Site For R-process Nucleosynthesis Research Articles

Collapsars as Sites of r-process Nucleosynthesis: Systematic Photometric Near-infrared Follow-up of Type Ic-BL Supernovae

One of the open questions following the discovery of GW170817 is whether neutron star (NS) mergers are the only astrophysical sites capable of producing r-process elements. Simulations have shown that 0.01–0.1 M ⊙ of r-process material could be generated in the outflows originating from the accretion disk surrounding the rapidly rotating black hole that forms as a remnant to both NS mergers and collapsing massive stars associated with long-duration gamma-ray bursts (collapsars). The hallmark signature of r-process nucleosynthesis in the binary NS merger GW170817 was its long-lasting near-infrared (NIR) emission, thus motivating a systematic photometric study of the light curves of broad-lined stripped-envelope (Ic-BL) supernovae (SNe) associated with collapsars. We present the first systematic study of 25 SNe Ic-BL—including 18 observed with the Zwicky Transient Facility and 7 from the literature—in the optical/NIR bands to determine what quantity of r-process material, if any, is synthesized in these explosions. Using semi-analytic models designed to account for r-process production in SNe Ic-BL, we perform light curve fitting to derive constraints on the r-process mass for these SNe. We also perform independent light curve fits to models without the r-process. We find that the r-process-free models are a better fit to the light curves of the objects in our sample. Thus, we find no compelling evidence of r-process enrichment in any of our objects. Further high-cadence infrared photometric studies and nebular spectroscopic analysis would be sensitive to smaller quantities of r-process ejecta mass or indicate whether all collapsars are completely devoid of r-process nucleosynthesis.

Bound Debris Expulsion from Neutron Star Merger Remnants

Many studies have found that neutron star mergers leave a fraction of the stars’ mass in bound orbits surrounding the resulting massive neutron star or black hole. This mass is a site of r-process nucleosynthesis and can generate a wind that contributes to a kilonova. However, comparatively little is known about the dynamics determining its mass or initial structure. Here we begin to investigate these questions, starting with the origin of the disk mass. Using tracer particle as well as discretized fluid data from numerical simulations, we identify where in the neutron stars the debris came from, the paths it takes in order to escape from the neutron stars’ interiors, and the times and locations at which its orbital properties diverge from those of neighboring fluid elements that end up remaining in the merged neutron star.

Nuclear quests for the r-process

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.

Lanthanide Features in Near-infrared Spectra of Kilonovae

The observations of GW170817/AT2017gfo have provided us with evidence that binary neutron star mergers are sites of r-process nucleosynthesis. However, the observed signatures in the spectra of GW170817/AT2017gfo have not been fully decoded, especially in the near-infrared (NIR) wavelengths. In this paper, we investigate the kilonova spectra over the entire wavelength range with the aim of elemental identification. We systematically calculate the strength of bound–bound transitions by constructing a hybrid line list that is accurate for important strong transitions and complete for weak transitions. We find that the elements on the left side of the periodic table, such as Ca, Sr, Y, Zr, Ba, La, and Ce, tend to produce prominent absorption lines in the spectra. This is because such elements have a small number of valence electrons and low-lying energy levels, resulting in strong transitions. By performing self-consistent radiative transfer simulations for the entire ejecta, we find that La iii and Ce iii appear in the NIR spectra, which can explain the absorption features at λ ∼ 12000–14000 Å in the spectra of GW170817/AT2017gfo. The mass fractions of La and Ce are estimated to be >2 × 10−6 and ∼(1–100) × 10−5, respectively. An actinide element Th can also be a source of absorption as the atomic structure is analogous to that of Ce. However, we show that Th iii features are less prominent in the spectra because of the denser energy levels of actinides compared to those of lanthanides.

What can neutron stars reveal about the equation of state of dense matter?

