arXiv subjects

Anthony Mezzacappa

Publications and source records attributed to Anthony Mezzacappa.

At least 73 records · Page 4Linked to original sources

arXiv2009

The Strange Prospects for Astrophysics

The implications of the formation of strange quark matter in neutron stars and in core-collapse supernovae is discussed with special emphasis on the possibility of having a strong first order QCD phase transition at high baryon densities. If strange quark matter is formed in core-collapse supernovae shortly after the bounce, it causes the launch of a second outgoing shock which is energetic enough to lead to a explosion. A signal for the formation of strange quark matter can be read off from the neutrino spectrum, as a second peak in antineutrinos is released when the second shock runs over the neutrinosphere.

astro-ph.HE↗

arXiv2009

Astrophysical Implications of the QCD phase transition

The possible role of a first order QCD phase transition at nonvanishing quark chemical potential and temperature for cold neutron stars and for supernovae is delineated. For cold neutron stars, we use the NJL model with nonvanishing color superconducting pairing gaps, which describes the phase transition to the 2SC and the CFL quark matter phases at high baryon densities. We demonstrate that these two phase transitions can both be present in the core of neutron stars and that they lead to the appearance of a third family of solution for compact stars. In particular, a core of CFL quark matter can be present in stable compact star configurations when slightly adjusting the vacuum pressure to the onset of the chiral phase transition from the hadronic model to the NJL model. We show that a strong first order phase transition can have strong impact on the dynamics of core collapse supernovae. If the QCD phase transition sets in shortly after the first bounce, a second outgoing shock wave can be generated which leads to an explosion. The presence of the QCD phase transition can be read off from the neutrino and antineutrino signal of the supernova.

astro-ph.SR↗

arXiv2006

Pulsar spins from an instability in the accretion shock of supernovae

Rotation-powered radio pulsars are born with inferred initial rotation periods of order 300 ms (some as short as 20 ms) in core-collapse supernovae. In the traditional picture, this fast rotation is the result of conservation of angular momentum during the collapse of a rotating stellar core. This leads to the inevitable conclusion that pulsar spin is directly correlated with the rotation of the progenitor star. So far, however, stellar theory has not been able to explain the distribution of pulsar spins, suggesting that the birth rotation is either too slow or too fast. Here we report a robust instability of the stalled accretion shock in core-collapse supernovae that is able to generate a strong rotational flow in the vicinity of the accreting proto-neutron star. Sufficient angular momentum is deposited on the proto-neutron star to generate a final spin period consistent with observations, even beginning with spherically symmetrical initial conditions. This provides a new mechanism for the generation of neutron star spin and weakens, if not breaks, the assumed correlation between the rotational periods of supernova progenitor cores and pulsar spin.

astro-ph↗

arXiv2005

Toward Five-dimensional Core-collapse Supernova Simulations

The computational difficulty of six-dimensional neutrino radiation hydrodynamics has spawned a variety of approximations, provoking a long history of uncertainty in the core-collapse supernova explosion mechanism. Under the auspices of the Terascale Supernova Initiative, we are honoring the physical complexity of supernovae by meeting the computational challenge head-on, undertaking the development of a new adaptive mesh refinement code for self-gravitating, six-dimensional neutrino radiation magnetohydrodynamics. This code--called {\em GenASiS,} for {\em Gen}eral {\em A}strophysical {\em Si}mulation {\em S}ystem--is designed for modularity and extensibility of the physics. Presently in use or under development are capabilities for Newtonian self-gravity, Newtonian and special relativistic magnetohydrodynamics (with `realistic' equation of state), and special relativistic energy- and angle-dependent neutrino transport--including full treatment of the energy and angle dependence of scattering and pair interactions.

astro-ph↗

arXiv2005

Conservative special relativistic radiative transfer for multidimensional astrophysical simulations: motivation and elaboration

Many astrophysical phenomena exhibit relativistic radiative flows. While velocities in excess of $v \sim 0.1c$ can occur in these systems, it has been common practice to approximate radiative transfer to $\cO(v/c)$. In the case of neutrino transport in core-collapse supernovae, this approximation gives rise to an inconsistency between the lepton number transfer and lab frame energy transfer, which have different $\cO(v/c)$ limits. A solution used in spherically symmetric $\cO(v/c)$ simulations has been to retain, for energy accounting purposes, the $\cO(v^2/c^2)$ terms in the lab frame energy transfer equation that arise from the $\cO(v/c)$ neutrino number transport equation. Avoiding the proliferation of such ``extra'' $\cO(v^2/c^2)$ terms in the absence of spherical symmetry motivates a special relativistic formalism, which we exhibit in coordinates sufficiently general to encompass Cartesian, spherical, and cylindrical coordinate systems.

