About

Alan C. Calder is a professor in the Physics and Astronomy Department at Stony Brook University and serves as Co-Director of the Institute for Advanced Computational Science (IACS). His research focuses on nuclear astrophysics, specifically simulating explosive astrophysical phenomena such as bright stellar explosions known as Type Ia supernovae. These supernovae are significant for their role in producing and disseminating heavy elements, which are important for galactic chemical evolution. Additionally, the light curves of Type Ia supernovae can be standardized and used as distance indicators in cosmology to investigate the expansion history of the Universe. Calder performs detailed simulations to explore how factors like the age or composition of the progenitor influence the brightness of these events, addressing systematic effects that contribute to uncertainties in cosmological studies. Prior to his current position, he held research appointments at the National Center for Supercomputing Applications and the University of Chicago, where he was involved with the Center for Astrophysical Thermonuclear Flashes. He also has experience as an instructor at the School of the Art Institute in Chicago.

Research topics

• Computer Science
• Programming language
• Operating system
• Art
• Software engineering
• Embedded system
• Visual arts
• Physics
• Parallel computing

Selected publications

• On the Importance of the Convective Urca Process in 3D Simulations of a Simmering White Dwarf

  ArXiv.org · 2026-02-24

  articleOpen accessSenior author

  Type Ia supernovae are bright thermonuclear explosions that are important to numerous areas of astronomy. However, the origins of these events are poorly understood. One proposed setting is that of a near Chandrasekhar mass white dwarf that undergoes runaway carbon burning in the core. During the thousand years leading up to the explosion, the white dwarf undergoes a simmering phase where slow carbon burning heats the core and drives convection. A poorly understood aspect of this phase is the convective Urca process, which links convection with weak nuclear reactions. We use the low Mach number code MAESTROeX to perform full 3D simulations as is required to accurately capture the turbulent convection. We present simulations with and without the A=23 convective Urca process, which have relaxed to a steady state. We characterize the effects of the convective Urca process on the neutrino losses, the nuclear energy generation, and the convective boundary. We find that the size of the convection zone is substantially reduced by the convective Urca process, though convection still extends past the Urca shell. Our findings on the structure of the convective zone and the compositional changes can be used to inform 1D stellar models that track the longer-timescale evolution.

  PublisherOA PDF

• Modeling X-ray Bursting Neutron Star Atmospheres

  arXiv (Cornell University) · 2026-02-03

  articleOpen accessSenior author

  We present a verification of a computational model, developed at the Los Alamos National Laboratory (LANL) for simulating radiation transfer in X-ray bursting neutron star atmospheres. We tested a baseline case and demonstrated strong agreement in the behavior of the outgoing spectrum's color-correction factor with earlier work and theoretical expectations. By analyzing the relationship between the simulation time and outgoing flux, we also demonstrated how the model calculates through a sequence of time-independent atmospheric snapshots, each iteratively refined, and uses them to progressively converge toward the correct atmospheric state (as would be observed during a burst). We examined the behavior of the outgoing flux across different optical depths and explored the physical explanations for deviations from a pure blackbody spectrum, attributed to frequency-dependent opacity sources. Additionally, we assessed the impact of Compton scattering, highlighting its role in redistributing photon energies.

  PublisherOA PDF

• On the Importance of the Convective Urca Process in 3D Simulations of a Simmering White Dwarf

  Open MIND · 2026-02-24

  preprintSenior author

  Type Ia supernovae are bright thermonuclear explosions that are important to numerous areas of astronomy. However, the origins of these events are poorly understood. One proposed setting is that of a near Chandrasekhar mass white dwarf that undergoes runaway carbon burning in the core. During the thousand years leading up to the explosion, the white dwarf undergoes a simmering phase where slow carbon burning heats the core and drives convection. A poorly understood aspect of this phase is the convective Urca process, which links convection with weak nuclear reactions. We use the low Mach number code MAESTROeX to perform full 3D simulations as is required to accurately capture the turbulent convection. We present simulations with and without the A=23 convective Urca process, which have relaxed to a steady state. We characterize the effects of the convective Urca process on the neutrino losses, the nuclear energy generation, and the convective boundary. We find that the size of the convection zone is substantially reduced by the convective Urca process, though convection still extends past the Urca shell. Our findings on the structure of the convective zone and the compositional changes can be used to inform 1D stellar models that track the longer-timescale evolution.

  DOI

• Modeling X-ray Bursting Neutron Star Atmospheres

  Open MIND · 2026-02-03

  preprintSenior author

  We present a verification of a computational model, developed at the Los Alamos National Laboratory (LANL) for simulating radiation transfer in X-ray bursting neutron star atmospheres. We tested a baseline case and demonstrated strong agreement in the behavior of the outgoing spectrum's color-correction factor with earlier work and theoretical expectations. By analyzing the relationship between the simulation time and outgoing flux, we also demonstrated how the model calculates through a sequence of time-independent atmospheric snapshots, each iteratively refined, and uses them to progressively converge toward the correct atmospheric state (as would be observed during a burst). We examined the behavior of the outgoing flux across different optical depths and explored the physical explanations for deviations from a pure blackbody spectrum, attributed to frequency-dependent opacity sources. Additionally, we assessed the impact of Compton scattering, highlighting its role in redistributing photon energies.

