arXiv subjects

Almudena Arcones

Publications and source records attributed to Almudena Arcones.

At least 19 recordsLinked to original sources

arXiv2026

Bayesian Inference of Stellar r-Process Abundances

The abundances of heavy elements observed in metal-poor, r-process-enhanced stars provide unique information about the astrophysical conditions in which the rapid neutron-capture process (r-process) occurs. We present a Bayesian framework to infer these conditions from stellar abundance patterns. Building on a large site-independent r-process survey, we use a Markov chain Monte Carlo (MCMC) sampler to fit observed abundances with weighted superpositions of abundances from nucleosynthesis calculations, each parameterized by an initial electron fraction, entropy, and expansion timescale. Applying this framework to the nearly complete r-process template star HD222925, we find that two components are required: A heavier H-component producing elements from the second to the third peak and a lighter L-component producing elements from the first to the second peak. Increasing the number of components does not significantly improve the agreement with observations. Extending the analysis to the limited-r stars HD128279 and HD122563, and to the solar r-process residuals, we find that their abundances can be reproduced by almost the same two components, albeit with different relative weights. Moreover, the lightest neutron-capture elements ($Z \lesssim 35$) in the Sun require additional contributions absent in HD222925. Residual discrepancies are concentrated in elements sensitive to observational systematics (e.g., Ag and Cd) or nuclear-physics inputs (around the third peak). Our study highlights the power of combining stellar abundances with nucleosynthesis calculations to constrain the astrophysical conditions of the r-process, and motivates further improvements in both observational data and nuclear-physics inputs.

astro-ph.SR↗

arXiv2026

HRMOS: A High-Resolution Multi-Object Spectrograph for the VLT

This White Paper presents the scientific rationale and instrument concept for HRMOS (High-Resolution Multi-Object Spectrograph), a next-generation instrument proposed for the ESO Very Large Telescope within the VLT 2030 roadmap. Current and planned facilities offer either multi-object spectroscopy or ultra-high spectral resolution, but not both. HRMOS fills this gap by combining very high spectral resolution, multi-object capability, and radial-velocity stability, enabling transformative studies in Galactic and extragalactic astrophysics. The baseline design provides a resolving power of R = 80000, radial-velocity precision of 10 m s-1 (goal: 5 m s-1), simultaneous observations of 50-60 targets, and broad optical coverage down to 385 nm. These capabilities enable precise measurements of elemental abundances, isotopic ratios, line profiles, and radial velocities for large stellar samples, including crowded fields, star clusters, the Galactic bulge, and nearby dwarf galaxies. HRMOS will address key questions on the age of the oldest stellar populations through nucleocosmochronology, the formation and survival of planetary systems, the assembly history of the Milky Way and satellites, the origin of the heaviest elements, stellar evolution, and the chemical and dynamical properties of the interstellar and circumgalactic medium. It will bridge large spectroscopic surveys and the next generation of extremely large telescopes, with strong synergies with 4MOST, Gaia, TESS, PLATO, the proposed Haydn mission, and future ELT instruments. Building on VLT/FLAMES heritage, HRMOS represents a strategic investment for European astronomy in the 2030s.

astro-ph.IM↗

arXiv2026

Propagating Uncertainties from Nuclear Physics to Gamma-rays in Core Collapse Supernovae

Nuclear yields are powerful probes of supernova explosions, their engines and their progenitors. In addition, as we improve our understanding of these explosions, we can use nuclear yields to probe dense matter and neutrino physics, both of which play a critical role in the central supernova engine. Especially with upcoming gamma-ray detectors that can directly detect radioactive isotopes out to increasing distances from gamma-rays emitted during their decay, nuclear yields have the potential to provide some of the most direct probes of supernova engines and stellar burning. To utilize these probes, we must understand and limit the uncertainties in their production. Uncertainties in the nuclear physics can be minimized by combining both laboratory experiments and nuclear theory. Similarly, astrophysical uncertainties caused by simplified explosion trajectories can be minimized by higher-fidelity stellar-evolution and supernova-engine models. This paper reviews the physics and astrophysics uncertainties in modeling nucleosynthetic yields, identifying the key areas of study needed to maximize the potential of supernova yields as probes of astrophysical transients and dense-matter physics.

