Anthony Mezzacappa - Academia.edu (original) (raw)

Papers by Anthony Mezzacappa

Proceedings of the International Astronomical Union, Jun 1, 2020

Motivated by their role as the direct or indirect source of many of the elements in the Universe,... more Motivated by their role as the direct or indirect source of many of the elements in the Universe, numerical modeling of core collapse supernovae began more than five decades ago. Progress toward ascertaining the explosion mechanism(s) has been realized through increasingly sophisticated models, as physics and dimensionality have been added, as physics and numerical modeling have improved, and as the leading computational resources available to modelers have become far more capable. The past five to ten years have witnessed the emergence of a consensus across the core collapse supernova modeling community that had not existed in the four decades prior. For the majority of progenitors-i.e., slowly rotating progenitors-the efficacy of the delayed shock mechanism, where the stalled supernova shock wave is revived by neutrino heating by neutrinos emanating from the proto-neutron star, has been demonstrated by all core collapse supernova modeling groups, across progenitor mass and metallicity. With this momentum, and now with a far deeper understanding of the dynamics of these events, the path forward is clear. While much progress has been made, much work remains to be done, but at this time we have every reason to be optimistic we are on track to answer one of the most important outstanding questions in astrophysics: How do massive stars end their lives?

Proceedings of 10th Symposium on Nuclei in the Cosmos — PoS(NIC X), Jul 29, 2009

Nuclear electron capture and the nuclear equation of state play important roles during the collap... more Nuclear electron capture and the nuclear equation of state play important roles during the collapse of a massive star and the subsequent supernova. The nuclear equation of state controls the nature of the bounce which initially forms the supernova shock while electron capture determines the location where the shock forms. Advances in nuclear structure theory have allowed a more realistic treatment of electron capture on heavy nuclei to be developed. We will review how this improvement has led to a change in our understanding of stellar core collapse, with electron capture on nuclei with masses larger than 50 found to dominate electron capture on free protons, resulting is significant changes in the hydrodynamics of core collapse and bounce. We will also demonstrate the impact of a variety of nuclear equations of state on supernova shock propagation. Of particular note is the interplay between the nuclear composition determined by the equation of state and nuclear electron capture.

Nucleation and Atmospheric Aerosols, 2007

The mechanism for core collapse supernova explosions remains undefined in detail and perhaps even... more The mechanism for core collapse supernova explosions remains undefined in detail and perhaps even in broad brush. Past multidimensional simulations point to the important role neutrino transport, fluid instabilities, rotation, and magnetic fields play, or may play, in generating core collapse supernova explosions, but the fundamental question as to whether or not these events are powered by neutrinos with the aid of some or all of these other phenomena or by magnetic fields or by a combination of both is unanswered. Here we present the results from two sets of simulations, in two and three spatial dimensions. In two dimensions, the simulations include multifrequency fluxlimited diffusion neutrino transport in the "ray-by-ray-plus" approximation, two-dimensional self gravity in the Newtonian limit, and nuclear burning through a 14-isotope alpha network. The threedimensional simulations are model simulations constructed to reflect the post stellar core bounce conditions during neutrino shock reheating at the onset of explosion. They are hydrodynamics-only models that focus on critical aspects of the shock stability and dynamics and their impact on the supernova mechanism and explosion. The two-dimensional simulations demonstrate the important role nuclear burning may play despite the relatively small total energy deposition behind the shock. The three-dimensional simulations demonstrate the need for three-dimensional multi-physics core collapse supernova models. In two dimensions, with the inclusion of nuclear burning, we obtain explosions (although in one case weak) for two progenitors (11 and 15 M ⊙ models). Moreover, in both cases the explosion is initiated when the inner edge of the oxygen layer accretes through the shock. Thus, the shock is not revived while in the iron core, as previously discussed in the literature. The three-dimensional studies of the development of the stationary accretion shock instability (SASI) demonstrate the fundamentally new dynamics allowed when simulations are performed in three spatial dimensions. The predominant l = 1 SASI mode gives way to a stable m = 1 mode, which in turn has significant ramifications for the distribution of angular momentum in the region between the shock and proto-neutron star and, ultimately, for the spin of the remnant neutron star. Moreover, the three-dimensional simulations make clear, given the increased number of degrees of freedom, that two-dimensional models are severely limited by artificially imposed symmetries.

