arXiv subjects

Alan C. Calder

Publications and source records attributed to Alan C. Calder.

At least 19 recordsLinked to original sources

arXiv2024

Benchmarking with Supernovae: A Performance Study of the FLASH Code

Astrophysical simulations are computation, memory, and thus energy intensive, thereby requiring new hardware advances for progress. Stony Brook University recently expanded its computing cluster "SeaWulf" with an addition of 94 new nodes featuring Intel Sapphire Rapids Xeon Max series CPUs. We present a performance and power efficiency study of this hardware performed with FLASH: a multi-scale, multi-physics, adaptive mesh-based software instrument. We extend this study to compare performance to that of Stony Brook's Ookami testbed which features ARM-based A64FX-700 processors, and SeaWulf's AMD EPYC Milan and Intel Skylake nodes. Our application is a stellar explosion known as a thermonuclear (Type Ia) supernova and for this 3D problem, FLASH includes operators for hydrodynamics, gravity, and nuclear burning, in addition to routines for the material equation of state. We perform a strong-scaling study with a 220 GB problem size to explore both single- and multi-node performance. Our study explores the performance of different MPI mappings and the distribution of processors across nodes. From these tests, we determined the optimal configuration to balance runtime and energy consumption for our application.

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arXiv2023

Sensitivity of 3D Convective Urca Simulations to Changes in Urca Reactions

A proposed setting for thermonuclear (Type Ia) supernovae is a white dwarf that has gained mass from a companion to the point of carbon ignition in the core. There is a simmering phase in the early stages of burning that involves the formation and growth of a core convection zone. One aspect of this phase is the convective Urca process, a linking of weak nuclear reactions to convection that may alter the composition and structure of the white dwarf. Convective Urca is not well understood and requires 3D fluid simulations to realistically model. Additionally, the convection is relatively slow (Mach number less than 0.005) so a low-Mach method is needed to make simulating computationally feasible. Using the MAESTROeX low-Mach hydrodynamics code, we investigate recent changes to how the weak reactions are modeled in the convective Urca simulations. We present results that quantify the changes to the reaction rates and their impact on the evolution of the simulation.

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arXiv2023

Challenges Modeling the Low-Luminosity Type Iax Supernovae

Numerical models allow the investigation of phenomena that cannot exist in a laboratory. Computational simulations are therefore essential for advancing our knowledge of astrophysics, however, the very nature of simulation requires making assumptions that can substantially affect their outcome. Here, we present the challenges faced when simulating dim thermonuclear explosions, Type Iax supernovae. This class of dim events produce a slow moving, sparse ejecta that presents challenges for simulation. We investigate the limitations of the equation of state and its applicability to the expanding, cooling ejecta. We also discuss how the "fluff", i.e. the low-density gas on the grid in lieu of vacuum, inhibits the ejecta as it expands. We explore how the final state of the simulation changes as we vary the character of the burning, which influences the outcome of the explosion. These challenges are applicable to a wide range of astrophysical simulations, and are important to discuss and overcome as a community.

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arXiv2023

Dimming the Lights: 2D Simulations of Deflagrations of Hybrid C/O/Ne White Dwarfs using FLASH

The dimmest and most numerous outlier of the Type Ia supernova population, Type Iax events, is increasingly being found in the results of observational campaigns. There is currently no single accepted model to describe these events. This 2D study explores the viability of modeling Type Iax events as a hybrid C/O/Ne white dwarf progenitor undergoing a deflagration using the multi-physics software FLASH. This hybrid was created using the stellar evolution code MESA, and its C-depleted core and mixed structure have demonstrated lower yields than traditional C/O progenitors in previous deflagration-to-detonation studies. To generate a sample, 30 "realizations" of this simulation were performed, the only difference being the shape of the initial matchhead used to start the deflagration. As consistent with earlier work, these realizations produce the familiar hot dense bound remnant surrounded by sparse ejecta. Our results indicate the majority of the star remains unburned (~70%) and bound (>90%). Our realizations produce total ejecta yields on the order of 10$^{-2}$ - 10$^{-1}$ solar masses, ejected $^{56}$Ni yields on the order of 10$^{-4}$ - 10$^{-2}$ solar masses, and ejecta kinetic energies on the order of 10$^{48}$ - 10$^{49}$ ergs. Compared to yields inferred from recent observations of the dimmest Type Iax events - SN 2007qd, SN 2008ha, SN 2010ae, SN 2019gsc, SN 2019muj, SN 2020kyg, and SN 2021fcg - our simulation produces comparable $^{56}$Ni yields, but too-small total yields and kinetic energies. Reignition of the remnant is also seen in some realizations.