astro-ph.HE↗

arXiv2025

r-Process Nucleosynthesis With Ab Initio Nuclear Masses Around The N=82 Shell Closure

Our understanding of the origin of heavy elements beyond iron relies on the rapid neutron capture process (r-process), which accounts for roughly half of their cosmic abundance. However, the extreme neutron-rich conditions required for the r-process involve many nuclei that remain experimentally inaccessible, making theoretical predictions essential. We explore the impact of nuclear masses calculated with the ab initio valence-space in-medium similarity renormalization group, focusing on the region around the N = 82 shell closure. We show for the first time that such ab initio mass calculations can be used to refine r-process predictions compared to global, but more phenomenological mass models. With the ab initio masses, the waiting point of the second r-process peak is strengthened, which leads to an overall slower nucleosynthesis flow, lower abundances of nuclei beyond the peak, and a stronger shift of the third r-process peak.

astro-ph.HE↗

arXiv2025

Complete survey of r-process conditions: the (un-)robustness of the r-process(-es)

Heavy elements are synthesized by the r-process in neutron star mergers and potentially in rare supernovae linked to strong magnetic fields. Expensive hydrodynamic simulations of these extreme environments are usually post-processed to calculate the nucleosynthesis. In contrast, here we follow a site-independent approach based on three key parameters: electron fraction, entropy, and expansion timescale. Our model reproduces the results based on hydrodynamic simulations. Moreover, the 120 000 astrophysical conditions analyzed allow us to systematically and generally explore the astrophysical conditions of the r-process, also beyond those found in current simulations. Our results show that a wide range of conditions produce very similar abundance patterns explaining the observed robustness of the r-process between the second and third peak. Furthermore, we cannot find a single condition that produces the full r-process from first to third peak. Instead, a superposition of at least two or three conditions or components is required to reproduce the typical r-process pattern as observed in the solar system and very old stars. The different final abundances are grouped into eight nucleosynthesis clusters, which can be used to select representative conditions for comparisons to observations and investigations of the nuclear physics input.

astro-ph.HE↗

arXiv2025

$^{56}$Ni production in long-lived binary neutron star merger remnants

We investigate the nucleosynthesis and kilonova emission based on numerical-relativity binary neutron star merger simulations that incorporate a two-moment neutrino-transport scheme. Unlike in previous works with simpler neutrino treatments, a massive, fast (up to $v=0.3c$), proton-rich neutrino-driven wind develops in the post-merger phase of the simulations as long as the merger remnant does not collapse to a black hole. We evolve the ejecta for 100 days after the merger using 2D ray-by-ray radiation-hydrodynamics simulations coupled in-situ to a complete nuclear network. The most abundant nucleosynthesis products are He, $^{56}$Ni, and $^{56}$Co. We find a total yield of $\sim 10^{-3} M_\odot$ of $^{56}$Ni for all mergers that produce massive neutron star remnants, independently of the mass ratio and equation of state. After a few days, the decay of $^{56}$Ni and later $^{56}$Co becomes the primary source of heating in the matter expanding above the remnant. As a result, the kilonova light curve flattens on timescales of days for polar observation angles. The observation of this effect could serve as smoking gun for the presence of a long-lived neutron star remnant in future kilonova observations.

astro-ph.HE↗

arXiv2025

Modeling the emission lines from r-process elements in Supernova nebulae

The origin of heavy r-process elements in the universe is still a matter of great debate, with a confirmed scenario being neutron star (NS) mergers. Additional relevant sites could be specific classes of events, such as gamma-ray burst (GRB) Supernovae (SNe), where a central engine could push neutron-rich material outwards, contributing to the ejecta of the massive exploding star. Here, we investigate our ability to infer the production of heavy elements in such scenarios, on the basis of the observed nebular emission. We solve the steady-state ionization, level population, and thermal balance, for optically thin ejecta in non-local thermodynamic equilibrium (NLTE), in order to explore the role of heavy elements in cooling the gas, and their imprint in the emergent spectrum a few hundreds days post-explosion. We find that heavy elements would be relevant in the cooling process of the nebula only if they account for at least $\sim1\%$ of the total ejected mass, at the typical kinetic temperatures of a few thousands K. However, even in the absence of such amount, a few $0.1\%$ of the total ejected mass could be instead sufficient to leave a detectable imprint around $\sim1-10~\mathrm{\mu m}$. This wavelength range, which would be relatively clean from features due to light elements, would be instead robustly populated by lines from heavy elements arising from forbidden transitions in their atomic fine structures. Hence, the new generation of telescopes, represented by the James Webb Space Telescope (JWST), will most likely allow for their detection.