Proceedings of The 32nd International Symposium on Lattice Field Theory — PoS(LATTICE2014), May 21, 2015

We have been working within the fundamental paradigm that core collapse supernovae (CCSNe) may be... more We have been working within the fundamental paradigm that core collapse supernovae (CCSNe) may be neutrino driven, since the first suggestion of this by Colgate and White nearly five decades ago. Computational models have become increasingly sophisticated, first in one spatial dimension assuming spherical symmetry, then in two spatial dimensions assuming axisymmetry, and now in three spatial dimensions with no imposed symmetries. The increase in the number of spatial dimensions has been accompanied by an increase in the physics included in the models, and an increase in the sophistication with which this physics has been modeled. Computation has played an essential role in the development of CCSN theory, not simply for the obvious reason that such multidimensional, multi-physics, nonlinear events cannot possibly be fully captured analytically, but for its role in discovery. In particular, the discovery of the standing accretion shock instability (SASI) through computation about a decade ago has impacted all simulations performed since then. Today, we appear to be at a threshold, where neutrinos, neutrino-driven convection, and the SASI, working together over time scales significantly longer than had been anticipated in the past, are able to generate explosions, and in some cases, robust explosions, in a number of axisymmetric models. But how will this play out in three dimensions? Early results from the first three-dimensional (3D), multi-physics simulation of the "Oak Ridge" group are promising. I will discuss the essential components of today's models and the requirements of realistic CCSN modeling, present results from our one-, two-, and three-dimensional models, place our models in context with respect to other efforts around the world, and discuss short-and long-term next steps.

Carolina Digital Repository (University of North Carolina at Chapel Hill), 1995

The instabilities that are believed to play an important role in the supernova mechanism are revi... more The instabilities that are believed to play an important role in the supernova mechanism are reviewed. We then investigate the dynamics of two of these instabilities, prompt convection and neutron ngers, and its consequences for the supernova outcome. We nd that prompt convection occurs immediately after shock propagation and is primarily entropy driven. A number of detailed one-dimensional spherically symmetric simulations of prompt convection are performed using a mixing length algorithm in a code coupling the core hydrodynamics with multigroup ux-limited di usion of neutrinos of all types. We nd that prompt convection does not have a signi cant e ect on the neutrino luminosities or spectra and, furthermore, that the core is not amenable to shock revival at this time. Consequently, we do not nd that prompt convection is important for the supernova mechanism. Our analysis of neutron ngers begins with a simple model of doubly di usive instabilities, applicable to salt and water, and is then extended to describe matter and neutrinos in a postshock core. The equations describing the stability of the latter are nonlinear, requiring a numerical approach. However, on general grounds, we nd that, in the usual thermodynamic setting that gets established beneath the neutrinospheres in the postshock core, there is a critical value of Y`, depending on and s, below which matter is stable to doubly di usive instabilities. Above this critical value, numerous numerical experiments show that the postshock core is most likely stable or, at worst, unstable to semiconvection rather than neutron ngers. We therefore conclude that the core is unlikely at any time to be unstable to neutron ngers and, consequently, that the latter play no role in the supernova mechanism.

Springer eBooks, 1997

Modeling the atmospheres of SNe Ia requires the solution of the NLTE radiative transfer equation.... more Modeling the atmospheres of SNe Ia requires the solution of the NLTE radiative transfer equation. We discuss the formulation of the radiative transfer equation in the co-moving frame. For characteristic velocities larger than ∼ 2000 km s −1 , the effects of advection on the synthetic spectra are non-negligible, and hence should be included in model calculations. We show that the time-independent or quasi-static approximation is adequate for SNe Ia near maximum light, as well as for most other astrophysical problems; e.g., hot stars, novae, and other types of supernovae. We examine the use of the Sobolev approximation in modeling moving atmospheres and find that the number of overlapping lines in the co-moving frame make the approximation suspect in models that predict both lines and continua. We briefly discuss the form of the Rosseland mean opacity in the co-moving frame, and present a formula that is easy to implement in radiation hydrodynamics calculations.

arXiv (Cornell University), Dec 30, 2010

Core-collapse supernova models depend on the details of the nuclear and weak interaction physics ... more Core-collapse supernova models depend on the details of the nuclear and weak interaction physics inputs just as they depend on the details of the macroscopic physics (transport, hydrodynamics, etc.), numerical methods, and progenitors. We present preliminary results from our ongoing comparison studies of nuclear and weak interaction physics inputs to core collapse supernova models using the spherically-symmetric, general relativistic, neutrino radiation hydrodynamics code Agile-Boltztran. We focus on comparisons of the effects of the nuclear EoS and the effects of improving the opacities, particularly neutrino-nucleon interactions. * Speaker.

