Astrophysical Simulations Research Articles

Measurement of the 91Zr(p,γ)92m Nb cross section motivated by type Ia supernova nucleosynthesis

The synthesis of heavy, proton rich isotopes is a poorly understood astrophysical process. Thermonuclear (type Ia) supernova explosions are among the suggested sites and the abundance of some isotopes present in the early Solar System may be used to test the models. 92Nb is such an isotope and one of the reactions playing a role in its synthesis is 91Zr(p,γ)92Nb. As no experimental cross sections were available for this reaction so far, nucleosynthesis models had to solely rely on theoretical calculations. In the present work the cross section of 91Zr(p,γ)92m Nb has been measured at astrophysical energies by activation. The results excellently confirm the predictions of cross sections and reaction rates for 91Zr(p,γ)92Nb, as used in astrophysical simulations.

The Progress of Three Astrophysics Simulation Methods: Monte-Carlo, PIC and MHD

Contemporarily, the dramatic progress that proceeded in astrophysical simulation stems from the computational upgrade. Simultaneously, the relative algorithms are from naive to mature. Some imaginary algorithms spring up like bamboo. The typical methods are Monte-Carlo, Particle-in-Cell, and Magnetohydrodynamic considering the relativistic effect. This paper reviews the typical progress of the above three methods as well as their hybrid method in astrophysics. These results offer a guideline for further research in astrophysics simulation.

Binding energy shifts from heavy-ion experiments in a nuclear statistical equilibrium model

Chemical constants extracted from \(^{124}\)Xe+ \(^{124}\)Sn collisions at 32 AMeV are compared to the predictions of an extended nuclear statistical equilibrium model including mean-field interactions and in-medium binding energy shifts for the light (\(Z\le 2\)) clusters. The ion species and density dependence of the in-medium modification is directly extracted from the experimental data. We show that the shift increases with the mass of the cluster and the density of the medium, and we provide a simple linear fit for future use in astrophysical simulations in the framework of the CompOSE data base. The resulting mass fractions are computed in representative thermodynamic conditions relevant for supernova and neutron star mergers. A comparison to the results of a similar analysis of the same data performed in the framework of a relativistic mean-field model shows a good agreement at low density, but significant discrepancies close to the Mott dissolution of clusters in the dense medium.

Towards extracting cosmic magnetic field structures from cosmic-ray arrival directions

We present a novel method to search for structures of coherently aligned patterns in ultra-high energy cosmic-ray arrival directions simultaneously across the entire sky. This method can be used to obtain information on the Galactic magnetic field, in particular the integrated component perpendicular to the line of sight, from cosmic-ray data only. Using a likelihood-ratio approach, neighboring cosmic rays are related by rotatable, elliptically shaped density distributions and the significance of their alignment with respect to circular distributions is evaluated. In this way, a vector field tangential to the celestial sphere is fitted which approximates the local deflections in cosmic magnetic fields if significant deflection structures are detected. The sensitivity of the method is evaluated on the basis of astrophysical simulations of the ultra-high energy cosmic-ray sky, where a discriminative power between isotropic and signal-induced scenarios is found.

Accelerating self-gravitating hydrodynamics simulations with adaptive force updates

ABSTRACT Many astrophysical hydrodynamics simulations must account for gravity, and evaluating the gravitational field at the positions of all resolution elements can incur significant cost. Typical algorithms update the gravitational field at the position of each resolution element every time the element is updated hydrodynamically, but the actual required update frequencies for hydrodynamics and gravity can be different in general. We show that the gravity calculation in hydrodynamics simulations can be optimized by only updating gravity on a time-scale dictated by the already determined maximum time-step for accurate gravity integration Δtgrav, while staying well within the typical error budget of hydro schemes and gravity solvers. Our implementation in the gizmo code uses the time-scale derived from the tidal tensor $t_{\rm tidal} = \Vert \mathbf {T}\Vert ^{-1/2}$ to determine Δtgrav and the force update frequency in turn, and uses the rate of change of acceleration evaluated by the gravity solver to construct a predictor of the acceleration for use between updates. We test the scheme on standard self-gravitating hydrodynamics test problems, finding solutions very close to the standard scheme while evaluating far fewer gravity forces, optimizing the simulations. We also demonstrate a $\sim 70{{\ \rm per\ cent}}$ speed-up in an example simulation of a giant molecular cloud. In general, this scheme introduces a new tunable parameter for obtaining an optimal compromise between accuracy and computational cost, in conjunction with, e.g. time-step tolerance, numerical resolution, and gravity solver tolerance.

