arXiv subjects

A. Arcones

Publications and source records attributed to A. Arcones.

At least 37 records · Page 2Linked to original sources

arXiv2014

The long-term evolution of neutron star merger remnants - I. The impact of r-process nucleosynthesis

We follow the longterm evolution of the dynamic ejecta of neutron star mergers for up to 100 years and over a density range of roughly 40 orders of magnitude. We include the nuclear energy input from the freshly synthesized, radioactively decaying nuclei in our simulations and study its effects on the remnant dynamics. Although the nuclear heating substantially alters the longterm evolution, we find that running nuclear networks over purely hydrodynamic simulations (i.e. without heating) yields actually acceptable nucleosynthesis results. The main dynamic effect of the radioactive heating is to quickly smooth out inhomogeneities in the initial mass distribution, subsequently the evolution proceeds self-similarly and after 100 years the remnant still carries the memory of the initial binary mass ratio. We also explore the nucleosynthetic yields for two mass ejection channels. The dynamic ejecta very robustly produce "strong" r-process elements with $A > 130$ with a pattern that is essentially independent of the details of the merging system. From a simple model we find that neutrino-driven winds yield "weak" r-process contributions with $50 < A < 130$ whose abundance patterns vary substantially between different merger cases. This is because their electron fraction, set by the ratio of neutrino luminosities, varies considerably from case to case. Such winds do not produce any $^{56}{\rm Ni}$, but a range of radioactive isotopes that are long-lived enough to produce a second, radioactively powered electromagnetic transient in addition to the "macronova" from the dynamic ejecta. While our wind model is very simple, it nevertheless demonstrates the potential of such neutrino-driven winds for electromagnetic transients and it motivates further, more detailed neutrino-hydrodynamic studies. The properties of the mentioned transients are discussed in more detail in a companion paper.

astro-ph.HE↗

arXiv2013

Nucleosynthesis of elements between Sr and Ag in neutron- and proton-rich neutrino-driven winds

Neutrino-driven winds that follow core collapse supernovae were thought to be the site where half of the heavy elements are produced by the r-process. Although recent hydrodynamic simulations show that the conditions in the wind are not enough for the r-process, lighter heavy elements like Sr, Y, and Zr can be produced. However, it is still not clear whether the conditions in the wind are slightly neutron rich or proton rich. Here we investigate the nucleosynthesis in neutrino-driven winds for both these conditions and systematically explore the impact of wind parameters on abundances. Our results show the difficulty of obtaining a robust abundance pattern in neutron-rich winds where also an over production of Sr, Y, and Zr is likely. In proton-rich conditions, the abundances smoothly change when varying wind parameters. Constraints for wind parameters and neutrino energies and luminosities will soon become available by combining nucleosynthesis studies, like the one presented here, with new and future experimental data and observations.

astro-ph.SR↗

arXiv2013

Europium production: neutron star mergers versus core-collapse supernovae

We have explored the Eu production in the Milky Way by means of a very detailed chemical evolution model. In particular, we have assumed that Eu is formed in merging neutron star (or neutron star black hole) binaries as well as in type II supernovae. We have tested the effects of several important parameters influencing the production of Eu during the merging of two neutron stars, such as: i) the time scale of coalescence, ii) the Eu yields and iii) the range of initial masses for the progenitors of the neutron stars. The yields of Eu from type II supernovae are very uncertain, more than those from coalescing neutron stars, so we have explored several possibilities. We have compared our model results with the observed rate of coalescence of neutron stars, the solar Eu abundance, the [Eu/Fe] versus [Fe/H] relation in the solar vicinity and the [Eu/H] gradient along the Galactic disc. Our main results can be summarized as follows: i) neutron star mergers can be entirely responsible for the production of Eu in the Galaxy if the coalescence time scale is no longer than 1 Myr for the bulk of binary systems, the Eu yield is around $3 \times 10^{-7}$ M$_\odot$, and the mass range of progenitors of neutron stars is 9-50 M$_\odot$; ii) both type II supernovae and merging neutron stars can produce the right amount of Eu if the neutron star mergers produce $2 \times 10^{-7}$ M$_\odot$ per system and type II supernovae, with progenitors in the range 20-50 M$_\odot$, produce yields of Eu of the order of $10^{-8}-10^{-9}$ M$_\odot$; iii) either models with only neutron stars producing Eu or mixed ones can reproduce the observed Eu abundance gradient along the Galactic disc.