arXiv (Cornell University), Aug 22, 2022

We present gravitational wave emission predictions based on three core collapse supernova simulat... more We present gravitational wave emission predictions based on three core collapse supernova simulations corresponding to three different progenitor masses. The masses span a large range, between 9.6 and 25 M , are all initially non-rotating, and are of two metallicities: zero and Solar. We compute both the temporal evolution of the gravitational wave strains for both the plus and the cross polarizations, as well as their spectral decomposition and characteristic strains. The temporal evolution of our zero metallicity, 9.6 M progenitor model is distinct from the temporal evolution of our Solar metallicity, 15 M progenitor model and our zero metallicity, 25 M progenitor model. In the former case, the high-frequency gravitational wave emission is largely confined to a brief time period ∼75 ms after bounce, whereas in the latter two cases, high-frequency emission does not commence until ∼125 ms after bounce or later. Nonetheless, the physical origin of the high-frequency emission in all three cases corresponds to convection in the proto-neutron star of both Schwarzschild and Ledoux type, and convective overshoot. The low-frequency emission in all three models exhibits very similar behavior. At frequencies below ∼250 Hz, gravitational waves are emitted by neutrino-driven convection and the SASI. This emission extends throughout the simulations when a gain region is present. In all three models, explosion is observed, at ∼125, ∼500, and ∼250 ms after bounce in the 9.6, 15, and 25 M progenitor models, respectively. At these times, the low-frequency gravitational wave emission is joined by very low frequency emission, below ∼10 Hz. These very low frequency episodes are the result of explosion and begin at the above designated explosion times in each of our models. Our characteristic strains tell us that in principle all three gravitational wave signals would be detectable by current-generation detectors for a supernova at a distance of 10 kpc. However, our 9.6 M progenitor model is a significantly weaker source of gravitational waves, with strain amplitudes approximately 5-10 times less than in our other two models. The characteristic strain for this model tells us that such a supernova would be detectable only within a much more narrow frequency range around the maximum sensitivity of today's detectors. Finally, in our 9.6 M progenitor model, we see very high frequency gravitational radiation, extending up to ∼ 2000 Hz. This feature results from the interaction of shock-and deleptonization-induced convection with perturbations introduced in the progenitor by nuclear burning during core collapse. While unique to the 9.6 M progenitor model analyzed here, this very high frequency emission may in fact be a generic feature of the predictions for the gravitational wave emission from all core collapse supernova models when simulations are performed with three-dimensional progenitors.

Bulletin of the American Physical Society, Apr 24, 2006

Bulletin of the American Physical Society, Nov 20, 2015

Core collapse supernovae are characterized by muti-dimensional dynamics. Studies have shown that ... more Core collapse supernovae are characterized by muti-dimensional dynamics. Studies have shown that the shock formed at core bounce always stalls. Until the development of axisymmetric (2D) simulations, little progress towards reviving the shock had been made. Modern simulations have given rise to the idea that both neutrino driven convection and the standing accretion shock instability (SASI) play pivotal roles in reviving the stalled shock. These mechanisms can increase the time material spends in the gain layer. The gain layer, the region near the stalled shock where net neutrino heating occurs, is dominated by turbulent flow. The turbulence in this region is necessary for maximizing the efficiency of the neutrino heating mechanism. Much of modern supernova theory is concerned with which of these two mechanisms plays a larger role in the revival of the stalled shock. We attempt to employ a singular value decomposition (SVD) in order to explore the relative contributions of the neutrino driven convection and SASI mechanisms. 1 Brandon will be conducting an oral presentation on the same abstract.

Bulletin of the American Physical Society, Apr 14, 2015

Knoxville-We are developing new computational methods for simulation of neutrino transport in cor... more Knoxville-We are developing new computational methods for simulation of neutrino transport in core-collapse supernovae, which is challenging since neutrinos evolve from being diffusive in the proto-neutron star to nearly free streaming in the critical neutrino heating region. To this end, we consider conservative formulations of the Boltzmann equation, 2 and aim to develop robust, high-order accurate methods. Runge-Kutta discontinuous Galerkin (DG) methods, 3 offer several attractive properties, including (i) high-order accuracy on a compact stencil and (ii) correct asymptotic behavior in the diffusion limit. We have recently developed a new DG method for the advection part for the transport solve, 4 which is high-order accurate and strictly preserves the physical bounds of the distribution function; i.e., f ∈ [0, 1]. We summarize the main ingredients of our bound-preserving DG method and discuss ongoing work to include neutrino-matter interactions in the scheme.

We give a brief overview of the status of core collapse supernova modeling, particularly as it pe... more We give a brief overview of the status of core collapse supernova modeling, particularly as it pertains to predictions of neutrino signatures for the next galactic or near extragalactic supernova. We also consider the implications of neutrino mass for both the supernova mechanism and neutrino signature predictions. DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that + its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. DISC LA1 M ER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.

