arXiv subjects

Jonas Lippuner

Publications and source records attributed to Jonas Lippuner.

At least 19 recordsLinked to original sources

arXiv2022

Parthenon -- a performance portable block-structured adaptive mesh refinement framework

On the path to exascale the landscape of computer device architectures and corresponding programming models has become much more diverse. While various low-level performance portable programming models are available, support at the application level lacks behind. To address this issue, we present the performance portable block-structured adaptive mesh refinement (AMR) framework Parthenon, derived from the well-tested and widely used Athena++ astrophysical magnetohydrodynamics code, but generalized to serve as the foundation for a variety of downstream multi-physics codes. Parthenon adopts the Kokkos programming model, and provides various levels of abstractions from multi-dimensional variables, to packages defining and separating components, to launching of parallel compute kernels. Parthenon allocates all data in device memory to reduce data movement, supports the logical packing of variables and mesh blocks to reduce kernel launch overhead, and employs one-sided, asynchronous MPI calls to reduce communication overhead in multi-node simulations. Using a hydrodynamics miniapp, we demonstrate weak and strong scaling on various architectures including AMD and NVIDIA GPUs, Intel and AMD x86 CPUs, IBM Power9 CPUs, as well as Fujitsu A64FX CPUs. At the largest scale on Frontier (the first TOP500 exascale machine), the miniapp reaches a total of $1.7\times10^{13}$ zone-cycles/s on 9,216 nodes (73,728 logical GPUs) at ~92% weak scaling parallel efficiency (starting from a single node). In combination with being an open, collaborative project, this makes Parthenon an ideal framework to target exascale simulations in which the downstream developers can focus on their specific application rather than on the complexity of handling massively-parallel, device-accelerated AMR.

cs.DC↗

arXiv2020

The landscape of disk outflows from black hole - neutron star mergers

We investigate mass ejection from accretion disks formed in mergers of black holes (BHs) and neutron stars (NSs). The third observing run of the LIGO/Virgo interferometers provided BH-NS candidate events that yielded no electromagnetic (EM) counterparts. The broad range of disk configurations expected from BH-NS mergers motivates a thorough exploration of parameter space to improve EM signal predictions. Here we conduct 27 high-resolution, axisymmetric, long-term hydrodynamic simulations of the viscous evolution of BH accretion disks that include neutrino emission/absorption effects and post-processing with a nuclear reaction network. In the absence of magnetic fields, these simulations provide a lower-limit to the fraction of the initial disk mass ejected. We find a nearly linear inverse dependence of this fraction on disk compactness (BH mass over initial disk radius). The dependence is related to the fraction of the disk mass accreted before the outflow is launched, which depends on the disk position relative to the innermost stable circular orbit. We also characterize a trend of decreasing ejected fraction and decreasing lanthanide/actinide content with increasing disk mass at fixed BH mass. This trend results from a longer time to reach weak freezout and an increasingly dominant role of neutrino absorption at higher disk masses. We estimate the radioactive luminosity from the disk outflow alone available to power kilonovae over the range of configurations studied, finding a spread of two orders of magnitude. For most of the BH-NS parameter space, the disk outflow contribution is well below the kilonova mass upper limits for GW190814.

astro-ph.HE↗

arXiv2020

Axisymmetric Radiative Transfer Models of Kilonovae

The detailed observations of GW170817 proved for the first time directly that neutron star mergers are a major production site of heavy elements. The observations could be fit by a number of simulations that qualitatively agree, but can quantitatively differ (e.g. in total r-process mass) by an order of magnitude. We categorize kilonova ejecta into several typical morphologies motivated by numerical simulations, and apply a radiative transfer Monte Carlo code to study how the geometric distribution of the ejecta shapes the emitted radiation. We find major impacts on both spectra and light curves. The peak bolometric luminosity can vary by two orders of magnitude and the timing of its peak by a factor of five. These findings provide the crucial implication that the ejecta masses inferred from observations around the peak brightness are uncertain by at least an order of magnitude. Mixed two-component models with lanthanide-rich ejecta are particularly sensitive to geometric distribution. A subset of mixed models shows very strong viewing angle dependence due to lanthanide "curtaining," which persists even if the relative mass of lanthanide-rich component is small. The angular dependence is weak in the rest of our models, but different geometric combinations of the two components lead to a highly diverse set of light curves. We identify geometry-dependent {P Cygni} features in late spectra that directly map out strong lines in the simulated opacity of neodymium, which can help to constrain the ejecta geometry and to directly probe the r-process abundances.