astro-ph.HE↗

arXiv2024

Chemical Evolution of R-process Elements in Stars (CERES): IV. An observational run-up of the third r-process peak with Hf, Os, Ir, and Pt

The third r-process peak (Os, Ir, Pt) is poorly understood due to observational challenges, with spectral lines located in the blue or near-ultraviolet region of stellar spectra. These challenges need to be overcome for a better understanding of the r-process in a broader context. To understand how the abundances of the third r-process peak are synthesised and evolve in the Universe, a homogeneous chemical analysis of metal-poor stars using high quality data observed in the blue region of the electromagnetic spectrum (< 400 nm) is necessary. We provide a homogeneous set of abundances for the third r-process peak (Os, Ir, Pt) and Hf, increasing by up to one order of magnitude their availability in the literature. A classical 1D, local thermodynamic equilibrium (LTE) analysis of four elements (Hf, Os, Ir, Pt) is performed, using ATLAS model atmospheres to fit synthetic spectra in high resolution (> 40,000), high signal-to-noise ratio, of 52 red giants observed with UVES/VLT. Due to the heavy line blending involved, a careful determination of upper limits and uncertainties is done. The observational results are compared with state-of-the-art nucleosynthesis models. Our sample displays larger abundances of Ir (Z=77) in comparison to Os (Z=76), which have been measured in a few stars in the past. The results also suggest decoupling between abundances of third r-process peak elements with respect to Eu (rare earth element) in Eu-poor stars. This seems to contradict a co-production scenario of Eu and the third r-process peak elements Os, Ir, and Pt in the progenitors of these objects. Our results are challenging to explain from the nucleosynthetic point of view: the observationally derived abundances indicate the need for an additional early, primary formation channel (or a non-robust r-process).

astro-ph.SR↗

arXiv2024

Neutrino-driven Core-collapse Supernova Yields in Galactic Chemical Evolution

We provide yields from 189 neutrino-driven core-collapse supernova (CCSN) simulations covering zero-age main sequence masses between 11 and 75 solar masses and three different metallicities. Our CCSN simulations have two main advantages compared to previous methods used for applications in Galactic chemical evolution (GCE). Firstly, the mass cut between remnant and ejecta evolves naturally. Secondly, the neutrino luminosities and thus the electron fraction are not modified. Both is key to obtain an accurate nucleosynthesis. We follow the composition with an in-situ nuclear reaction network including the 16 most abundant isotopes and use the yields as input in a GCE model of the Milky Way. We adopt a GCE which takes into account infall of gas as well as nucleosynthesis from a large variety of stellar sources. The GCE model is calibrated to reproduce the main features of the solar vicinity. For the CCSN models, we use different calibrations and propagate the uncertainty. We find a big impact of the CCSN yields on our GCE predictions. We compare the abundance ratios of C, O, Ne, Mg, Si, S, Ar, Ca, Ti, and Cr with respect to Fe to an observational data set as homogeneous as possible. From this, we conclude that at least half of the massive stars have to explode to match the observed abundance ratios. If the explosions are too energetic, the high amount of iron will suppress the abundance ratios. With this, we demonstrate how GCE models can be used to constrain the evolution and deaths of massive stars.

astro-ph.HE↗

arXiv2024

Impact of nuclear matter properties on the nucleosynthesis and the kilonova from binary neutron star merger ejecta