Neutron stars are astrophysical objects of extremes, reaching the highest densities we can observe in the cosmos, and probing matter under conditions that cannot be recreated in terrestrial experiments. In August 2017, the first neutron-star merger has been observed, which provided compelling evidence that these events are an important site for r-process nucleosynthesis. Furthermore, the gravitational-wave signal of such events might shed light upon the nature of strongly interacting matter in the neutron-star core. To understand these remarkable events, reliable nuclear physics input is essential. In this contribution, I explain how to use chiral effective field theory and advanced many-body methods to provide a consistent and systematic approach to strongly inter- acting systems from nuclei to neutron stars with controlled theoretical uncertainties. I will discuss recent results for the equation of state relevant for the nuclear astrophysics of neutron stars and neutron-star mergers.

νbhlight: Radiation GRMHD for Neutrino-driven Accretion Flows

Abstract The 2017 detection of the in-spiral and merger of two neutron stars was a landmark discovery in astrophysics. We now know that such mergers are central engines of short gamma-ray bursts and sites of r-process nucleosynthesis, where the heaviest elements in our universe are formed. In the coming years, we expect many more such mergers. Modeling such systems presents a significant computational challenge along with the observational one. To meet this challenge, we present , a scheme for solving general relativistic magnetohydrodynamics with energy-dependent neutrino transport in full (3 + 1) dimensions, facilitated by Monte Carlo methods. We present a suite of tests demonstrating the accuracy, efficacy, and necessity of our scheme. We demonstrate the potential of our scheme by running a sample calculation in a domain of interest—the dynamics and composition of the accretion disk formed by a binary neutron star merger.

Neutron star mergers as sites of r-process nucleosynthesis and short gamma-ray bursts

Neutron star mergers have been long considered as promising sites of heavy [Formula: see text]-process nucleosynthesis. We overview the observational evidence supporting this scenario including: the total amount of [Formula: see text]-process elements in the galaxy, extreme metal-poor stars, geological radioactive elemental abundances, dwarf galaxies and short gamma-ray bursts (sGRBs). Recently, the advanced LIGO and Virgo observatories discovered a gravitational-wave signal of a neutron star merger, GW170817, as well as accompanying multi-wavelength electromagnetic (EM) counterparts. The ultra-violet, optical and near infrared (n/R) observations point to [Formula: see text]-process elements that have been synthesized in the merger ejecta. The rate and ejected mass inferred from GW170817 and the EM counterparts are consistent with other observations. We however, find that, within the simple one zone chemical evolution models (based on merger rates with reasonable delay time distributions as expected from evolutionary models, or from observations of sGRBs), it is difficult to reconcile the current observations of the Eu abundance history of galactic stars for [Fe/H] [Formula: see text]. This implies that to account for the role of mergers in the galactic chemical evolution, we need a galactic model with multiple populations that have different spatial distributions and/or varying formation rates.

2016 Nier Prize for Gregory A. Brennecka

2016 Nier Prize for Gregory A. Brennecka

Neutrino-heated winds from rotating protomagnetars

We calculate the steady-state properties of neutrino-driven winds from strongly magnetized, rotating protoneutron stars (PNSs; ‘protomagnetars’) under the assumption that the outflow geometry is set by the force-free magnetic field of an aligned dipole. Our goal is to assess protomagnetars as sites of r-process nucleosynthesis and gamma-ray burst engines using a more realistic outflow geometry than assumed in previous works. One-dimensional solutions calculated along flux tubes corresponding to different polar field lines are stitched together to determine the global properties of the flow at a given neutrino luminosity and rotation period. Protomagnetars with rotation periods of P ∼ 2–5 ms are shown to produce outflows more favourable for the production of third-peak r-process nuclei due to their much shorter expansion times through the seed nucleus formation region, yet only moderately lower entropies, as compared to normal spherical PNS winds. Protomagnetars with moderately rapid birth periods P ∼ 3–5 ms may thus represent a promising galactic r-process site which is compatible with a variety of other observations, including the recent discovery of possible magnetar-powered supernovae in metal-poor galaxies. We also confirm previous results that the outflows from protomagnetars with P ∼ 1–2 ms can achieve maximum Lorentz factors Γmax ∼ 100–1000 in the range necessary to power gamma-ray bursts (GRBs). The implications of GRB jets with a heavy nuclei-dominated composition as sources of ultrahigh energy cosmic rays are also addressed.