astro-ph.HE↗

arXiv2019

The Impact of Nuclear Physics Uncertainties on Galactic Chemical Evolution Predictions

Modeling the evolution of the elements in the Milky Way is a multidisciplinary and challenging task. In addition to simulating the 13 billion years evolution of our Galaxy, chemical evolution simulations must keep track of the elements synthesized and ejected from every astrophysical site of interest (e.g., supernova, compact binary merger). The elemental abundances of such ejecta, which are a fundamental input for chemical evolution codes, are usually taken from theoretical nucleosynthesis calculations performed by the nuclear astrophysics community. Therefore, almost all chemical evolution predictions rely on the nuclear physics behind those calculations. In this proceedings, we highlight the impact of nuclear physics uncertainties on galactic chemical evolution predictions. We demonstrate that nuclear physics and galactic evolution uncertainties both have a significant impact on interpreting the origin of neutron-capture elements in our Solar System. Those results serve as a motivation to create and maintain collaborations between the fields of nuclear astrophysics and galaxy evolution.

astro-ph.GA↗

arXiv2019

Full Transport Model of GW170817-Like Disk Produces a Blue Kilonova

The 2017 detection of the inspiral and merger of two neutron stars in gravitational waves and gamma rays was accompanied by a quickly-reddening transient. Such a transient was predicted to occur following a rapid neutron capture (r-process) nucleosynthesis event, which synthesizes neutron-rich, radioactive nuclei and can take place in both dynamical ejecta and in the wind driven off the accretion torus formed after a neutron star merger. We present the first three-dimensional general relativistic, full transport neutrino radiation magnetohydrodynamics (GRRMHD) simulations of the black hole-accretion disk-wind system produced by the GW170817 merger. We show that the small but non-negligible optical depths lead to neutrino transport globally coupling the disk electron fraction, which we capture by solving the transport equation with a Monte Carlo method. The resulting absorption drives up the electron fraction in a structured, continuous outflow, with electron fraction as high as $Y_e\sim 0.4$ in the extreme polar region. We show via nuclear reaction network and radiative transfer calculations that nucleosynthesis in the disk wind will produce a blue kilonova.

astro-ph.HE↗

arXiv2019

Gamma-rays from kilonova: a potential probe of r-process nucleosynthesis

The mergers of compact binaries with at least one neutron star component are the potential leading sites of the production and ejection of $r$-process elements. Discoveries of galactic binary pulsars, short gamma-ray bursts, and gravitational-wave detections have all been constraining the rate of these events, while the gravitational wave plus broadband electromagnetic coverage of binary neutron star merger (GW170817) has also placed constraints on the properties (mass and composition) of the merger ejecta. But uncertainties and ambiguities in modeling the optical and infrared emission make it difficult to definitively measure the distribution of heavy isotopes in these mergers. In contrast, gamma rays emitted in the decay of these neutron-rich ejecta may provide a more direct measurement of the yields. We calculate the gamma production in remnants of neutron star mergers, considering two epochs: a kilonova epoch, lasting about two weeks, and a much later epoch of tens and hundreds of thousands of years after the merger. For the kilonova epoch, when the expanding ejecta is still only partially transparent to gamma radiation, we use 3D radiative transport simulations to produce the spectra. We show that the gamma-ray spectra associated with beta- and alpha-decay provide a fingerprint of the ejecta properties and, for a sufficiently nearby remnant, may be detectable, even for old remnants. We compare our gamma spectra with the potential detection limits of next generation detectors, including the Lunar Occultation Explorer (LOX), the All-sky Medium Energy Gamma-ray Observatory (AMEGO), and the Compton Spectrometer and Imager (COSI). We show that fission models can be discriminated via the presence of short-lived fission fragments in the remnant spectra.

astro-ph.HE↗

arXiv2019

Composition Effects on Kilonova Spectra and Light Curves: I

The merger of neutron star binaries is believed to eject a wide range of heavy elements into the universe. By observing the emission from this ejecta, scientists can probe the ejecta properties (mass, velocity and composition distributions). The emission (a.k.a. kilonova) is powered by the radioactive decay of the heavy isotopes produced in the merger and this emission is reprocessed by atomic opacities to optical and infra-red wavelengths. Understanding the ejecta properties requires calculating the dependence of this emission on these opacities. The strong lines in the optical and infra-red in lanthanide opacities have been shown to significantly alter the light-curves and spectra in these wavelength bands, arguing that the emission in these wavelengths can probe the composition of this ejecta. Here we study variations in the kilonova emission by varying individual lanthanide (and the actinide uranium) concentrations in the ejecta. The broad forest of lanthanide lines makes it difficult to determine the exact fraction of individual lanthanides. Nd is an exception. Its opacities above 1 micron are higher than other lanthanides and observations of kilonovae can potentially probe increased abundances of Nd. Similarly, at early times when the ejecta is still hot (first day), the U opacity is strong in the 0.2-1 micron wavelength range and kilonova observations may also be able to constrain these abundances.