Material expelled from binary neutron star (BNS) mergers can harbor r-process nucleosynthesis and power a Kilonova (KN), both intimately related to the astrophysical conditions of the ejection. In turn such conditions indirectly depend on the equation of state (EOS) describing matter inside the neutron star. Therefore, in principle the above observables can hold valuable information on nuclear matter, as the merger gravitational wave signal already does. In this work, we consider the outcome of a set of BNS merger simulations employing different finite-temperature nuclear EOSs. The latter are obtained from a Skyrme-type interaction model where nuclear properties, such as the incompressibility and the nucleon effective mass at saturation density, are systematically varied. We post-process the ejecta using a reaction network coupled with a semi-analytic KN model, to asses the sensitivity on the input EOS of the final yields and the KN light curves. Both of them are found to be non-trivially influenced by the EOS, with the overall outcome being dominated by the heterogeneous outflows from the remnant disk, hosting a variable degree of neutron-rich material. The dynamical ejecta can be more directly related to the EOS parameters considered, however, we find its role in the yields production and the KN emission too entangled with the other ejecta components, in order to infer solid correlations. This result highlights the strong degeneracy that intervenes between the merger outcome and the behaviour of the intrinsic nuclear matter, and places itself as a limit to the employment of EOS-constraining approaches of such kind.

astro-ph.HE↗

arXiv2024

Do not forget the electrons: Extending moderately-sized nuclear networks for multidimensional hydrodynamic codes

We present here an extended nuclear network, with 90 species, designed for being coupled with hydrodynamic simulations, which includes neutrons, protons, electrons, positrons, and the corresponding neutrino and anti-neutrino emission. This network is also coupled with temperature, making it extremely robust and, together with its size, unique of its kind. The inclusion of electron captures on free protons makes the network very appropriate for multidimensional studies of Type Ia supernova explosions, especially when the exploding object is a massive white dwarf. The results obtained with the proposed medium-sized network compare fairly well, to a few percent, with those computed with the extended network WinNet (> 2000 isotopes) in scenarios reproducing the gross physical conditions of current Type Ia supernova explosion models. In those cases where the carbon and oxygen fuel ignites at high density, the high-temperature plateau typical of the nuclear statistical equilibrium regime is well defined and stable, allowing large integration time steps. We show that the inclusion of electron captures on free protons substantially improves the estimation of the electron fraction of the mixture. Therefore, the pressure is better determined than in networks where electron captures are excluded, which will ultimately lead to more reliable hydrodynamic models. Explosive combustion of helium at low density, occurring near the surface layer of a white dwarf, is also better described with the proposed network, which gives nuclear energy generation rates much closer to WinNet than typical reduced alpha networks.

astro-ph.SR↗

arXiv2023

Effects of nuclear matter properties in neutron star mergers

The dynamics in mergers of binary neutron star (BNS) systems depend sensitively on the equation of state (EOS) of dense matter. This has profound implications on the emission of gravitational waves (GWs) and the ejection of matter in the merger and post-merger phases and is thus of high interest for multi-messenger astronomy. Today, a variety of nuclear EOSs are available with various underlying microphysical models. This calls for a study to focus on EOS effects from different physical nuclear matter properties and their influence on BNS mergers. We perform simulations of equal-mass BNS mergers with a set of 9 different EOSs based on Skyrme density functionals. In the models, we systematically vary the effective nucleon mass, incompressibility, and symmetry energy at saturation density. This allows us to investigate the influence of specific nuclear matter properties on the dynamics of BNS mergers. We analyze the impact of these properties on the merger dynamics, the fate of the remnant, disk formation, ejection of matter, and gravitational wave emission. Our results indicate that some aspects of the merger are sensitive to the EOS around saturation density while others are sensitive to the behavior towards higher densities, e.g., characterized by the slope of the pressure as a function of density. The detailed density dependence of the EOS thus needs to be taken into account to describe its influence on BNS mergers.