Well-balanced treatment of gravity in astrophysical fluid dynamics simulations at low Mach numbers

Context.Accurate simulations of flows in stellar interiors are crucial to improving our understanding of stellar structure and evolution. Because the typically slow flows are merely tiny perturbations on top of a close balance between gravity and the pressure gradient, such simulations place heavy demands on numerical hydrodynamics schemes.Aims.We demonstrate how discretization errors on grids of reasonable size can lead to spurious flows orders of magnitude faster than the physical flow. Well-balanced numerical schemes can deal with this problem.Methods.Three such schemes were applied in the implicit, finite-volume SEVEN-LEAGUEHYDROcode in combination with a low-Mach-number numerical flux function. We compare how the schemes perform in four numerical experiments addressing some of the challenges imposed by typical problems in stellar hydrodynamics.Results.We find that theα-βand deviation well-balancing methods can accurately maintain hydrostatic solutions provided that gravitational potential energy is included in the total energy balance. They accurately conserve minuscule entropy fluctuations advected in an isentropic stratification, which enables the methods to reproduce the expected scaling of convective flow speed with the heating rate. The deviation method also substantially increases accuracy of maintaining stationary orbital motions in a Keplerian disk on long timescales. The Cargo–LeRoux method fares substantially worse in our tests, although its simplicity may still offer some merits in certain situations.Conclusions.Overall, we find the well-balanced treatment of gravity in combination with low Mach number flux functions essential to reproducing correct physical solutions to challenging stellar slow-flow problems on affordable collocated grids.

The AGORA High-resolution Galaxy Simulations Comparison Project. III. Cosmological Zoom-in Simulation of a Milky Way–mass Halo

Abstract We present a suite of high-resolution cosmological zoom-in simulations to z = 4 of a 1012 M ⊙ halo at z = 0, obtained using seven contemporary astrophysical simulation codes (Art-I, Enzo, Ramses, Changa, Gadget-3, Gear, and Gizmo) widely used in the numerical galaxy formation community. The physics prescriptions for gas cooling and heating and star formation are the same as the ones used in our previous Assembling Galaxies of Resolved Anatomy (AGORA) disk comparison but now account for the effects of cosmological processes such as the expansion of the universe, intergalactic gas inflow, and the cosmic ultraviolet background radiation emitted by massive stars and quasars. In this work, we introduce the most careful comparison yet of galaxy formation simulations run by different code groups, together with a series of four calibration steps each of which is designed to reduce the number of tunable simulation parameters adopted in the final run. In the first two steps, we methodically calibrate the gas physics, such as cooling and heating, in simulations without star formation. In the third step, we seek agreement on the total stellar mass produced with the common star formation prescription used in the AGORA disk comparison, in stellar-feedback-free simulations. In the last calibration step, we activate stellar feedback, where each code group is asked to set the feedback prescription to as close to the most widely used one in its code community as possible, while aiming for convergence in the stellar mass at z = 4 to the values predicted by semiempirical models. After all the participating code groups successfully complete the calibration steps, we achieve a suite of cosmological simulations with similar mass assembly histories down to z = 4. With numerical accuracy that resolves the internal structure of a target halo (≲100 physical pc at z = 4), we find that the codes overall agree well with one another, e.g., in gas and stellar properties, but also show differences, e.g., in circumgalactic medium (CGM) properties. We argue that, if adequately tested in accordance with our proposed calibration steps and common parameters, high-resolution cosmological zoom-in simulations can have robust and reproducible results. New code groups are invited to join and enrich this comparison by generating equivalent models or to test the code’s compatibility on their own, by adopting the common initial conditions, the common easy-to-implement physics package, and the proposed calibration steps. Further analyses of the zoom-in simulations presented here will be presented in forthcoming reports from the AGORA Collaboration, including studies of the CGM, simulations by additional codes, and results at lower redshift.