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arXiv2023

A Further Study of Linux Kernel Hugepages on A64FX with FLASH, an Astrophysical Simulation Code

We present an expanded study of the performance of FLASH when using Linux Kernel Hugepages on Ookami, an HPE Apollo 80 A64FX platform. FLASH is a multi-scale, multi-physics simulation code written principally in modern Fortran and makes use of the PARAMESH library to manage a block-structured adaptive mesh. Our initial study used only the Fujitsu compiler to utilize standard hugepages (hp), but further investigation allowed us to utilize hp for multiple compilers by linking to the Fujitsu library libmpg and transparent hugepages (thp) by enabling it at the node level. By comparing the results of hardware counters and in-code timers, we found that hp and thp do not significantly impact the runtime performance of FLASH. Interestingly, there is a significant reduction in the TLB misses, differences in cache and memory access counters, and strange behavior is observed when using thp.

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arXiv2022

Performance of an Astrophysical Radiation Hydrodynamics Code under Scalable Vector Extension Optimization

We present results of a performance study of an astrophysical radiation hydrodynamics code, V2D, on the Arm-based A64FX processor developed by Fujitsu. The code solves sparse linear systems, a task for which the A64FX architecture should be well suited. We performed the performance analysis study on Ookami, an Apollo 80 platform utilizing the A64FX processor. We explored several compilers and performance analysis packages and found the code did not perform as expected under scalable vector extension optimization, suggesting that a "deeper dive" into analyzing the code is worthwhile. However, a simple driver program that exercised basic sparse linear algebra routines used by V2D did show significant speedup with the use of the scalable vector extension optimization. We present the initial results from the study which used V2D on a relatively simple test problem that emphasized the repeated solution of sparse linear systems.

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arXiv2022

On Using Linux Kernel Huge Pages with FLASH, an Astrophysical Simulation Code

We present efforts at improving the performance of FLASH, a multi-scale, multi-physics simulation code principally for astrophysical applications, by using huge pages on Ookami, an HPE Apollo 80 A64FX platform. FLASH is written principally in modern Fortran and makes use of the PARAMESH library to manage a block-structured adaptive mesh. We explored options for enabling the use of huge pages with several compilers, but we were only able to successfully use huge pages when compiling with the Fujitsu compiler. The use of huge pages substantially reduced the number of translation lookaside buffer misses, but overall performance gains were marginal.

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arXiv2021

Ookami: Deployment and Initial Experiences

Ookami is a computer technology testbed supported by the United States National Science Foundation. It provides researchers with access to the A64FX processor developed by Fujitsu in collaboration with RIKΞN for the Japanese path to exascale computing, as deployed in Fugaku, the fastest computer in the world. By focusing on crucial architectural details, the ARM-based, multi-core, 512-bit SIMD-vector processor with ultrahigh-bandwidth memory promises to retain familiar and successful programming models while achieving very high performance for a wide range of applications. We review relevant technology and system details, and the main body of the paper focuses on initial experiences with the hardware and software ecosystem for micro-benchmarks, mini-apps, and full applications, and starts to answer questions about where such technologies fit into the NSF ecosystem.

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arXiv2019

Type Ia Supernova Explosions from Hybrid Carbon-Oxygen-Neon White Dwarf Progenitors That Have Mixed During Cooling

The creation of "hybrid" white dwarfs, made of a C-O core within a O-Ne shell has been proposed, and studies indicate that ignition in the C-rich central region makes these viable progenitors for thermonuclear (type Ia) supernovae. Recent work found that the C-O core is mixed with the surrounding O-Ne as the white dwarf cools prior to accretion, which results in lower central C fractions in the massive progenitor than previously assumed. To further investigate the efficacy of hybrid white dwarfs as progenitors of thermonuclear supernovae, we performed simulations of thermonuclear supernovae from a new series of hybrid progenitors that include the effects of mixing during cooling. The progenitor white dwarf model was constructed with the one-dimensional stellar evolution code MESA and represented a star evolved through the phase of unstable interior mixing followed by accretion until it reached conditions for the ignition of carbon burning. This MESA model was then mapped to a two-dimensional initial condition for explosions simulated with FLASH. For comparison, similar simulations were performed for a traditional C-O progenitor white dwarf. By comparing the yields of the explosions, we find that, as with earlier studies, the lower C abundance in the hybrid progenitor compared to the traditional C-O progenitor leads to a lower average yield of 56Ni. Although the unmixed hybrid WD showed a similar decrement also in total iron group yield, the mixed case does not and produces a smaller fraction of iron group elements in the form of 56Ni. We attribute this to the higher central density required for ignition and the location, center or off-center, of deflagration ignition.

