astrophysical site(s) of the r-process are uncertain, with candidates such as neutron star mergers and magneto-rotational supernovae predicting different event rates, delay times, and heavy-element yields. Galactic chemical evolution models constrain these properties by comparing model predictions with observed abundances. We explore, in a systematic and data-driven way, the astrophysical conditions under which r-process enrichment can reproduce the observed trends of multiple neutron-capture elements in the Milky Way. Rather than assuming a fixed site, we adopt a flexible, parametric approach to test whether a common set of r-process parameters can explain the chemical evolution of several heavy elements. We compute a grid of one-infall, homogeneous models varying: Eu yield per event, r-process event rate, enrichment delay time, and progenitor mass range. For each of the $\sim 1.5 \times 10^5$ models, we predict [X/Fe] vs. [Fe/H] trends by scaling Eu yields with the solar r-process pattern. A multi-objective optimisation based on Pareto fronts identifies models that best reproduce the abundance trends. Best-fitting models favour short delay times ($\leq 30\ \rm Myr$), low-mass progenitors ($\sim 20-25\ \rm M_\odot$), and an effective Eu injection of $\sim 2 \times 10^{-7}\ \rm M_\odot$ per event. Stars more massive than $\sim 80\ \rm M_\odot$ are too rare to dominate the enrichment. While heavy elements can be reproduced, lighter ones show stronger conflicts with Eu, reflecting that the solar r-process scaling relation becomes less valid toward lighter elements. No single class of r-process events, under solar-scaled yields, can explain light and heavy neutron-capture elements; at least two components are required: a main r-process consistent with solar and r-rich stars, and a weaker component producing enhanced light r-process elements, similar to that observed in r-poor stars.

astro-ph.GA↗

arXiv2025

Impact of newly measured $\beta$\nobreakdash-delayed neutron emitters around \myisoSimp{78}{Ni} on light element nucleosynthesis in the neutrino-wind following a neutron star merger

Neutron emission probabilities and half-lives of 37 beta-delayed neutron emitters from 75Ni to 92Br were measured at the RIKEN Nishina Center in Japan, including 11 one-neutron and 13 two-neutron emission probabilities and 6 half-lives measured for the first time, which supersede theoretical estimates. These nuclei lie in the path of the weak r-process occurring in neutrino-driven winds from the accretion disk formed after the merger of two neutron stars, synthesizing elements in the A~80 abundance peak. The presence of such elements dominates the accompanying kilonova emission over the first few days and has been identified in the AT2017gfo event, associated with the gravitational wave detection GW170817. Abundance calculations based on over 17000 simulated trajectories describing the evolution of matter properties in the merger outflows show that the new data lead to an increase of 50-70 percent in the abundance of Y, Zr, Nb, and Mo. This enhancement is large compared to the scatter of relative abundances observed in old very metal-poor stars and is therefore significant in the comparison with other possible astrophysical processes contributing to light-element production. These results underline the importance of including experimental decay data for very neutron-rich beta-delayed neutron emitters into r-process models.

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arXiv2024

Nucleosynthesis in magnetorotational supernovae: impact of the magnetic field configuration

The production of heavy elements is one of the main by-products of the explosive end of massive stars. A long sought goal is finding differentiated patterns in the nucleosynthesis yields, which could permit identifying a number of properties of the explosive core. Among them, the traces of the magnetic field topology are particularly important for \emph{extreme} supernova explosions, most likely hosted by magnetorotational effects. We investigate the nucleosynthesis of five state-of-the-art magnetohydrodynamic models with fast rotation that have been previously calculated in full 3D and that involve an accurate neutrino transport (M1). One of the models does not contain any magnetic field and synthesizes elements around the iron group, in agreement with other CC-SNe models in literature. All other models host a strong magnetic field of the same intensity, but with different topology. For the first time, we investigate the nucleosynthesis of MR-SNe models with a quadrupolar magnetic field and a 90 degree tilted dipole. We obtain a large variety of ejecta compositions reaching from iron nuclei to nuclei up to the third r-process peak. We assess the robustness of our results by considering the impact of different nuclear physics uncertainties such as different nuclear masses, $\beta^{-}$-decays and $\beta^{-}$-delayed neutron emission probabilities, neutrino reactions, fission, and a feedback of nuclear energy on the temperature. We find that the qualitative results do not change with different nuclear physics input. The properties of the explosion dynamics and the magnetic field configuration are the dominant factors determining the ejecta composition.