Exascale models of stellar explosions: Quintessential multi-physics simulation

The ExaStar project aims to deliver an efficient, versatile, and portable software ecosystem for multi-physics astrophysics simulations run on exascale machines. The code suite is a component-based multi-physics toolkit, built on the capabilities of current simulation codes (in particular Flash-X and Castro), and based on the massively parallel adaptive mesh refinement framework AMReX. It includes modules for hydrodynamics, advanced radiation transport, thermonuclear kinetics, and nuclear microphysics. The code will reach exascale efficiency by building upon current multi- and many-core packages integrated into an orchestration system that uses a combination of configuration tools, code translators, and a domain-specific asynchronous runtime to manage performance across a range of platform architectures. The target science includes multi-physics simulations of astrophysical explosions (such as supernovae and neutron star mergers) to understand the cosmic origin of the elements and the fundamental physics of matter and neutrinos under extreme conditions.

Evaluating modern system dynamics software for use in astrophysical simulations

System dynamics (SD) software provides a diagrammatic way of modelling integral and differential relations in a system. In the current literature, SD software is typically applied to problems in the social sciences. However, the methods utilised in SD are not constrained to these fields, warranting novel applications in the physical sciences. Previous literature has applied SD software to time-dependent systems, but this method of modelling problems in physics has not proved popular in research, possibly due to limitations within the software available. The primary contribution of this paper is to demonstrate the application of current SD software to astrophysical phenomena in which the independent variable is not time, namely in modelling the internal structure of zero-age main-sequence stars. This demonstrates an innovative model application with the possibility for future adaptations, and extension to other astrophysical phenomena. The ease of the modelling process and validity of results for such a system is demonstrated, however a number of complications associated with current SD software were encountered. Suggestions are made as to how these may be overcome in future software iterations to make SD software more accessible to researchers in astrophysics.

Mixing matters

ABSTRACT All hydrodynamical simulations of turbulent astrophysical phenomena require sub-grid scale models to properly treat energy dissipation and metal mixing. We present the first implementation and application of an anisotropic eddy viscosity and metal mixing model in Lagrangian astrophysical simulations, including a dynamic procedure for the model parameter. We compare these two models directly to the common Smagorinsky and dynamic variant. Using the mesh-free finite mass method as an example, we show that the anisotropic model is best able to reproduce the proper Kolmogorov inertial range scaling in homogeneous, isotropic turbulence. Additionally, we provide a method to calibrate the metal mixing rate that ensures numerical convergence. In our first application to cosmological simulations, we find that all models strongly impact the early evolution of galaxies, leading to differences in enrichment and thermodynamic histories. The anisotropic model has the strongest impact, with little difference between the dynamic variant and the constant-coefficient variant. We also find that the metal distribution functions in the circumgalactic gas are significantly tighter at all redshifts, with the anisotropic model providing the tightest distributions. This is contrary to a recent study that found metal mixing to be relatively unimportant on cosmological scales. In all of our experiments, the constant-coefficient Smagorinsky and anisotropic models rivalled their dynamic counterparts, suggesting that the computationally inexpensive constant-coefficient models are viable alternatives in cosmological contexts.