astro-ph↗

arXiv2005

The Long Term: Six-dimensional Core-collapse Supernova Models

The computational difficulty of six-dimensional neutrino radiation hydrodynamics has spawned a variety of approximations, provoking a long history of uncertainty in the core-collapse supernova explosion mechanism. Under the auspices of the Terascale Supernova Initiative, we are honoring the physical complexity of supernovae by meeting the computational challenge head-on, undertaking the development of a new adaptive mesh refinement code for self-gravitating, six-dimensional neutrino radiation magnetohydrodynamics. This code--called {\em GenASiS}, for {\em Gen}eral {\em A}strophysical {\em Si}mulation {\em S}ystem--is designed for modularity and extensibility of the physics. Presently in use or under development are capabilities for Newtonian self-gravity, Newtonian and special relativistic magnetohydrodynamics (with `realistic' equation of state), and special relativistic energy- and angle-dependent neutrino transport--including full treatment of the energy and angle dependence of scattering and pair interactions.

astro-ph↗

arXiv2005

The Spherical Accretion Shock Instability in the Linear Regime

We use time-dependent, axisymmetric, hydrodynamic simulations to study the linear stability of the stalled, spherical accretion shock that arises in the post-bounce phase of core-collapse supernovae. We show that this accretion shock is stable to radial modes, with decay rates and oscillation frequencies in close agreement with the linear stability analysis of Houck and Chevalier. For non-spherical perturbations we find that the l=1 mode is always unstable for parameters appropriate to core-collapse supernovae. We also find that the l=2 mode is unstable, but typically has a growth rate smaller than that for l=1. Furthermore, the l=1 mode is the only mode found to transition into a nonlinear stage in our simulations. This result provides a possible explanation for the dominance of an l=1 'sloshing' mode seen in many two-dimensional simulations of core-collapse supernovae.

astro-ph↗

arXiv2004

APS Neutrino Study: Report of the Neutrino Astrophysics and Cosmology Working Group

In 2002, Ray Davis and Masatoshi Koshiba were awarded the Nobel Prize in Physics ``for pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos.'' However, while astronomy has undergone a revolution in understanding by synthesizing data taken at many wavelengths, the universe has only barely been glimpsed in neutrinos, just the Sun and the nearby SN 1987A. An entire universe awaits, and since neutrinos can probe astrophysical objects at densities, energies, and distances that are otherwise inaccessible, the results are expected to be particularly exciting. Similarly, the revolution in quantitative cosmology has heightened the need for very precise tests that depend on the effects of neutrinos, and prominent among them is the search for the effects of neutrino mass, since neutrinos are a small but known component of the dark matter. In this report, we highlight some of the key opportunties for progress in neutrino astrophysics and cosmology, and the implications for other areas of physics.

astro-ph↗

arXiv2004

The Core Collapse Supernova Mechanism: Current Models, Gaps, and the Road Ahead

The pursuit of the core collapse supernova explosion mechanism continues. While such efforts have been undertaken over the last four decades, it is only in the last decade that multidimensional models have been developed, and only in the last few years that significant components of supernova models, such as the neutrino transport, have been modeled with sufficient realism. We can now identify what are arguably the important components that must be included in any realistic three-dimensional supernova model. The challenge we now face is to include all of these in the models.

astro-ph↗

arXiv2002

Conservative formulations of general relativistic kinetic theory

Experience with core-collapse supernova simulations shows that accurate accounting of total particle number and 4-momentum can be a challenge for computational radiative transfer. This accurate accounting would be facilitated by the use of particle number and 4-momentum transport equations that allow transparent conversion between volume and surface integrals in both configuration and momentum space. Such conservative formulations of general relativistic kinetic theory in multiple spatial dimensions are presented in this paper, and their relevance to core-collapse supernova simulations is described.

astro-ph↗

arXiv2002

Stability of Standing Accretion Shocks, With an Eye Toward Core Collapse Supernovae

We examine the stability of standing, spherical accretion shocks. Accretion shocks arise in core collapse supernovae (the focus of this paper), star formation, and accreting white dwarfs and neutron stars. We present a simple analytic model and use time-dependent hydrodynamics simulations to show that this solution is stable to radial perturbations. In two dimensions we show that small perturbations to a spherical shock front can lead to rapid growth of turbulence behind the shock, driven by the injection of vorticity from the now non-spherical shock. We discuss the ramifications this instability may have for the supernova mechanism.