astro-ph.HE↗

arXiv2023

Core-collapse supernova simulations with reduced nucleosynthesis networks

We present core-collapse supernova simulations including nuclear reaction networks that impact explosion dynamics and nucleosynthesis. The different composition treatment can lead to changes in the neutrino heating in the vicinity of the shock by modifying the number of nucleons and thus the neutrino-opacity of the region. This reduces the ram pressure outside the shock and allows an easier expansion. The energy released by the nuclear reactions during collapse also slows down the accretion and aids the shock expansion. In addition, nuclear energy generation in the postshocked matter produces up to $20\%$ more energetic explosions. Nucleosynthesis is affected due to the different dynamic evolution of the explosion. Our results indicate that the energy generation from nuclear reactions helps to sustain late outflows from the vicinity of the proto-neutron star, synthesizing more neutron-rich species. Furthermore, we show that there are systematic discrepancies between the ejecta calculated with in-situ and ex-situ reaction networks. These differences stem from the intrinsic characteristics of evolving the composition in hydrodynamic simulations or calculating it with Lagrangian tracer particles. The mass fractions of some Ca, Ti, Cr, and Fe isotopes are consistently underproduced in postprocessing calculations, leading to different nucleosynthesis paths. Our results suggest that large in-situ nuclear reaction networks are important for a realistic feedback of the energy generation, the neutrino heating, and a more accurate ejecta composition.

astro-ph.HE↗

arXiv2022

Magnetorotational supernovae: A nucleosynthetic analysis of sophisticated 3D models

Magnetorotational supernovae are a rare type of core-collapse supernovae where the magnetic field and rotation play a central role in the dynamics of the explosion. We present the post-processed nucleosynthesis of state-of-the-art neutrino-MHD supernova models that follow the post explosion evolution for few seconds. We find three different dynamical mechanisms to produce heavy r-process elements: i) a prompt ejection of matter right after core bounce, ii) neutron-rich matter that is ejected at late times due to a reconfiguration of the protoneutronstar shape, iii) small amount of mass ejected with high entropies in the center of the jet. We investigate total ejecta yields, including the ones of unstable nuclei such as $^{26}$Al, $^{44}$Ti, $^{56}$Ni, and $^{60}$Fe. The obtained $^{56}$Ni masses vary between $0.01 - 1\,\mathrm{M_\odot}$. The latter maximum is compatible with hypernova observations. Furthermore, all of our models synthesize Zn masses in agreement with observations of old metal-poor stars. We calculate simplified light curves to investigate whether our models can be candidates for superluminous supernovae. The peak luminosities obtained from taking into account only nuclear heating reach up to a few $\sim 10^{43} \,\mathrm{erg\,s^{-1}}$. Under certain conditions, we find a significant impact of the $^{66}$Ni decay chain that can raise the peak luminosity up to $\sim 38\%$ compared to models including only the $^{56}$Ni decay chain. This work reinforces the theoretical evidence on the critical role of magnetorotational supernovae to understand the occurrence of hypernovae, superluminous supernovae, and the synthesis of heavy elements.

astro-ph.HE↗

arXiv2022

Chemical Evolution of R-process Elements in Stars (CERES). I. Stellar parameters and chemical abundances from Na to Zr

Aims. The Chemical Evolution of R-process Elements in Stars (CERES) project aims to provide a homogeneous analysis of a sample of metal-poor stars ([Fe/H]<-1.5). We present the stellar parameters and the chemical abundances of elements up to Zr for a sample of 52 giant stars.Methods. We relied on a sample of high signal-to-noise UVES spectra. We determined stellar parameters from Gaia photometry and parallaxes. Chemical abundances were derived using spectrum synthesis and model atmospheres.Results. We determined chemical abundances of 26 species of 18 elements: Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Y, and Zr. For several stars, we were able to measure both neutral and ionised species, including Si, Sc, Mn, and Zr. We have roughly doubled the number of measurements of Cu for stars at [Fe/H] <= -2.5. The homogeneity of the sample made it possible to highlight the presence of two Zn-rich stars ([Zn/Fe]~+0.7), one r-rich and the other r-poor. We report the existence of two branches in the [Zn/Fe] versus [Ni/Fe] plane and suggest that the high [Zn/Fe] branch is the result of hypernova nucleosynthesis. We discovered two stars with peculiar light neutron-capture abundance patterns: CES1237+1922 (also known as BS 16085-0050), which is ~1 dex underabundant in Sr, Y, and Zr with respect to the other stars in the sample, and CES2250-4057 (also known as HE 2247-4113), which shows a ~1 dex overabundance of Sr with respect to Y and Zr.Conclusions. The high quality of our dataset allowed us to measure hardly detectable ions. This can provide guidance in the development of line formation computations that take deviations from local thermodynamic equilibrium and hydrodynamical effects into account.