Neutrinos in core-collapse supernovae and nucleosynthesis

Neutrino interactions are a key component of the mechanism for supernova explosions. They also play an important role in the associated nucleosynthesis. Nevertheless, a number of issues remain to be resolved. In this review we summarize research by our group and others on some of these issues. Foremost is the role of neutrinos in the explosion itself. Much effort is now invested in understanding the transport of neutrinos from the core and the role of neutrino heated convection in the outer envelopes. In addition to the neutrino transport issues, we review some current topics in neutrino-induced nucleosynthesis (the ν- and νp-processes). We review some current issues regarding the synthesis of the ν-process isotopes 180Ta and 138La and summarize the application of the ν-process chronometers, 136, 138Ce, 138La, and 92Nb. We also consider the light ν-process isotopes 7Li and 11B and summarize how neutrino oscillations and nucleosynthesis might constrain the neutrino mass hierarchy. We will also review the current dilemma regarding the nucleosynthesis of heavy elements by rapid neutron capture in the neutrino energized wind above the nascent neutron star. We also review the crucial role of neutrino reactions and heating above the high-temperature accretion disc around the black hole of a failed supernova (collapsar model). This collapsar model for long-duration gamma ray bursts is a possible site for r-process nucleosynthesis. We present numerical r-process calculations in the context of a magnetohydrodynamics + neutrino pair heated collapsar simulation. Neutrino heating of the jet is crucial for achieving the required relativistic outflow and at the same time generating material with a high neutron excess. During the late time evolution of the jet an r-process-like abundance distribution is formed within neutrino heated regions of the relativistic outflowing jet. Indeed, sufficient mass is ejected within the flow to account for the observed r-process abundance distribution along with the large dispersion in r-process elements observed in metal-poor halo stars. Finally, we discuss the nuclear physics issues and possible role of relic supernova neutrinos in resolving the supernova rate problem.

Alfven Wave–driven Proto–Neutron Star Winds andr‐Process Nucleosynthesis

We propose magnetic proto-neutron star (PNS) winds driven by Alfvén waves, as well as the neutrino heating, as an appropriate site for r-process nucleosynthesis. Alfvén waves excited by surface motions of a PNS propagate outwardly, and they heat and accelerate the wind by dissipation. Compared with the wind purely driven by neutrino heating, a larger entropy per baryon and shorter dynamical timescale are achieved, which favors the r-process. We study reasonable cases in which the wave amplitude is 10% of the Alfvén speed at the surface to find that a PNS with surface field strength ≳5 × 1014 G gives suitable wind properties for the r-process, provided that the dissipation length of the wave is at most ~10 times the PNS radius. We also compare properties of transcritical and subcritical winds in light of the r-process. We finally discuss possibilities of detections of γ-rays from radioactive nuclei and absorption lines due to Ba in supernova remnants that possess magnetars.

Supernova 1987A bound on neutrino spectra forr-process nucleosynthesis

The neutrino driven wind during a core collapse supernova is an attractive site for r-process nucleosynthesis. The initial electron fraction ${Y}_{e}$ in the wind depends on observable neutrino energies and luminosities. The mean antineutrino energy is limited by supernova SN 1987A data while lepton number conservation constrains the ratio of antineutrino to neutrino luminosities. If ${Y}_{e},$ in the wind, is to be suitable for rapid neutron capture nucleosynthesis, then the mean electron neutrino energy could be significantly lower than that predicted in present supernova simulations. Alternatively, there could be a rapid increase of the mean antineutrino energy at late times. However, this is not seen in the SN 1987A data. Finally, there could be new neutrino physics such as oscillations to sterile neutrinos.