astro-ph.HE↗

arXiv2019

Impact of Pulsar and Fallback Sources on Multifrequency Kilonova Models

We explore the impact of pulsar electromagnetic dipole and fallback accretion emission on the luminosity of a suite of kilonova models. The pulsar models are varied over pulsar magnetic field strength, pulsar lifetime, ejecta mass, and elemental abundances; the fallback models are varied over fallback accretion rate and ejecta mass. For the abundances, we use Fe and Nd as representatives of the wind and dynamical ejecta, respectively. We simulate radiative transfer in the ejecta in either 1D spherical or 2D cylindrical spatial geometry. For the grid of 1D simulations, the mass fraction of Nd is 0, $10^{-4}$, or $10^{-3}$ and the rest is Fe. Our models that fit the bolometric luminosity of AT 2017gfo (the kilonova associated with the first neutron star merger discovered in gravitational waves, GW170817) do not simultaneously fit the B, V, and I time evolution. However, we find that the trends of the evolution in B and V magnitudes are better matched by the fallback model relative to the pulsar model, implying the time dependence of the remnant source influences the color evolution. Further exploration of the parameter space and model deficiencies is needed before we can describe AT 2017gfo with a remnant source.

astro-ph.HE↗

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↗

arXiv2017

R-process Nucleosynthesis from Three-Dimensional Magnetorotational Core-Collapse Supernovae

We investigate r-process nucleosynthesis in three-dimensional (3D) general-relativistic magnetohydrodynamic simulations of rapidly rotating strongly magnetized core collapse. The simulations include a microphysical finite-temperature equation of state and a leakage scheme that captures the overall energetics and lepton number exchange due to postbounce neutrino emission and absorption. We track the composition of the ejected material using the nuclear reaction network SkyNet. Our results show that the 3D dynamics of magnetorotational core-collapse supernovae (CCSN) are important for their nucleosynthetic signature. We find that production of r-process material beyond the second peak is reduced by a factor of 100 when the magnetorotational jets produced by the rapidly rotating core undergo a kink instability. Our results indicate that 3D magnetorotationally powered CCSNe are a robust r-process source only if they are obtained by the collapse of cores with unrealistically large precollapse magnetic fields of order $10^{13}$G. Additionally, a comparison simulation that we restrict to axisymmetry, results in overly optimistic r-process production for lower magnetic field strengths.

astro-ph.HE↗

arXiv2017

The Origin of r-Process Elements in the Milky Way

Some of the heavy elements, such as gold and europium (Eu), are almost exclusively formed by the rapid neutron capture process (r-process). However, it is still unclear which astrophysical site between core-collapse supernovae and neutron star - neutron star (NS-NS) mergers produced most of the r-process elements in the universe. Galactic chemical evolution (GCE) models can test these scenarios by quantifying the frequency and yields required to reproduce the amount of europium (Eu) observed in galaxies. Although NS-NS mergers have become popular candidates, their required frequency (or rate) needs to be consistent with that obtained from gravitational wave measurements. Here we address the first NS-NS merger detected by LIGO/Virgo (GW170817) and its associated Gamma-ray burst and analyze their implication on the origin of r-process elements. The range of NS-NS merger rate densities of 320-4740 Gpc$^{-3}$ yr$^{-1}$ provided by LIGO/Virgo is remarkably consistent with the range required by GCE to explain the Eu abundances in the Milky Way with NS-NS mergers, assuming the solar r-process abundance pattern for the ejecta. Under the same assumption, this event has produced about 1-5 Earth masses of Eu, and 3-13 Earth masses of gold. When using theoretical calculations to derive Eu yields, constraining the role of NS-NS mergers becomes more challenging because of nuclear astrophysics uncertainties. This is the first study that directly combines nuclear physics uncertainties with GCE calculations. If GW170817 is a representative event, NS-NS mergers can produce Eu in sufficient amounts and are likely to be the main r-process site.