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arXiv2019

Modeling subgrid combustion processes in simulations of thermonuclear supernovae

Supernovae of type Ia are thought to arise from the thermonuclear incineration of a carbon-oxygen white dwarf stellar remnant. However, the detailed explosion scenario and stellar evolutionary origin scenario -- or scenarios -- which lead to observed supernovae are still quite uncertain. One of the principal tests of proposed scenarios is comparison with the explosion products inferred, for example, from the spectrum of the supernovae. Making this comparison requires computation of the combustion dynamics and products through simulation of proposed scenarios. Here we discuss two specific proposed explosion scenarios, the deflagration-detonation transition and the helium shell double detonation, With these two examples in mind, we proceed to discuss challenges to computational modeling of the combustion taking place in these explosions. Both subsonically and supersonically propagating reaction fronts are discussed, called deflagrations and detonations respectively. Several major stages of the combustion occur on length and time scales that are many orders of magnitude smaller than those accessible in simulations of the explosion. Models which attempt to capture this sub-grid behavior and the verification of those models is briefly discussed.

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arXiv2019

Thermonuclear (Type Ia) Supernovae and Progenitor Evolution

Thermonuclear (type Ia) supernovae are bright stellar explosions with the unique property that the light curves can be standardized, allowing them to be used as distance indicators for cosmological studies. Many fundamental questions bout these events remain, however. We provide a critique of our present understanding of these and present results of simulations assuming the single-degenerate progenitor model consisting of a white dwarf that has gained mass from a stellar companion. We present results from full three-dimensional simulations of convection with weak reactions comprising the A=23 Urca process in the progenitor white dwarf.

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arXiv2018

Quantification of Incertitude in Black Box Simulation Codes

We present early results from a study addressing the question of how one treats the propagation of incertitude, that is, epistemic uncertainty, in input parameters in astrophysical simulations. As an example, we look at the propagation of incertitude in control parameters for stellar winds in MESA stellar evolution simulations. We apply two methods of incertitude propagation, the Cauchy Deviates method and the Quadratic Response Surface method, to quantify the output uncertainty in the final white dwarf mass given a range of values for wind parameters. The methodology we apply is applicable to the problem of propagating input incertitudes through any simulation code treated as a "black box," i.e. a code for which the algorithmic details are either inaccessible or prohibitively complicated. We have made the tools developed for this study freely available to the community.

astro-ph.IM↗

arXiv2016

Cosmic Chandlery with Thermonuclear Supernovae

Thermonuclear (Type Ia) supernovae are bright stellar explosions, the light curves of which can be calibrated to allow for use as "standard candles" for measuring cosmological distances. Contemporary research investigates how the brightness of an event may be influenced by properties of the progenitor system that follow from properties of the host galaxy such as composition and age. The goals are to better understand systematic effects and to assess the intrinsic scatter in the brightness, thereby reducing uncertainties in cosmological studies. We present the results from ensembles of simulations in the single-degenerate paradigm addressing the influence of age and metallicity on the brightness of an event and compare our results to observed variations of brightness that correlate with properties of the host galaxy. We also present results from "hybrid" progenitor models that incorporate recent advances in stellar evolution.

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arXiv2016

Model Atmospheres for X-ray Bursting Neutron Stars

The hydrogen and helium accreted by X-ray bursting neutron stars is periodically consumed in runaway thermonuclear reactions that cause the entire surface to glow brightly in X-rays for a few seconds. With models of the emission, the mass and radius of the neutron star can be inferred from the observations. By simultaneously probing neutron star masses and radii, X-ray bursts are one of the strongest diagnostics of the nature of matter at extremely high densities. Accurate determinations of these parameters are difficult, however, due to the highly non-ideal nature of the atmospheres where X-ray bursts occur. Observations from X-ray telescopes such as RXTE and NuStar can potentially place strong constraints on nuclear matter once uncertainties in atmosphere models have been reduced. Here we discuss current progress on modeling atmospheres of X-ray bursting neutron stars and some of the challenges still to be overcome.

astro-ph.HE↗

arXiv2016

A Tracer Method for Computing Type Ia Supernova Yields: Burning Model Calibration, Reconstruction of Thickened Flames, and Verification for Planar Detonations