Thermal effects in hot and dilute homogeneous asymmetric nuclear matter

We present a comprehensive analysis of hot and dilute isospin-asymmetric nuclear matter employing the temperature-dependent effective-relativistic mean-field theory (E-RMF). The E-RMF is applied to study the effect of $\delta$ and $\omega-\rho$ meson cross-coupling on the thermal properties of asymmetric nuclear matter using two recently developed IOPB-I and G3 parameter sets. These sets are known to reproduce the nuclear matter properties in agreement with various experimental and observational constraints. We consider the nuclear matter to be homogeneous and study the equation of state (EoS) for densities, temperature and asymmetry which are relevant for astrophysical simulations such as supernovae explosion. The effect of temperature is investigated in reference to the density-dependent free symmetry energy and its higher-order derivatives using the well known parabolic approximation. The larger value of $\lambda_\omega$ cross-coupling in G3 in addition to the $\delta$ meson coupling in G3 smoothen the free symmetry energy. Thermal effects on various state variables are examined at fixed temperature and isospin asymmetry by separating their T=0 and the finite-T expressions. The thermal effects are mainly governed by effective mass with larger effective mass estimating larger thermal contribution. The effect of temperature on isothermal and isentropic incompressibility is discussed which is in harmony with various available microscopic calculations. The liquid-gas phase transition properties are examined in asymmetric matter with two conserved charges in the context of different slope parameter and comparable symmetry energy in IOPB-I and G3 set. The spinodal instability, binodal curve and critical properties are found to be influenced by the slope parameter $L_{sym}$.

Inconsistencies arising from the coupling of galaxy formation sub-grid models to pressure-smoothed particle hydrodynamics

ABSTRACT Smoothed particle hydrodynamics (SPH) is a Lagrangian method for solving the fluid equations that is commonplace in astrophysics, prized for its natural adaptivity and stability. The choice of variable to smooth in SPH has been the topic of contention, with smoothed pressure (P-SPH) being introduced to reduce errors at contact discontinuities relative to smoothed density schemes. Smoothed pressure schemes produce excellent results in isolated hydrodynamics tests; in more complex situations however, especially when coupling to the ‘sub-grid’ physics and multiple time-stepping used in many state-of-the-art astrophysics simulations, these schemes produce large force errors that can easily evade detection as they do not manifest as energy non-conservation. Here, two scenarios are evaluated: the injection of energy into the fluid (common for stellar feedback) and radiative cooling. In the former scenario, force and energy conservation errors manifest (of the same order as the injected energy), and in the latter large force errors that change rapidly over a few time-steps lead to instability in the fluid (of the same order as the energy lost to cooling). Potential ways to remedy these issues are explored with solutions generally leading to large increases in computational cost. Schemes using a density-based formulation do not create these instabilities and as such it is recommended that they are preferred over pressure-based solutions when combined with an energy diffusion term to reduce errors at contact discontinuities.

Conceptual Analogies Between Multi-Scale Feeding and Feedback Cycles in Supermassive Black Hole and Cancer Environments.

Adopting three physically-motivated scales (“micro” – “meso” – “macro”, which refer to mpc – kpc – Mpc, respectively) is paramount for achieving a unified theory of multiphase active galactic nuclei feeding and feedback, and it represents a keystone for astrophysical simulations and observations in the upcoming years. In order to promote this multi-scale idea, we have decided to adopt an interdisciplinary approach, exploring the possible conceptual similarities between supermassive black hole feeding and feedback cycles and the dynamics occurring in human cancer microenvironment.