astro-ph.HE↗

arXiv2023

Neutrino-Driven Outflows and the Elemental Abundance Patterns of Very Metal-Poor Stars

The elemental abundances between strontium and silver ($Z = 38-47$) observed in the atmospheres of very metal-poor stars (VMP) in the Galaxy may contain the fingerprint of the weak $r$-process and $\nu p$-process occurring in early core-collapse supernovae explosions. In this work, we combine various astrophysical conditions based on a steady-state model to cover the richness of the supernova ejecta in terms of entropy, expansion timescale, and electron fraction. The calculated abundances based on different combinations of conditions are compared with stellar observations with the aim of constraining supernova ejecta conditions. We find that some conditions of the neutrino-driven outflows consistently reproduce the observed abundances of our sample. In addition, from the successful combinations, the neutron-rich trajectories better reproduce the observed abundances of Sr-Zr ($Z= 38-40$), while the proton-rich ones, Mo-Pd ($Z= 42-47$).

astro-ph.HE↗

arXiv2023

The nuclear reaction network WinNet

We present the state-of-the-art single-zone nuclear reaction network WinNet that 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 is publicly available and open source at GitHub and Zenodo.

astro-ph.IM↗

arXiv2022

Horizons: Nuclear Astrophysics in the 2020s and Beyond

Nuclear Astrophysics is a field at the intersection of nuclear physics and astrophysics, which seeks to understand the nuclear engines of astronomical objects and the origin of the chemical elements. This white paper summarizes progress and status of the field, the new open questions that have emerged, and the tremendous scientific opportunities that have opened up with major advances in capabilities across an ever growing number of disciplines and subfields that need to be integrated. We take a holistic view of the field discussing the unique challenges and opportunities in nuclear astrophysics in regards to science, diversity, education, and the interdisciplinarity and breadth of the field. Clearly nuclear astrophysics is a dynamic field with a bright future that is entering a new era of discovery opportunities.

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arXiv2022

Constraining nucleosynthesis in neutrino-driven winds: observations, simulations and nuclear physics

A promising astrophysical site to produce the lighter heavy elements of the first $r$-process peak ($Z = 38-47$) is the moderately neutron rich ($0.4 < Y_e < 0.5$) neutrino-driven ejecta of explosive environments, such as core-collapse supernovae and neutron star mergers, where the weak $r$-process operates. This nucleosynthesis exhibits uncertainties from the absence of experimental data from $(α,xn)$ reactions on neutron-rich nuclei, which are currently based on statistical model estimates. In this work, we report on a new study of the nuclear reaction impact using a Monte Carlo approach and improved $(α,xn)$ rates based on the Atomki-V2 $α$ Optical Model Potential ($α$OMP). We compare our results with observations from an up-to-date list of metal-poor stars with [Fe/H] $<$ -1.5 to find conditions of the neutrino-driven wind where the lighter heavy elements can be synthesized. We identified a list of $(α,xn)$ reaction rates that affect key elemental ratios in different astrophysical conditions. Our study aims on motivating more nuclear physics experiments on $(α, xn)$ reactions using current and the new generation of radioactive beam facilities and also more observational studies of metal-poor stars.

astro-ph.HE↗

arXiv2021

Post-explosion evolution of core-collapse supernovae

We investigate the post-explosion phase in core-collapse supernovae with 2D hydrodynamical simulations and a simple neutrino treatment. The latter allows us to perform 46 simulations and follow the evolution of the 32 successful explosions during several seconds. We present a broad study based on three progenitors (11.2 $M_\odot$, 15 $M_\odot$, and 27 $M_\odot$), different neutrino-heating efficiencies, and various rotation rates. We show that the first seconds after shock revival determine the final explosion energy, remnant mass, and properties of ejected matter. Our results suggest that a continued mass accretion increases the explosion energy even at late times. We link the late-time mass accretion to initial conditions such as rotation strength and shock deformation at explosion time. Only some of our simulations develop a neutrino-driven wind that survives for several seconds. This indicates that neutrino-driven winds are not a standard feature expected after every successful explosion. Even if our neutrino treatment is simple, we estimate the nucleosynthesis of the exploding models for the 15 $M_\odot$ progenitor after correcting the neutrino energies and luminosities to get a more realistic electron fraction.