Physical review, Feb 9, 2023

arXiv (Cornell University), Jul 20, 2023

We present numerical results from a parameter study of the standing accretion shock instability (... more We present numerical results from a parameter study of the standing accretion shock instability (SASI), investigating the impact of general relativity (GR) on the dynamics. Using GR hydrodynamics and gravity, and non-relativistic (NR) hydrodynamics and gravity, in an idealized model setting, we vary the initial radius of the shock and, by varying its mass and radius in concert, the proto-neutron star (PNS) compactness. We investigate two regimes expected in a post-bounce core-collapse supernova (CCSN): one meant to resemble a relatively low-compactness configuration and one meant to resemble a relatively high-compactness configuration. We find that GR leads to a longer SASI oscillation period, with ratios between the GR and NR cases as large as 1.29 for the high-compactness suite. We also find that GR leads to a slower SASI growth rate, with ratios between the GR and NR cases as low as 0.47 for the high-compactness suite. We discuss implications of our results for CCSN simulations.

Springer eBooks, 2005

Supernova simulations to date have assumed that during core collapse electron captures (EC) occur... more Supernova simulations to date have assumed that during core collapse electron captures (EC) occur dominantly on free protons, while captures on heavy nuclei are Pauli-blocked and are ignored. Using microscopic calculations we show that the EC rates on heavy nuclei are large enough that, in contrast to previous assumptions, electron capture on nuclei dominates over capture on free protons. This leads to significant changes in core collapse simulations.

Bulletin of the American Physical Society, 2014

Core-collapse supernovae are driven by a multidimensional neutrino radiation hydrodynamic (RHD) e... more Core-collapse supernovae are driven by a multidimensional neutrino radiation hydrodynamic (RHD) engine, and full simulation ultimately requires symmetry-free three-dimensional (3D) RHD simulation. We present ongoing 3D simulation with our multidimensional RHD supernova code CHIMERA that includes all of the most important physical components. The 3D simulation will be compared to completed axisymmetric (2D) simulations that have shown robust explosions in agreement with observational measurements. The impact of symmetry (dimension) and its consequences for our understanding of the explosion mechanism will be discussed in the context of current simulations.

Journal of Computational Physics, 2019

Building on the framework of Zhang & Shu [1, 2], we develop a realizability-preserving method to ... more Building on the framework of Zhang & Shu [1, 2], we develop a realizability-preserving method to simulate the transport of particles (fermions) through a background material using a two-moment model that evolves the angular moments of a phase space distribution function f. The two-moment model is closed using algebraic moment closures; e.g., as proposed by Cernohorsky & Bludman [3] and Banach & Larecki [4]. Variations of this model have recently been used to simulate neutrino transport in nuclear astrophysics applications, including core-collapse supernovae and compact binary mergers. We employ the discontinuous Galerkin (DG) method for spatial discretization (in part to capture the asymptotic diffusion limit of the model) combined with implicit-explicit (IMEX) time integration to stably bypass short timescales induced by frequent interactions between particles and the background. Appropriate care is taken to ensure the method preserves strict algebraic bounds on the evolved moments (particle density and flux) as dictated by Pauli's exclusion principle, which demands a bounded distribution function (i.e., f ∈ [0, 1]). This realizability-preserving scheme combines a suitable CFL condition, a realizability-enforcing limiter, a closure procedure based on Fermi-Dirac statistics, and an IMEX scheme whose stages can be written as a convex combination of forward Euler steps combined with a backward Euler step. The IMEX scheme is formally only first-order accurate, but works well in the diffusion limit, and-without interactions with the background-reduces to the optimal second-order strong stability-preserving explicit Runge-Kutta scheme of Shu & Osher [5]. Numerical results demonstrate the realizability-preserving properties of the scheme. We also demonstrate that the use of algebraic moment closures not based on Fermi-Dirac statistics can lead to unphysical moments in the context of fermion transport.

The Evolution of Galaxies, 2002

Thermonuclear Supernovae, 1997

Journal of Physics: Conference Series, 2006

First results from a fully self-consistent, temperature-dependent equation of state that spans th... more First results from a fully self-consistent, temperature-dependent equation of state that spans the whole density range of neutron stars and supernova cores are presented. The equation of state (EoS) is calculated using a mean-field Hartree-Fock method in three dimensions (3D). The nuclear interaction is represented by the phenomenological Skyrme model in this work, but the EoS can be obtained in our framework for any suitable form of the nucleon-nucleon effective interaction. The scheme we employ naturally allows effects such as (i) neutron drip, which results in an external neutron gas, (ii) the variety of exotic nuclear shapes expected for extremely neutron heavy nuclei, and (iii) the subsequent dissolution of these nuclei into nuclear matter. In this way, the equation of state is calculated across phase transitions without recourse to interpolation techniques between density regimes described by different physical models. EoS tables are calculated in the wide range of densities, temperature and proton/neutron ratios on the ORNL NCCS XT3, using up to 2000 processors simultaneously.