Spatially resolved spectroscopy across stellar surfaces

Context. High-precision stellar analyses require hydrodynamic 3D modeling. Such models predict changes across stellar disks of spectral line shapes, asymmetries, and wavelength shifts. For testing models in stars other than the Sun, spatially resolved observations are feasible from differential spectroscopy during exoplanet transits, retrieving spectra of those stellar surface segments that successively become hidden behind the transiting planet, as demonstrated in Papers I, II, and III. Aims. Synthetic high-resolution spectra over extended spectral regions are now available from 3D models. Similar to other ab initio simulations in astrophysics, these data contain patterns that have not been specifically modeled but may be revealed after analyses to be analogous to those of a large volume of observations. Methods. From five 3D models spanning Teff = 3964–6726 K (spectral types ~K8 V–F3 V), synthetic spectra at hyper-high resolution (λ/Δλ >1 000 000) were analyzed. Selected Fe I and Fe II lines at various positions across stellar disks were searched for characteristic patterns between different types of lines in the same star and for similar lines between different stars. Results. Spectral-line patterns are identified for representative photospheric lines of different strengths, excitation potentials, and ionization levels, thereby encoding the hydrodynamic 3D structure. Line profiles and bisectors are shown for various stars at different positions across stellar disks. Absolute convective wavelength shifts are obtained as differences to 1D models, where such shifts do not occur. Conclusions. Observable relationships for line properties are retrieved from realistically complex synthetic spectra. Such patterns may also test very detailed 3D modeling, including non-LTE effects. While present results are obtained at hyper-high spectral resolution, the subsequent Paper V examines their practical observability at realistically lower resolutions, and in the presence of noise.

Neutron capture on $$^{16}$$O within the framework of RMF + ACCC + BCS for astrophysical simulations

Neutron capture on $$^{16}$$ O may serve as a neutron poison in certain nucleosynthesis scenarios. We revisit this reaction at energy ranges of astrophysical interest, employing a novel theoretical approach that self-consistently treats capture through bound and resonant levels. Our covariant density functional theory is based on a relativistic mean field (RMF) theory with contributions from resonant orbitals included via the analytical continuation of the coupling constant (ACCC), and pairing correlations included via the resonant Bardeen–Cooper–Schrieffer (BCS) technique. We employ this RMF + ACCC + BCS approach to extract bound states, resonant states, and pairing correlations in $$^{17}$$ O in a self-consistent microscopic way. We calculate $$^{16}$$ O(n, $$\gamma $$ ) $$^{17}$$ O direct capture cross sections resulting from neutron E1 transitions from scattering states to bound states, resonant cross sections from a Breit–Wigner formalism, and Maxwellian-averaged cross sections and thermonuclear reaction rates for astrophysical applications. We use different effective interactions to determine the viability of our approach to determine the $$^{17}$$ O level structure and corresponding reaction rates. Comparisons to measurements and database values are given.

A Proper Discretization of Hydrodynamic Equations in Cylindrical Coordinates for Astrophysical Simulations

Abstract Cylindrical coordinates are often used in computational fluid dynamics, particularly when one is considering gas flow accreting onto a central object. Although cylindrical coordinates have several advantages in describing rotation, they have apparent singularity along the axis at the coordinate origin (z-axis). This singularity introduces difficulties in numerical simulations. First, it is difficult to reproduce the flow across the z-axis. Second, the time step is extremely shortened by the CFL condition near the z-axis because the numerical cell thereof is narrow in the azimuthal direction for a given angular resolution. Here, we propose a new discretization scheme to overcome these difficulties. In our new scheme, we consider changes in the direction of the unit vector within a cell when evaluating the flux across each cell surface. Besides, we evaluate the source term in the radial component of the momentum equation from the thermal and dynamic pressures acting on the azimuthal cell surface. The new scheme is designed to be free-stream-preserving so that flow with uniform density, pressure, and velocity is an exact solution of the discretized equation. These improvements are essential to using a lower angular resolution in the innermost area and thus to elongating each time step. Our examples demonstrate that the innermost circular region around the axis can be resolved by only six numerical cells. We present an application to an accreting compact star surrounded by a disk, in addition to Sod shock tube and rotating outflow tests.

Information, Entropy Decrease and Simulations of Astrophysical Evolutions

Entropy is a great development in science. We proposed that entropy decrease due to internal interactions in the isolated system is possible. We define the entangled scale, which mainly involves the number n and entangled degree. Since coherence, entanglement and correlation are all internal interactions in information systems, we discuss quantitatively entropy decrease along coherence, and entropy increase only for incoherence. From beginning quantum heat engine, we must systematically study quantum thermodynamics. Based on some astrophysical simulation models, they shown that the universe evolves from disorder to structures, which correspond to entropy decrease. This is consistence with theoretical result. The simulation must be an isolated system only using internal gravitational interactions.