astro-ph.HE↗

arXiv2021

Extreme r-process enhanced stars at high metallicity in Fornax

We present and discuss three extremely r-process enhanced stars located in the massive dwarf spheriodal galaxy Fornax. These stars are very unique with an extreme Eu enrichment ($1.25\le \mathrm{[Eu/Fe]} \le 1.45$) at high metallicities ($-1.3 \le \mathrm{[Fe/H]} \le -0.8$). They have the largest Eu abundances ever observed in a dwarf galaxy opening new opportunities to further understand the origin of heavy elements formed by the r-process. We derive stellar abundances of Co, Zr, La, Ce, Pr, Nd, Er, and Lu using 1-dimensional, local thermodynamic equilibrium (LTE) codes and model atmospheres in conjunction with state-of-the art yield predictions. We derive Zr in the largest sample of stars (105) known to date in a dwarf galaxy. Accurate stellar abundances combined with a careful assessment of the yield predictions have revealed three metal-rich stars in Fornax showing a pure r-process pattern. We define a new class of stars, namely Eu-stars, as r-II stars (i.e., [Eu/Fe]$>1$) at high metallicities (i.e., $\mathrm{[Fe/H]}\gtrsim -1.5$). The stellar abundance pattern contains Lu, observed for the first time in a dwarf galaxy, and reveals that a late burst of star formation has facilitated extreme r-process enhancement late in the galaxy's history ($<4$\,Gyr ago). Due to the large uncertainties associated with the nuclear physics input in the yield predictions, we cannot yet determine the r-process site leading to the three Eu-stars in Fornax. Our results demonstrate that extremely r-rich stars are not only associated with ultra faint low-mass dwarf galaxies, but can be born also in massive dwarf galaxies.

astro-ph.GA↗

arXiv2021

Low Energy measurement of the $^{96}\mathrm{Zr}(α,n)^{99}\mathrm{Mo}$ reaction cross section and its impact on weak r-process nucleosynthesis

Lighter heavy elements beyond iron and up to around silver can form in neutrino-driven ejecta in core-collapse supernovae and neutron star mergers. Slightly neutron-rich conditions favour a weak r-process that follows a path close to stability. Therefore, the beta decays are slow compared to the expansion time scales, and ($α$,n) reactions become critical to move matter towards heavier nuclei. The rates of these reactions are calculated with the statistical model and their main uncertainty, at energies relevant for the weak r-process, is the $α$+nucleus optical potential. There are several sets of parameters to calculate the $α$+nucleus optical potential leading to large deviations for the reaction rates, exceeding even one order of magnitude. Recently the $^{96}$Zr($α$,n)$^{99}$Mo reaction has been identified as a key reaction that impacts the production of elements from Ru to Cd. Here, we present the first cross section measurement of this reaction at energies (6.22 MeV $\leq$ E$_\mathrm{c.m.}$ $\leq$ 12.47 MeV) relevant for the weak r-process. The new data provide a stringent test of various model predictions which is necessary to improve the precision of the weak r-process network calculations. The strongly reduced reaction rate uncertainty leads to very well-constrained nucleosynthesis yields for $Z = 44 - 48$ isotopes under different neutrino-driven wind conditions.

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arXiv2020

Nuclear physics uncertainties in neutrino-driven, neutron-rich supernova ejecta

Neutrino-driven ejecta in core collapse supernovae (CCSNe) offer an interesting astrophysical scenario where lighter heavy elements between Sr and Ag can be synthesized. Previous studies emphasized the important role that ($α,n$) reactions play in the production of these elements, particularly in neutron-rich and alpha-rich environments. In this paper, we have investigated the sensitivity of elemental abundances to specific ($α,n$) reaction-rate uncertainties under different astrophysical conditions. Following a Monte Carlo nucleosynthesis study with over 36 representative astrophysical wind conditions, we have identified the most important reactions based on their impact on the final elemental abundances. Experimental studies of these reactions will reduce the nucleosynthesis uncertainties and make it possible to use observations to understand the origin of lighter heavy elements and the astrophysical conditions where they are formed.