astro-ph.GA↗

arXiv2013

High resolution simulations of the head-on collision of white dwarfs

The direct impact of white dwarfs has been suggested as a plausible channel for type Ia supernovae. In spite of their (a priori) rareness, in highly populated globular clusters and in galactic centers, where the amount of white dwarfs is considerable, the rate of violent collisions between two of them might be non-negligible. Even more, there are indications that binary white dwarf systems orbited by a third stellar-mass body have an important chance to induce a clean head-on collision. Therefore, this scenario represents a source of contamination for the supernova light-curves sample that it is used as standard candles in cosmology, and it deserves further investigation. Some groups have conducted numerical simulations of this scenario, but their results show several differences. In this paper we address some of the possible sources of these differences, presenting the results of high resolution hydrodynamical simulations jointly with a detailed nuclear post-processing of the nuclear abundances, to check the viability of white dwarf collisions to produce significant amounts of 56Ni. To that purpose, we use a 2D-axial symmetric smoothed particle hydrodynamic code to obtain a resolution considerably higher than in previous studies. In this work, we also study how the initial mass and nuclear composition affect the results. The gravitational wave emission is also calculated, as this is a unique signature of this kind of events. All calculated models produce a significant amount of 56Ni, ranging from 0.1 Msun to 1.1 Msun, compatible not only with normal-Branch type Ia supernova but also with the subluminous and super-Chandrasekhar subset. Nevertheless, the distribution mass-function of white dwarfs favors collisions among 0.6-0.7 Msun objects, leading to subluminous events.

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arXiv2013

Approaching the precursor nuclei of the third r-process peak with RIBs

The rapid neutron nucleosynthesis process involves an enormous amount of very exotic neutron-rich nuclei, which represent a theoretical and experimental challenge. Two of the main decay properties that affect the final abundance distribution the most are half-lives and neutron branching ratios. Using fragmentation of a primary $^{238}$U beam at GSI we were able to measure such properties for several neutron-rich nuclei from $^{208}$Hg to $^{218}$Pb. This contribution provides a short update on the status of the data analysis of this experiment, together with a compilation of the latest results published in this mass region, both experimental and theoretical. The impact of the uncertainties connected with the beta-decay rates and with beta-delayed neutron emission is illustrated on the basis of $r$-process network calculations. In order to obtain a reasonable reproduction of the third $r$-process peak, it is expected that both half-lives and neutron branching ratios are substantially smaller, than those based on FRDM+QRPA, commonly used in $r$-process model calculations. Further measurements around $N\sim126$ are required for a reliable modelling of the underlying nuclear structure, and for performing more realistic $r$-process abundance calculations.

nucl-ex↗

arXiv2012

On the astrophysical robustness of neutron star merger r-process

In this study we explore the nucleosynthesis in the dynamic ejecta of compact binary mergers. We are particularly interested in the question how sensitive the resulting abundance patterns are to the parameters of the merging system. Therefore, we systematically investigate combinations of neutron star masses in the range from 1.0 to 2.0 \Msun and, for completeness, we compare the results with those from two simulations of a neutron star black hole merger. The ejecta masses vary by a factor of five for the studied systems, but all amounts are (within the uncertainties of the merger rates) compatible with being a major source of cosmic r-process. The ejecta undergo a robust r-process nucleosynthesis which produces all the elements from the second to the third peak in close-to-solar ratios. Most strikingly, this r-process is extremely robust, all 23 investigated binary systems yield practically identical abundance patterns. This is mainly the result of the ejecta being extremely neutron rich (\ye $\approx0.04$) and the r-process path meandering along the neutron drip line so that the abundances are determined entirely by nuclear rather than by astrophysical properties. This robustness together with the ease with which both the second and third peak are reproduced make compact binary mergers the prime candidate for the source of the observed unique heavy r-process component.