We refine our previously introduced parameterized model for explosive carbon-oxygen fusion during thermonuclear supernovae (SN Ia) by adding corrections to post-processing of recorded Lagrangian fluid element histories to obtain more accurate isotopic yields. Deflagration and detonation products are verified for propagation in a uniform density medium. A new method is introduced for reconstructing the temperature-density history within the artificially thick model deflagration front. We obtain better than 5\% consistency between the electron capture computed by the burning model and yields from post-processing. For detonations, we compare to a benchmark calculation of the structure of driven steady-state planar detonations performed with a large nuclear reaction network and error-controlled integration. We verify that, for steady-state planar detonations down to a density of 5x10^6 g/cc, our post processing matches the major abundances in the benchmark solution typically to better than 10% for times greater than 0.01 s after the shock front passage. As a test case to demonstrate the method, presented here with post-processing for the first time, we perform a two dimensional simulation of a SN Ia in the Chandrasekhar-mass deflagration-detonation transition (DDT) scenario. We find that reconstruction of deflagration tracks leads to slightly more complete silicon burning than without reconstruction. The resulting abundance structure of the ejecta is consistent with inferences from spectroscopic studies of observed SNe Ia. We confirm the absence of a central region of stable Fe-group material for the multi-dimensional DDT scenario. Detailed isotopic yields are tabulated and only change modestly when using deflagration reconstruction.

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arXiv2016

On Measuring the Metallicity of a Type Ia Supernova's Progenitor

In Type Ia Supernovae (\sneia), the relative abundances of chemical elements are affected by the neutron excess in the composition of the progenitor white dwarf. Since these products leave signatures in the spectra near maximum light, spectral features may be used to constrain the composition of the progenitor. We calculate the nucleosynthetic yields for three \snia simulations, assuming single degenerate, Chandrasekhar mass progenitors, for a wide range of progenitor metallicities, and calculate synthetic light curves and spectra to explore correlations between progenitor metallicity and the strength of spectral features. We use two 2D simulations of the deflagration-detonation-transition scenario with different $^{56}$Ni yields and the W7 simulation to control for differences between explosion models and total yields. While the overall yields of intermediate mass elements (16 $<$ A $\leq$ 40) differ between the three cases, trends in the yields are similar. With increasing metallicity, $^{28}$Si yields remain nearly constant, $^{40}$Ca yields decline, and Ti and $^{54}$Fe yields increase. In the synthetic spectra, we identify two features at 30 days post explosion that appear to deepen with progenitor metallicity: a Ti feature around 4200\,Å and a Fe feature around 5200\,Å\@. In all three simulations, their pseudo equivalent widths show a systematic trend with progenitor metallicity. This suggests that these two features may allow differentiation among progenitor metallicities of observed \sneia and potentially help reduce the intrinsic Hubble scatter.

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arXiv2016

Type Ia Supernova Explosions from Hybrid Carbon-Oxygen-Neon White Dwarf Progenitors

Motivated by recent results in stellar evolution that predict the existence of hybrid white dwarf (WD) stars with a C-O core inside an O-Ne shell, we simulate thermonuclear (Type Ia) supernovae from these hybrid progenitors. We use the FLASH code to perform multidimensional simulations in the deflagration to detonation transition (DDT) explosion paradigm. Our hybrid progenitor models were produced with the MESA stellar evolution code and include the effects of the Urca process, and we map the progenitor model to the FLASH grid. We performed a suite of DDT simulations over a range of ignition conditions consistent with the progenitor's thermal and convective structure assuming multiple ignition points. To compare the results from these hybrid WD stars to previous results from C-O white dwarfs, we construct a set of C-O WD models with similar properties and similarly simulate a suite of explosions. We find that despite significant variability within each suite, trends distinguishing the explosions are apparent in their $^{56}$Ni yields and the kinetic properties of the ejecta. We comment on the feasibility of these explosions as the source of some classes of observed subluminous events.

astro-ph.HE↗

arXiv2016

White Dwarf Mergers on Adaptive Meshes I. Methodology and Code Verification

The Type Ia supernova progenitor problem is one of the most perplexing and exciting problems in astrophysics, requiring detailed numerical modeling to complement observations of these explosions. One possible progenitor that has merited recent theoretical attention is the white dwarf merger scenario, which has the potential to naturally explain many of the observed characteristics of Type Ia supernovae. To date there have been relatively few self-consistent simulations of merging white dwarf systems using mesh-based hydrodynamics. This is the first paper in a series describing simulations of these systems using a hydrodynamics code with adaptive mesh refinement. In this paper we describe our numerical methodology and discuss our implementation in the compressible hydrodynamics code CASTRO, which solves the Euler equations, and the Poisson equation for self-gravity, and couples the gravitational and rotation forces to the hydrodynamics. Standard techniques for coupling gravitation and rotation forces to the hydrodynamics do not adequately conserve the total energy of the system for our problem, but recent advances in the literature allow progress and we discuss our implementation here. We present a set of test problems demonstrating the extent to which our software sufficiently models a system where large amounts of mass are advected on the computational domain over long timescales. Future papers in this series will describe our treatment of the initial conditions of these systems and will examine the early phases of the merger to determine its viability for triggering a thermonuclear detonation.

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