astro-ph↗

arXiv2001

NewtonPlus: Approximating Relativistic Effects in Supernova Simulations

We propose an approximation to full relativity that captures the main gravitational effects of dynamical importance in supernovae. The conceptual link between this formalism and the Newtonian limit is such that it could likely be implemented relatively easily in existing multidimensional Newtonian gravitational hydrodynamics codes employing a Poisson solver. As a test of the formalism's utility, we display results for rapidly rotating (and therefore highly deformed) neutron stars.

astro-ph↗

arXiv2001

NewtonPlus: Approximate Relativity for Supernova Simulations

We propose an approximation to general relativity that captures the main gravitational effects of dynamical importance in supernovae. The conceptual link between this formalism and the Newtonian limit is such that it could likely be implemented in existing multidimensional Newtonian gravitational hydrodynamics codes employing a Poisson solver. As a test of the formalism's utility, we display results for rapidly rotating (and therefore highly deformed) neutron stars.

astro-ph↗

arXiv2001

Probing the gravitational well: No supernova explosion in spherical symmetry with general relativistic Boltzmann neutrino transport

We report on the stellar core collapse, bounce, and postbounce evolution of a 13 solar mass star in a self-consistent general relativistic spherically symmetric simulation based on Boltzmann neutrino transport. We conclude that approximations to exact neutrino transport and omission of general relativistic effects were not alone responsible for the failure of numerous preceding attempts to model supernova explosions in spherical symmetry. Compared to simulations in Newtonian gravity, the general relativistic simulation results in a smaller shock radius. We however argue that the higher neutrino luminosities and rms energies in the general relativistic case could lead to a larger supernova explosion energy.

astro-ph↗

arXiv2001

Conservative General Relativistic Radiation Hydrodynamics in Spherical Symmetry and Comoving Coordinates

The description of general relativistic radiation hydrodynamics in spherical symmetry is presented in natural coordinate choices. For hydrodynamics, comoving coordinates are chosen, and the momentum phase space for the radiation particles is described in comoving frame four momenta. We also investigate a description of the momentum phase space in terms of particle impact parameter and energy at infinity and derive a simple approximation to the general relativistic Boltzmann equation. Further developed are, however, the exact equations in comoving coordinates, because the description of the interaction between matter and radiation particles is best described in the closely related orthonormal basis comoving with the fluid elements. We achieve a conservative and concise formulation of radiation hydrodynamics that is well suited for numerical implementation by a variety of methods. The contribution of radiation to the general relativistic jump conditions at shock fronts is discussed, and artificial viscosity is consistently included in the derivations in order to support approaches relying on this option.

astro-ph↗

arXiv2001

General Relativistic Simulations of Stellar Core Collapse and Postbounce Evolution with Boltzmann Neutrino Transport

We present self-consistent general relativistic simulations of stellar core collapse, bounce, and postbounce evolution for 13, 15, and 20 solar mass progenitors in spherical symmetry. Our simulations implement three-flavor Boltzmann neutrino transport and standard nuclear physics. The results are compared to our corresponding simulations with Newtonian hydrodynamics and O(v/c) Boltzmann transport.

astro-ph↗

arXiv2001

Nuclear Reaction Rate Uncertainties and their effects on Nova Nucleosynthesis Modeling

The nucleosynthesis and other observable consequences of a nova outburst depend sensitively on the details of the thermonuclear runaway which initiates the outburst. One important source of uncertainty in our current models is the nuclear reaction data used as input for the evolutionary calculations. We present preliminary results of the first analyses of the impact on nova nucleosynthesis of all reaction rate uncertainties considered simultaneously.

astro-ph↗

arXiv2000

Simulation of the Spherically Symmetric Stellar Core Collapse, Bounce, and Postbounce Evolution of a 13 Solar Mass Star with Boltzmann Neutrino Transport, and Its Implications for the Supernova Mechanism

With exact three-flavor Boltzmann neutrino transport, we simulate the stellar core collapse, bounce, and postbounce evolution of a 13 solar mass star in spherical symmetry, the Newtonian limit, without invoking convection. In the absence of convection, prior spherically symmetric models, which implemented approximations to Boltzmann transport, failed to produce explosions. We are motivated to consider exact transport to determine if these failures were due to the transport approximations made and to answer remaining fundamental questions in supernova theory. The model presented here is the first in a sequence of models beginning with different progenitors. In this model, a supernova explosion is not obtained. We discuss the ramifications of our results for the supernova mechanism.

astro-ph↗