astro-ph.SR↗

arXiv2012

Neutrino-driven wind simulations and nucleosynthesis of heavy elements

Neutrino-driven winds, which follow core-collapse supernova explosions, present a fascinating nuclear astrophysics problem that requires understanding advanced astrophysics simulations, the properties of matter and neutrino interactions under extreme conditions, the structure and reactions of exotic nuclei, and comparisons against forefront astronomical observations. The neutrino-driven wind has attracted vast attention over the last 20 years as it was suggested to be a candidate for the astrophysics site where half of the heavy elements are produced via the r-process. In this review, we summarize our present understanding of neutrino-driven winds from the dynamical and nucleosynthesis perspectives. Rapid progress has been made during recent years in understanding the wind with improved simulations and better micro physics. The current status of the fields is that hydrodynamical simulations do not reach the extreme conditions necessary for the r-process and the proton or neutron richness of the wind remains to be investigated in more detail. However, nucleosynthesis studies and observations point already to neutrino-driven winds to explain the origin of lighter heavy elements, such as Sr, Y, Zr.

astro-ph.SR↗

arXiv2011

Impact of supernova dynamics on the νp-process

We study the impact of the late time dynamical evolution of ejecta from core-collapse supernovae on νp-process nucleosynthesis. Our results are based on hydrodynamical simulations of neutrino wind ejecta. Motivated by recent two-dimensional wind simulations, we vary the dynamical evolution during the νp-process and show that final abundances strongly depend on the temperature evolution. When the expansion is very fast, there is not enough time for antineutrino absorption on protons to produce enough neutrons to overcome the β-decay waiting points and no heavy elements beyond A=64 are produced. The wind termination shock or reverse shock dramatically reduces the expansion speed of the ejecta. This extends the period during which matter remains at relatively high temperatures and is exposed to high neutrino fluxes, thus allowing for further (p,γ) and (n,p) reactions to occur and to synthesize elements beyond iron. We find that the νp-process starts to efficiently produce heavy elements only when the temperature drops below ~3 GK. At higher temperatures, due to the low alpha separation energy of 60Zn (S_α = 2.7 MeV) the reaction 59Cu(p,α)56Ni is faster than the reaction 59Cu(p,γ)60Zn. This results in the closed NiCu cycle that we identify and discuss here for the first time. We also investigate the late phase of the νp-process when the temperatures become too low to maintain proton captures. Depending on the late neutron density, the evolution to stability is dominated by βdecays or by (n,γ) reactions. In the latter case, the matter flow can even reach the neutron-rich side of stability and the isotopic composition of a given element is then dominated by neutron-rich isotopes.

astro-ph.SR↗

arXiv2011

Nuclear correlations and the r-process

We show that long-range correlations for nuclear masses have a significant effect on the synthesis of heavy elements by the r-process. As calculated by Delaroche et al. [1], these correlations suppress magic number effects associated with minor shells. This impacts the calculated abundances before the third r-process peak (at mass number A~195), where the abundances are low and form a trough. This trough and the position of the third abundance peak are strongly affected by the masses of nuclei in the transition region between deformed and spherical. Based on different astrophysical environments, our results demonstrate that a microscopic theory of nuclear masses including correlations naturally smoothens the separation energies, thus reducing the trough and improving the agreement with observed solar system abundances.

nucl-th↗

arXiv2011

Production of Light Element Primary Process nuclei in neutrino-driven winds

We present first comparisons between Light Element Primary Process (LEPP) abundances observed in some ultra metal poor (UMP) stars and nucleosynthesis calculations based on long-time hydrodynamical simulations of core-collapse supernovae and their neutrino-driven wind. UMP star observations indicate Z>38 elements include the contributions of at least two nucleosynthesis components: r-process nuclei that are synthesized by rapid neutron capture in a yet unknown site and LEPP elements (mainly Sr, Y, Zr). We show that neutrino-driven wind simulations can explain the observed LEPP pattern. We explore in detail the sensitivity of the calculated abundances to the electron fraction, which is a key nucleosynthesis parameter but poorly known due to uncertainties in neutrino interactions and transport. Our results show that the observed LEPP pattern can be reproduced in proton- and neutron-rich winds.

astro-ph.GA↗

arXiv2011

Dynamical r-process studies within the neutrino-driven wind scenario and its sensitivity to the nuclear physics input