The impact of superconductivity and the Hall effect in models of magnetised neutron stars

AbstractEquilibrium configurations of the internal magnetic field of a pulsar play a key role in modelling astrophysical phenomena from glitches to gravitational wave emission. In this paper, we present a numerical scheme for solving the Grad–Shafranov equation and calculating equilibrium configurations of pulsars, accounting for superconductivity in the core of the neutron star, and for the Hall effect in the crust of the star. Our numerical code uses a finite difference method in which the source term appearing in the Grad–Shafranov equation, which is used to model the magnetic equilibrium is non-linear. We obtain solutions by linearising the source and applying an under-relaxation scheme at each step of computation to improve the solver’s convergence. We have developed our code in both C++ and Python, and our numerical algorithm can further be adapted to solve any non-linear PDEs appearing in other areas of computational astrophysics. We produce mixed toroidal–poloidal field configurations, and extend the portion of parameter space that can be investigated with respect to previous studies. We find that in even in the more extreme cases, the magnetic energy in the toroidal component does not exceed approximately 5% of the total. We also find that if the core of the star is superconducting, the toroidal component is entirely confined to the crust of the star, which has important implications for pulsar glitch models which rely on the presence of a strong toroidal field region in the core of the star, where superfluid vortices pin to superconducting fluxtubes.

The Littlewood-Paley $ pth $-order moments in three-dimensional MHD turbulence

In this paper, we consider the Littlewood-Paley \begin{document}$ p $\end{document} th-order ( \begin{document}$ 1\le p ) moments of the three-dimensional MHD periodic equations, which are defined by the infinite-time and space average of \begin{document}$ L^p $\end{document} -norm of velocity and magnetic fields involved in the spectral cut-off operator \begin{document}$ \dot\Delta_m $\end{document} . Our results imply that in some cases, \begin{document}$ k^{-\frac{1}{3}} $\end{document} is an upper bound at length scale \begin{document}$ 1/k $\end{document} . This coincides with the scaling law of many observations on astrophysical systems and simulations in terms of 3D MHD turbulence.

LOC program for line radiative transfer

Context. Radiative transfer (RT) modelling is part of many astrophysical simulations. It is used to make synthetic observations and to assist the analysis of observations. We concentrate on modelling the radio lines emitted by the interstellar medium. In connection with high-resolution models, this can be a significant computationally challenge. Aims. Our aim is to provide a line RT program that makes good use of multi-core central processing units (CPUs) and graphics processing units (GPUs). Parallelisation is essential to speed up computations and to enable large modelling tasks with personal computers. Methods. The program LOC is based on ray-tracing (i.e. not Monte Carlo) and uses standard accelerated lambda iteration methods for faster convergence. The program works on 1D and 3D grids. The 1D version makes use of symmetries to speed up the RT calculations. The 3D version works with octree grids, and to enable calculations with large models, is optimised for low memory usage. Results. Tests show that LOC results agree with other RT codes to within ∼2%. This is typical of code-to-code differences, which are often related to different interpretations of the model set-up. LOC run times compare favourably especially with those of Monte Carlo codes. In 1D tests, LOC runs were faster by up to a factor ∼20 on a GPU than on a single CPU core. In spite of the complex path calculations, a speed-up of up to ∼10 was also observed for 3D models using octree discretisation. GPUs enable calculations of models with hundreds of millions of cells, as are encountered in the context of large-scale simulations of interstellar clouds. Conclusions. LOC shows good performance and accuracy and is able to handle many RT modelling tasks on personal computers. It is written in Python, with only the computing-intensive parts implemented as compiled OpenCL kernels. It can therefore also a serve as a platform for further experimentation with alternative RT implementation details.