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arXiv2018

Unusual neutron-capture nucleosynthesis in a carbon-rich Galactic bulge star

Metal-poor stars in the Galactic halo often show strong enhancements in carbon and/or neutron-capture elements. However, the Galactic bulge is notable for its paucity of carbon-enhanced metal-poor (CEMP) and/or CH-stars, with only two such objects known to date. This begs the question whether the processes that produced their abundance distribution were governed by a comparable nucleosynthesis in similar stellar sites as for their more numerous counterparts in the halo. Recently, two contenders of such stars were discovered in the bulge, at [Fe/H] = $-1.5$ and $-$2.5 dex, both of which show enhancements in [C/Fe] of 0.4 and 1.4 dex, [Ba/Fe] in excess of 1.3 dex, and also elevated nitrogen. The more metal-poor of the stars is matched by standard $s$-process nucleosynthesis in low-mass Asymptotic Giant Branch (AGB) polluters. The other star shows an abnormally high [Rb/Fe] ratio. Here, we investigate the origin of the abundance peculiarities in the Rb-rich star by new, detailed measurements of heavy element abundances and by comparing the chemical element ratios of 36 species to models of neutron-capture nucleosynthesis. The $i$-process with intermediate neutron densities between those of the $s$- and $r$-processes has been previously found to provide good matches of CEMP stars with enhancements in both $r$- and $s$-process elements, rather than invoking a superposition of yields from the respective individual processes. However, the peculiar bulge star is incompatible with a pure $i$-process from a single ingestion event. Instead, it can, statistically, be better reproduced by models accounting for two proton ingestion events, or by an $i$-process component in combination with $s$-process nucleosynthesis in low-to-intermediate mass AGB stars, indicating multiple polluters. [abridged]

astro-ph.GA↗

arXiv2018

FRIB and the GW170817 Kilonova

In July 2018 an FRIB Theory Alliance program was held on the implications of GW170817 and its associated kilonova for r-process nucleosynthesis. Topics of discussion included the astrophysical and nuclear physics uncertainties in the interpretation of the GW170817 kilonova, what we can learn about the astrophysical site or sites of the r process from this event, and the advances in nuclear experiment and theory most crucial to pursue in light of the new data. Here we compile a selection of scientific contributions to the workshop, broadly representative of progress in r-process studies since the GW170817 event.

astro-ph.HE↗

arXiv2018

r-Process Nucleosynthesis: Connecting Rare-Isotope Beam Facilities with the Cosmos

This is an exciting time for the study of r-process nucleosynthesis. Recently, a neutron star merger GW170817 was observed in extraordinary detail with gravitational waves and electromagnetic radiation from radio to gamma rays. The very red color of the associated kilonova suggests that neutron star mergers are an important r-process site. Astrophysical simulations of neutron star mergers and core collapse supernovae are making rapid progress. Detection of both, electron neutrinos and antineutrinos from the next galactic supernova will constrain the composition of neutrino-driven winds and provide unique nucleosynthesis information. Finally FRIB and other rare-isotope beam facilities will soon have dramatic new capabilities to synthesize many neutron-rich nuclei that are involved in the r-process. The new capabilities can significantly improve our understanding of the r-process and likely resolve one of the main outstanding problems in classical nuclear astrophysics. However, to make best use of the new experimental capabilities and to fully interpret the results, a great deal of infrastructure is needed in many related areas of astrophysics, astronomy, and nuclear theory. We will place these experiments in context by discussing astrophysical simulations and observations of r-process sites, observations of stellar abundances, galactic chemical evolution, and nuclear theory for the structure and reactions of very neutron-rich nuclei. This review paper was initiated at a three-week International Collaborations in Nuclear Theory program in June 2016 where we explored promising r-process experiments and discussed their likely impact, and their astrophysical, astronomical, and nuclear theory context.