We use results from long-time core-collapse supernovae simulations to investigate the impact of the late time evolution of the ejecta and of the nuclear physics input on the calculated r-process abundances. Based on the latest hydrodynamical simulations, heavy r-process elements cannot be synthesized in the neutrino-driven winds that follow the supernova explosion. However, by artificially increasing the wind entropy, elements up to A=195 can be made. In this way one can reproduce the typical behavior of high-entropy ejecta where the r-process is expected to occur. We identify which nuclear physics input is more important depending on the dynamical evolution of the ejecta. When the evolution proceeds at high temperatures (hot r-process), an (n,g)-(g,n) equilibrium is reached. While at low temperature (cold r-process) there is a competition between neutron captures and beta decays. In the first phase of the r-process, while enough neutrons are available, the most relevant nuclear physics input are the nuclear masses for the hot r-process and the neutron capture and beta-decay rates for the cold r-process. At the end of this phase, the abundances follow a steady beta flow for the hot r-process and a steady flow of neutron captures and beta decays for the cold r-process. After neutrons are almost exhausted, matter decays to stability and our results show that in both cases neutron captures are key for determining the final abundances, the position of the r-process peaks, and the formation of the rare-earth peak. In all the cases studied, we find that the freeze out occurs in a timescale of several seconds.

astro-ph.SR↗

arXiv2010

Explosive nucleosynthesis in core-collapse supernovae

The specific mechanism and astrophysical site for the production of half of the elements heavier than iron via rapid neutron capture (r-process) remains to be found. In order to reproduce the abundances of the solar system and of the old halo stars, at least two components are required: the heavy r-process nuclei (A>130) and the weak r-process which correspond to the lighter heavy nuclei (A<130). In this work, we present nucleosynthesis studies based on trajectories of hydrodynamical simulations for core-collapse supernovae and their subsequent neutrino-driven winds. We show that the weak r-process elements can be produced in neutrino-driven winds and we relate their abundances to the neutrino emission from the nascent neutron star. Based on the latest hydrodynamical simulations, heavy r-process elements cannot be synthesized in the neutrino-driven winds. However, by artificially increasing the wind entropy, elements up to A=195 can be made. In this way one can mimic the general behavior of an ejecta where the r-process occurs. We use this to study the impact of the nuclear physics input (nuclear masses, neutron capture cross sections, and beta-delayed neutron emission) and of the long-time dynamical evolution on the final abundances.

astro-ph.SR↗

arXiv2010

Explosive nucleosynthesis: nuclear physics impact using neutrino-driven wind simulations

We present nucleosynthesis studies based on hydrodynamical simulations of core-collapse supernovae and their subsequent neutrino-driven winds. Although the conditions found in these simulations are not suitable for the rapid neutron capture (r-process) to produce elements heavier than A$\sim$130, this can be solved by artificially increasing the wind entropy. In this way one can mimic the general behavior of an ejecta where the r-process occurs. We study the impact of the long-time dynamical evolution and of the nuclear physics input on the final abundances and show that different nuclear mass models lead to significant variations in the abundances. These differences can be linked to the behavior of nuclear masses far from stability. In addition, we have analyzed in detail the effect of neutron capture and beta-delayed neutron emission when matter decays back to stability. In all our studied cases, freeze out effects are larger than previously estimated and produce substantial changes in the post freeze out abundances.

astro-ph.SR↗

arXiv2010

Electron fraction constraints based on Nuclear Statistical Equilibrium with beta equilibrium

The electron-to-nucleon ratio or electron fraction is a key parameter in many astrophysical studies. Its value is determined by weak-interaction rates that are based on theoretical calculations subject to several nuclear physics uncertainties. Consequently, it is important to have a model independent way of constraining the electron fraction value in different astrophysical environments. Here we show that nuclear statistical equilibrium combined with beta equilibrium can provide such a constraint. We test the validity of this approximation in presupernova models and give lower limits for the electron fraction in type Ia supernova and accretion-induced collapse.

astro-ph.SR↗

arXiv2010

Nucleosynthesis-relevant conditions in neutrino-driven supernova outflows. II. The reverse shock in two-dimensional simulations