astro-ph.GA↗

arXiv2017

SkyNet: A modular nuclear reaction network library

Almost all of the elements heavier than hydrogen that are present in our solar system were produced by nuclear burning processes either in the early universe or at some point in the life cycle of stars. In all of these environments, there are dozens to thousands of nuclear species that interact with each other to produce successively heavier elements. In this paper, we present SkyNet, a new general-purpose nuclear reaction network that evolves the abundances of nuclear species under the influence of nuclear reactions. SkyNet can be used to compute the nucleosynthesis evolution in all astrophysical scenarios where nucleosynthesis occurs. SkyNet is free and open-source and aims to be easy to use and flexible. Any list of isotopes can be evolved and SkyNet supports various different types of nuclear reactions. SkyNet is modular so that new or existing physics, like nuclear reactions or equations of state, can easily be added or modified. Here, we present in detail the physics implemented in SkyNet with a focus on a self-consistent transition to and from nuclear statistical equilibrium (NSE) to non-equilibrium nuclear burning, our implementation of electron screening, and coupling of the network to an equation of state. We also present comprehensive code tests and comparisons with existing nuclear reaction networks. We find that SkyNet agrees with published results and other codes to an accuracy of a few percent. Discrepancies, where they exist, can be traced to differences in the physics implementations.

astro-ph.HE↗

arXiv2017

Signatures of hypermassive neutron star lifetimes on r-process nucleosynthesis in the disk ejecta from neutron star mergers

We investigate the nucleosynthesis of heavy elements in the winds ejected by accretion disks formed in neutron star mergers. We compute the element formation in disk outflows from hypermassive neutron star (HMNS) remnants of variable lifetime, including the effect of angular momentum transport in the disk evolution. We employ long-term axisymmetric hydrodynamic disk simulations to model the ejecta, and compute r-process nucleosynthesis with tracer particles using a nuclear reaction network containing $\sim 8000$ species. We find that the previously known strong correlation between HMNS lifetime, ejected mass, and average electron fraction in the outflow is directly related to the amount of neutrino irradiation on the disk, which dominates mass ejection at early times in the form of a neutrino-driven wind. Production of lanthanides and actinides saturates at short HMNS lifetimes ($\lesssim 10$ ms), with additional ejecta contributing to a blue optical kilonova component for longer-lived HMNSs. We find good agreement between the abundances from the disk outflow alone and the solar r-process distribution only for short HMNS lifetimes ($\lesssim 10$ ms). For longer lifetimes, the rare-earth and third r-process peaks are significantly under-produced compared to the solar pattern, requiring additional contributions from the dynamical ejecta. The nucleosynthesis signature from a spinning black hole (BH) can only overlap with that from a HMNS of moderate lifetime ($\lesssim 60$ ms). Finally, we show that angular momentum transport not only contributes with a late-time outflow component, but that it also enhances the neutrino-driven component by moving material to shallower regions of the gravitational potential, in addition to providing additional heating.

astro-ph.HE↗

arXiv2017

Neutrino-heated winds from millisecond proto-magnetars as sources of the weak r-process

We explore heavy element nucleosynthesis in neutrino-driven winds from rapidly-rotating, strongly magnetized proto-neutron stars for which the magnetic dipole is aligned with the rotation axis, and the field is assumed to be a static force-free configuration. We process the proto-magnetar wind trajectories calculated by Vlasov et al 2014 through the r-process nuclear reaction network SkyNet using contemporary models for the evolution of the wind electron fraction during the proto-neutron star cooling phase. Although we do not find a successful second or third peak r-process for any rotation period P, we show that proto-magnetars with P around 1-5 ms produce heavy element abundance distributions that extend to higher nuclear mass number than from otherwise equivalent spherical winds (with the mass fractions of some elements enhanced by factors of 100-1000). The heaviest elements are synthesized by outflows emerging along flux tubes which graze the closed zone and pass near the equatorial plane outside the light cylinder. Due to dependence of the nucleosynthesis pattern on the magnetic field strength and rotation rate of the proto-neutron star, natural variations in these quantities between core collapse events could contribute to the observed diversity of the abundances of weak r-process nuclei in metal-poor stars. Further diversity, including possibly even a successful third-peak r-process, could be achieved for misaligned rotators with non-zero magnetic inclination with respect to the rotation axis. If proto-magnetars are central engines for GRBs, their relativistic jets should contain a high mass fraction of heavy nuclei of characteristic average mass number A of order 100, providing a possible source for ultra-high energy cosmic rays comprised of heavy nuclei with an energy spectrum that extends beyond the nominal GZK cut-off for protons or iron nuclei.

astro-ph.HE↗

arXiv2016

Dynamics, nucleosynthesis, and kilonova signature of black hole - neutron star merger ejecta