astro-ph.SR↗

arXiv2017

The role of weak interactions in dynamic ejecta from binary neutron star mergers

Weak reactions are critical for the neutron richness of the matter dynamically ejected after the merger of two neutron stars. The neutron richness, defined by the electron fraction (Ye), determines which heavy elements are produced by the r-process and thus directly impacts the kilonova light curve. In this work, we have performed a systematic and detailed post-processing study of the impact of weak reactions on the distribution of the electron fraction and of the entropy on the dynamic ejecta obtained from an equal mass neutron star binary merger simulated in full general relativity and with microscopic equation of state. Previous investigations indicated that shocks increase Ye, however our results show that shocks can also decrease Ye, depending on their thermodynamical conditions. Moreover, we have found that neutrino absorption are key and need to be considered in future simulations. We also demonstrated that the angular dependence of the neutrino luminosity and the spatial distribution of the ejecta can lead to significant difference in the electron fraction distribution. In addition to the detailed study of the Ye evolution and its dependences, we have performed nucleosynthesis calculations. They clearly point to the necessity of improving the neutrino treatment in current simulations to be able to predict the contribution of neutron star mergers to the chemical history of the universe and to reliable calculate their kilonova light curves.

astro-ph.HE↗

arXiv2015

First measurement of several $β$-delayed neutron emitting isotopes beyond N=126

The $β$-delayed neutron emission probabilities of neutron rich Hg and Tl nuclei have been measured together with $β$-decay half-lives for 20 isotopes of Au, Hg, Tl, Pb and Bi in the mass region N$\gtrsim$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 $β$-decay strength distribution. In doing so, it provides important constraints to global theoretical models currently used in $r$-process nucleosynthesis.

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arXiv2015

How many nucleosynthesis processes exist at low metallicity?

Abundances of low-metallicity stars offer a unique opportunity to understand the contribution and conditions of the different processes that synthesize heavy elements. Many old, metal-poor stars show a robust abundance pattern for elements heavier than Ba, and a less robust pattern between Sr and Ag. Here we probe if two nucleosynthesis processes are sufficient to explain the stellar abundances at low metallicity, and we carry out a site independent approach to separate the contribution from these two processes or components to the total observationally derived abundances. Our approach provides a method to determine the contribution of each process to the production of elements such as Sr, Zr, Ba, and Eu. We explore the observed star-to-star abundance scatter as a function of metallicity that each process leads to. Moreover, we use the deduced abundance pattern of one of the nucleosynthesis components to constrain the astrophysical conditions of neutrino-driven winds from core-collapse supernovae.

astro-ph.SR↗

arXiv2014

Local and global effects of beta decays on r-process

Nuclear beta decay rates are an essential ingredient in simulations of the astrophysical r-process. Most of these rates still rely on theoretical modeling. However, modern radioactive ion-beam facilities have allowed to measure beta half lives of some nuclei on or close to the r-process path. These data indicate that r-process half lives are in general shorter than anticipated in the standard theoretical predictions based on the Finite Range Droplet Model (FRDM). The data have also served as important constraints for improved predictions of half lives based on continuum QRPA calculations on top of the energy-density functional theory. Although these calculations are yet limited to spherical nuclei, they include the important r-process waiting point nuclei close to and at the neutron magic numbers $N=50, 82$ and 126. We have studied the impact of these new experimental and theoretical half lives on r-process nucleosynthesis within the two astrophysical sites currently favored for the r process: the neutrino-driven wind from the freshly born neutron star in a supernova explosion and the ejecta of the merger of two neutron stars. We find that the, in general, shorter beta decay rates have several important effects on the dynamics of r-process nucleosynthesis. At first, the matter flow overcomes the waiting point nuclei faster enhancing matter transport to heavier nuclei. Secondly, the shorter half lives result also in a faster consumption of neutrons resulting in important changes of the conditions at freeze-out with consequences for the final r-process abundances. Besides these global effects on the r-process dynamics, the new half lives also lead to some local changes in the abundance distributions.

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