After the initiation of the explosion of core-collapse supernovae, neutrinos emitted from the nascent neutron star drive a supersonic baryonic outflow. This neutrino-driven wind interacts with the more slowly moving, earlier supernova ejecta forming a wind termination shock (or reverse shock), which changes the local wind conditions and their evolution. Important nucleosynthesis processes (alpha-process, charged-particle reactions, r-process, and vp-process) occur or might occur in this environment. The nucleosynthesis depends on the long-time evolution of density, temperature, and expansion velocity. Here we present two-dimensional hydrodynamical simulations with an approximate description of neutrino-transport effects, which for the first time follow the post-bounce accretion, onset of the explosion, wind formation, and the wind expansion through the collision with the preceding supernova ejecta. Our results demonstrate that the anisotropic ejecta distribution has a great impact on the position of the reverse shock, the wind profile, and the long-time evolution. This suggests that hydrodynamic instabilities after core bounce and the consequential asymmetries may have important effects on the nucleosynthesis-relevant conditions in the neutrino-heated baryonic mass flow from proto-neutron stars.

astro-ph.SR↗

arXiv2010

Electromagnetic Counterparts of Compact Object Mergers Powered by the Radioactive Decay of R-process Nuclei

The most promising astrophysical sources of kHz gravitational waves (GWs) are the inspiral and merger of binary neutron star(NS)/black hole systems. Maximizing the scientific return of a GW detection will require identifying a coincident electro-magnetic (EM) counterpart. One of the most likely sources of isotropic EM emission from compact object mergers is a supernova-like transient powered by the radioactive decay of heavy elements synthesized in ejecta from the merger. We present the first calculations of the optical transients from compact object mergers that self-consistently determine the radioactive heating by means of a nuclear reaction network; using this heating rate, we model the light curve with a one dimensional Monte Carlo radiation transfer calculation. For an ejecta mass ~1e-2 M_sun[1e-3 M_sun] the resulting light curve peaks on a timescale ~ 1 day at a V-band luminosity nu L_nu ~ 3e41[1e41] ergs/s (M_V = -15[-14]); this corresponds to an effective "f" parameter ~3e-6 in the Li-Paczynski toy model. We argue that these results are relatively insensitive to uncertainties in the relevant nuclear physics and to the precise early-time dynamics and ejecta composition. Due to the rapid evolution and low luminosity of NS merger transients, EM counterpart searches triggered by GW detections will require close collaboration between the GW and astronomical communities. NS merger transients may also be detectable following a short-duration Gamma-Ray Burst or "blindly" with present or upcoming optical transient surveys. Because the emission produced by NS merger ejecta is powered by the formation of rare r-process elements, current optical transient surveys can directly constrain the unknown origin of the heaviest elements in the Universe.

astro-ph.HE↗

arXiv2009

The effects of r-process heating on fall-back accretion in compact object mergers

We explore the effects of r-process nucleosynthesis on fall-back accretion in neutron star(NS)-NS and black hole-NS mergers, and the resulting implications for short-duration gamma-ray bursts (GRBs). Though dynamically important, the energy released during the r-process is not yet taken into account in merger simulations. We use a nuclear reaction network to calculate the heating (due to beta-decays and nuclear fission) experienced by material on the marginally-bound orbits nominally responsible for late-time fall-back. Since matter with longer orbital periods t_orb experiences lower densities, for longer periods of time, the total r-process heating rises rapidly with t_orb, such that material with t_orb > 1 seconds can become completely unbound. Thus, r-process heating fundamentally changes the canonical prediction of an uninterrupted power-law decline in the fall-back rate dM/dt at late times. When the timescale for r-process to complete is > 1 second, the heating produces a complete cut-off in fall-back accretion after ~ 1 second; if robust, this would imply that fall-back accretion cannot explain the late-time X-ray flaring observed following some short GRBs. However, for a narrow, but physically plausible, range of parameters, fall-back accretion can resume after ~ 10 s, despite having been strongly suppressed for ~ 1-10 s after the merger. This suggests the intriguing possibility that the gap observed between the prompt and extended emission in short GRBs is a manifestation of r-process heating.

astro-ph.HE↗

arXiv2008

R-process nucleosynthesis calculations with complete nuclear physics input

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.

astro-ph↗