  DOI

• Approximating Convective Urca Cooling in a Simmering White Dwarf

  Journal of Physics Conference Series · 2025-04-01 · 1 citations

  articleOpen access

  Abstract Type Ia supernovae are bright thermonuclear explosions that play important roles in many areas of astronomy such as cosmology and galaxy evolution. The near Chandrasekhar mass white dwarf is a potential progenitor for these supernovae. This model entails a white dwarf accreting material from a companion and gaining mass to the point of igniting carbon fusion in the core. The onset of carbon fusion, called the simmering phase, drives convection and alters the evolution of the white dwarf as it approaches the thermonuclear explosion. A key factor during this phase is the convective Urca process which links convection with weak nuclear reactions that leak energy from the star. To study the effects of the convective Urca process, it is vital to accurately model the turbulent convection in the core. This necessitates 3D hydrodynamic simulations which are computationally expensive. As a point of comparison and to aid in exploring initial conditions, we use the “quick mixing” approximation, which assumes convective mixing is efficient enough to produce a uniform composition in the convection zone. Utilizing this approximation, we can predict the ratio of the A = 23 Urca pair as well as the resulting neutrino loss rates without running full 3D simulations. We compare the results of a 3D hydrodynamic simulation, run using the low Mach number hydrodynamic code MAESTROeX, to the quick mixing calculation. Additionally, we investigate how varying the size of the convection zone influences the convective Urca process and sets approximate bounds on reasonable initial conditions.

  PublisherDOI

• 3D Convective Urca Process in a Simmering White Dwarf

  The Astrophysical Journal · 2025-01-28 · 1 citations

  articleOpen access

  Abstract 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. In the early stages of carbon burning, called the simmering phase, energy released by the reactions in the core drive 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, which may alter the composition and structure of the white dwarf. The convective Urca process is not well understood and requires 3D fluid simulations to properly model the turbulent convection, an inherently 3D process. Because the neutron excess of the fluid both sets and is set by the extent of the convection zone, the realistic steady state can only be determined in simulations with real 3D mixing processes. Additionally, the convection is relatively slow (Mach number less than 0.005) and thus a low Mach number method is needed to model the flow over many convective turnovers. Using the MAESTROeX low Mach number hydrodynamic software, we present the first full-star 3D simulations of the A = 23 convective Urca process, spanning hundreds of convective turnover times. Our findings on the extent of mixing across the Urca shell, the characteristic velocities of the flow, the energy-loss rates due to neutrino emission, and the structure of the convective boundary can be used to inform 1D stellar models that track the longer-timescale evolution.

  PublisherDOI

• What Time Taught Us: Monitoring a Computing Technology Testbed Across Multiple Years

  Lecture notes in computer science · 2025-11-23

  book-chapter
  PublisherDOI

• 3D Convective Urca Process in a Simmering White Dwarf

  arXiv (Cornell University) · 2024-12-10

  preprintOpen access

  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. In the early stages of carbon burning, called the simmering phase, energy released by the reactions in the core drive 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, which may alter the composition and structure of the white dwarf. The convective Urca process is not well understood and requires 3D fluid simulations to properly model the turbulent convection, an inherently 3D process. Because the neutron excess of the fluid both sets and is set by the extent of the convection zone, the realistic steady state can only be determined in simulations with real 3D mixing processes. Additionally, the convection is relatively slow (Mach number less than 0.005) and thus a low Mach number method is needed to model the flow over many convective turnovers. Using the MAESTROeX low Mach number hydrodynamic software, we present the first full star 3D simulations of the A=23 convective Urca process, spanning hundreds of convective turnover times. Our findings on the extent of mixing across the Urca shell, the characteristic velocities of the flow, the energy loss rates due to neutrino emission, and the structure of the convective boundary can be used to inform 1D stellar models that track the longer-timescale evolution.

  PublisherOA PDFDOI

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

  Journal of Physics Conference Series · 2024-04-01

  articleOpen access

  Abstract 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.

  PublisherOA PDFDOI

• Benchmarking with Supernovae: A Performance Study of the FLASH Code

  2024-07-17

  preprintOpen access

  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.

  PublisherDOI

Recent grants

• White Dwarf Mergers as Progenitors of Type Ia Supernovae

  NSF · $444k · 2012–2017

Frequent coauthors

• Dean M. Townsley

  University of Alabama

  172 shared

• F. X. Timmes

  166 shared

• Edward F. Brown

  Rensselaer Polytechnic Institute

  158 shared

• Aaron P. Jackson

  125 shared

• David A. Chamulak

  Lockheed Martin (United States)

  71 shared

• Brendan K. Krueger

  Astrophysique, Instrumentation et Modélisation

  69 shared

• M. Zingale

  Stony Brook University

  60 shared

• P. M. Ricker

  56 shared

Education

• Ph.D., Computer Science

  University of California, Los Angeles

  1990

• M.S., Computer Science

  University of California, Los Angeles

  1986

• B.S., Computer Science

  University of California, Los Angeles

  1984