We investigate the ejecta from black hole - neutron star mergers by modeling the formation and interaction of mass ejected in a tidal tail and a disk wind. The outflows are neutron-rich, giving rise to optical/infrared emission powered by the radioactive decay of $r$-process elements (a kilonova). Here we perform an end-to-end study of this phenomenon, where we start from the output of a fully-relativistic merger simulation, calculate the post-merger hydrodynamical evolution of the ejecta and disk winds including neutrino physics, determine the final nucleosynthetic yields using post-processing nuclear reaction network calculations, and compute the kilonova emission with a radiative transfer code. We study the effects of the tail-to-disk mass ratio by scaling the tail density. A larger initial tail mass results in fallback matter becoming mixed into the disk and ejected in the subsequent disk wind. Relative to the case of a disk without dynamical ejecta, the combined outflow has lower mean electron fraction, faster speed, larger total mass, and larger absolute mass free of high-opacity Lanthanides or Actinides. In most cases, the nucleosynthetic yield is dominated by the heavy $r$-process contribution from the unbound part of the tidal tail. A Solar-like abundance distribution can however be obtained when the total mass of the dynamical ejecta is comparable to the mass of the disk outflows. The kilonova has a characteristic duration of 1 week and a luminosity of ~$10^{41}$ erg/s, with orientation effects leading to variations of a factor ~2 in brightness. At early times (< 1 day) the emission includes an optical component from the (hot) Lanthanide-rich material, but the spectrum evolves quickly to the infrared thereafter.

astro-ph.HE↗

arXiv2016

Post-Outburst Radio Observations of the High Magnetic Field Pulsar PSR J1119-6127

We have carried out high frequency radio observations of the high magnetic field pulsar PSR J1119-6127 following its recent X-ray outburst. While initial observations showed no evidence of significant radio emission, subsequent observations detected pulsed emission across a large frequency band. In this letter, we report on the initial disappearance of the pulsed emission and its prompt reactivation and dramatic evolution over several months of observation. The periodic pulse profile at S-band (2.3 GHz) after reactivation exhibits a multi-component emission structure, while the simultaneous X-band (8.4 GHz) profile shows a single emission peak. Single pulses were also detected at S-band near the main emission peaks. We present measurements of the spectral index across a wide frequency bandwidth, which captures the underlying changes in the radio emission profile of the neutron star. The high frequency radio detection, unusual emission profile, and observed variability suggest similarities with magnetars, which may independently link the high energy outbursts to magnetar-like behavior.

astro-ph.HE↗

arXiv2016

SpECTRE: A Task-based Discontinuous Galerkin Code for Relativistic Astrophysics

We introduce a new relativistic astrophysics code, SpECTRE, that combines a discontinuous Galerkin method with a task-based parallelism model. SpECTRE's goal is to achieve more accurate solutions for challenging relativistic astrophysics problems such as core-collapse supernovae and binary neutron star mergers. The robustness of the discontinuous Galerkin method allows for the use of high-resolution shock capturing methods in regions where (relativistic) shocks are found, while exploiting high-order accuracy in smooth regions. A task-based parallelism model allows efficient use of the largest supercomputers for problems with a heterogeneous workload over disparate spatial and temporal scales. We argue that the locality and algorithmic structure of discontinuous Galerkin methods will exhibit good scalability within a task-based parallelism framework. We demonstrate the code on a wide variety of challenging benchmark problems in (non)-relativistic (magneto)-hydrodynamics. We demonstrate the code's scalability including its strong scaling on the NCSA Blue Waters supercomputer up to the machine's full capacity of 22,380 nodes using 671,400 threads.

astro-ph.HE↗

arXiv2016

Simulations of inspiraling and merging double neutron stars using the Spectral Einstein Code

We present results on the inspiral, merger, and post-merger evolution of a neutron star - neutron star (NSNS) system. Our results are obtained using the hybrid pseudospectral-finite volume Spectral Einstein Code (SpEC). To test our numerical methods, we evolve an equal-mass system for $\approx 22$ orbits before merger. This waveform is the longest waveform obtained from fully general-relativistic simulations for NSNSs to date. Such long (and accurate) numerical waveforms are required to further improve semi-analytical models used in gravitational wave data analysis, for example the effective one body models. We discuss in detail the improvements to SpEC's ability to simulate NSNS mergers, in particular mesh refined grids to better resolve the merger and post-merger phases. We provide a set of consistency checks and compare our results to NSNS merger simulations with the independent BAM code. We find agreement between them, which increases confidence in results obtained with either code. This work paves the way for future studies using long waveforms and more complex microphysical descriptions of neutron star matter in SpEC.

gr-qc↗