astro-ph.GA↗

arXiv2021

Evolution of neutron capture elements in dwarf galaxies

We study the evolution of Eu and Ba abundances in local group dwarf spheroidal and ultra faint dwarf galaxies by means of detailed chemical evolution models and compare our results with new sets of homogeneous abundances. The adopted models include gas infall and outflow and have been previously tested. We investigate several production scenarios for r-process elements: merging neutron stars and magneto-rotational driven supernovae. Production of Ba through the main s-process acting in low- and intermediate- mass stars is considered as well. We also test different sets of nucleosynthesis yields. For merging neutron stars we adopt either a constant and short delay time for merging or a delay time distribution function. Our simulations show that: i) if r-process elements are produced only by a quick source, it is possible to reproduce the [Eu/Fe] vs [Fe/H], but those models fail in reproducing the [Ba/Fe] vs [Fe/H]. ii) If r-process elements are produced only with longer delays the opposite happens. iii) If both a quick source and a delayed one are adopted, such as magneto-rotational driven supernovae and merging neutron stars with a delay time distribution, the [Eu/Fe] abundance pattern is successfully reproduced, but models still fail in reproducing the [Ba/Fe]. iv) On the other hand, the characteristic abundances of Reticulum II can be reproduced only if both the Eu and the r-process fraction of Ba are produced on short and constant time delays during a single merging event. We discuss also other possible interpretations, including an inhomogeneous mixing of gas which might characterize this galaxy.

astro-ph.GA↗

arXiv2021

129I and 247Cm in Meteorites Constrain the Last Astrophysical Source of Solar r-process Elements

The composition of the early Solar System can be inferred from meteorites. Many elements heavier than iron were formed by the rapid neutron-capture process (r process), but the astrophysical sources where this occurred remain poorly understood. We demonstrate that the near-identical half-lives ($\simeq$ 15.6 Myr) of the radioactive r-process nuclei 129I and 247Cm preserve their ratio, irrespective of the time between production and incorporation into the Solar System. We constrain the last r-process source by comparing the measured meteoritic 129I / 247Cm = 438 $\pm$ 184 to nucleosynthesis calculations based on neutron star merger and magneto-rotational supernova simulations. Moderately neutron-rich conditions, often found in merger disk ejecta simulations, are most consistent with the meteoritic value. Uncertain nuclear physics data limit our confidence in this conclusion.

astro-ph.SR↗

arXiv2021

Nucleosynthesis in magneto-rotational supernovae

We present the nucleosynthesis of magneto-rotational supernovae (MR-SNe) including neutrino-driven and magneto-rotational-driven ejecta based, for the first time, on 2D simulations with accurate neutrino transport. The models analysed here have different rotation and magnetic fields, allowing us to explore the impact of these two key ingredients. The accurate neutrino transport of the simulations is critical to analyse the slightly neutron-rich and proton-rich ejecta that are similar to the, also neutrino-driven, ejecta in standard supernovae. In the model with strong magnetic field, the r-process produces heavy elements up to the third r-process peak ($A\sim 195$), in agreement with previous works. This model presents a jet-like explosion with proton-rich jets surrounded by neutron-rich material where the r-process occurs. We have estimated a lower limit for $^{56}$Ni of $2.5\times10^{-2} M_\odot$, which is still well below the expected hypernova value. Longer simulations including the accretion disc evolution are required to get a final prediction. In addition, we have found that the late evolution is critical in a model with weak magnetic field in which late-ejected neutron-rich matter produces elements up to the second r-process peak. Even if we cannot yet provide conclusions for hypernova nucleosynthesis, our results agree with observations of old stars and radioactive isotopes in supernova remnants. This makes MR-SNe a good additional scenario to neutron star mergers for the synthesis of heavy elements and brings us closer to understand their origin and the role of MR-SNe in the early Galaxy nucleosynthesis.

astro-ph.HE↗