The Einstein Toolkit: A Community Computational Infrastructure for Relativistic Astrophysics

We describe the Einstein Toolkit, a community-driven, freely accessible computational infrastructure intended for use in numerical relativity, relativistic astrophysics, and other applications. The Toolkit, developed by a collaboration involving researchers from multiple institutions around the world, combines a core set of components needed to simulate astrophysical objects such as black holes, compact objects, and collapsing stars, as well as a full suite of analysis tools. The Einstein Toolkit is currently based on the Cactus Framework for high-performance computing and the Carpet adaptive mesh refinement driver. It implements spacetime evolution via the BSSN evolution system and general-relativistic hydrodynamics in a finite-volume discretization. The toolkit is under continuous development and contains many new code components that have been publicly released for the first time and are described in this article. We discuss the motivation behind the release of the toolkit, the philosophy underlying its development, and the goals of the project. A summary of the implemented numerical techniques is included, as are results of numerical test covering a variety of sample astrophysical problems.Comment: 62 pages, 20 figure

Modules for Experiments in Stellar Astrophysics (MESA)

Stellar physics and evolution calculations enable a broad range of research in astrophysics. Modules for Experiments in Stellar Astrophysics (MESA) is a suite of open source libraries for a wide range of applications in computational stellar astrophysics. A newly designed 1-D stellar evolution module, MESA star, combines many of the numerical and physics modules for simulations of a wide range of stellar evolution scenarios ranging from very-low mass to massive stars, including advanced evolutionary phases. MESA star solves the fully coupled structure and composition equations simultaneously. It uses adaptive mesh refinement and sophisticated timestep controls, and supports shared memory parallelism based on OpenMP. Independently usable modules provide equation of state, opacity, nuclear reaction rates, and atmosphere boundary conditions. Each module is constructed as a separate Fortran 95 library with its own public interface. Examples include comparisons to other codes and show evolutionary tracks of very low mass stars, brown dwarfs, and gas giant planets; the complete evolution of a 1 Msun star from the pre-main sequence to a cooling white dwarf; the Solar sound speed profile; the evolution of intermediate mass stars through the thermal pulses on the He-shell burning AGB phase; the interior structure of slowly pulsating B Stars and Beta Cepheids; evolutionary tracks of massive stars from the pre-main sequence to the onset of core collapse; stars undergoing Roche lobe overflow; and accretion onto a neutron star. Instructions for downloading and installing MESA can be found on the project web site (http://mesa.sourceforge.net/).Comment: 110 pages, 39 figures; submitted to ApJS; visit the MESA website at http://mesa.sourceforge.ne

Massive Computation for Understanding Core-Collapse Supernova Explosions

How do massive stars explode? Progress toward the answer is driven by increases in compute power. Petascale supercomputers are enabling detailed 3D simulations of core-collapse supernovae that are elucidating the role of fluid instabilities, turbulence, and magnetic field amplification in supernova engines

Numerical Methods for the Simulation of Dynamical Mass Transfer in Binaries

We describe computational tools that have been developed to simulate dynamical mass transfer in semi-detached, polytropic binaries that are initially executing synchronous rotation upon circular orbits. Initial equilibrium models are generated with a self-consistent field algorithm; models are then evolved in time with a parallel, explicit, Eulerian hydrodynamics code with no assumptions made about the symmetry of the system. Poisson's equation is solved along with the equations of ideal fluid mechanics to allow us to treat the nonlinear tidal distortion of the components in a fully self-consistent manner. We present results from several standard numerical experiments that have been conducted to assess the general viability and validity of our tools, and from benchmark simulations that follow the evolution of two detached systems through five full orbits (up to approximately 90 stellar dynamical times). These benchmark runs allow us to gauge the level of quantitative accuracy with which simulations of semi-detached systems can be performed using presently available computing resources. We find that we should be able to resolve mass transfer at levels $\dot{M} / M > few x 10^-5$ per orbit through approximately 20 orbits with each orbit taking about 30 hours of computing time on parallel computing platforms.Comment: 34 pages, 20 eps figures, submitted to ApJ

Simulazioni della Coalescenza di Stelle di Neutroni Binarie nell'Era dell'Astrofisica Multimessaggera

The recent ground-breaking detection of gravitational waves (GW) from the merger of two neutron stars (NS), known as the GW170817 event, along with the observations of electromagnetic counterparts across the entire spectrum including a short gamma-ray burst (SGRB) and a radioactively powered kilonova, has given birth to the era of multimessenger astrophysics with GW sources. In order to probe the underlying physical mechanisms at play in such systems, it is necessary to employ fully general relativistic magnetohydrodynamic (GRMHD) simulations, including effects of magnetic fields and neutrino emission/reabsorption for a more realistic description. In the first part of this Thesis, we introduce our newly-developed GRMHD code Spritz, that solves the GRMHD equations in 3D Cartesian coordinates and on a dynamical spacetime. We present its salient features including the staggered formulation of the vector potential as well as support for any arbitrary equation of state (EOS), followed by a series of tests for code validation. We then describe the implementation of an approximate neutrino leakage scheme in Spritz, shedding some light on the involved equations, physical assumptions, and implemented numerical methods including higher order schemes, along with a large battery of general relativistic tests performed with and without magnetic fields and/or neutrino leakage. Since flux-conserving GRMHD codes like Spritz depend upon a technical algorithm to recover the fundamental `primitive' variables from the evolved `conserved' ones, which is often error-prone, we propose a new robust, accurate and efficient conservative-to-primitive variable recovery scheme named `RePrimAnd', along with the proof of existence of a solution and its uniqueness. As a next natural step, we implemented this scheme in Spritz, and performed a number of demanding GRMHD tests including critical cases like a NS collapse to a black hole (BH) as well as the evolution of a BH-accretion disk system. The second part of the thesis focusses instead on the application of GRMHD codes to perform magnetized BNS merger simulations. In particular, using the WhiskyMHD code, we present a detailed study of BNS merger simulations forming a long-lived NS remnant and including long post-merger evolution. Exploring this `magnetar scenario' allows us to address some of the open questions in the context of the SGRB and accompanying kilonova of the GW170817 event. Finally, we also discuss the results of the first magnetized BNS merger simulation performed with Spritz and the RePrimAnd scheme, concluding with an outlook on the next steps.The recent ground-breaking detection of gravitational waves (GW) from the merger of two neutron stars (NS), known as the GW170817 event, along with the observations of electromagnetic counterparts across the entire spectrum including a short gamma-ray burst (SGRB) and a radioactively powered kilonova, has given birth to the era of multimessenger astrophysics with GW sources. In order to probe the underlying physical mechanisms at play in such systems, it is necessary to employ fully general relativistic magnetohydrodynamic (GRMHD) simulations, including effects of magnetic fields and neutrino emission/reabsorption for a more realistic description. In the first part of this Thesis, we introduce our newly-developed GRMHD code Spritz, that solves the GRMHD equations in 3D Cartesian coordinates and on a dynamical spacetime. We present its salient features including the staggered formulation of the vector potential as well as support for any arbitrary equation of state (EOS), followed by a series of tests for code validation. We then describe the implementation of an approximate neutrino leakage scheme in Spritz, shedding some light on the involved equations, physical assumptions, and implemented numerical methods including higher order schemes, along with a large battery of general relativistic tests performed with and without magnetic fields and/or neutrino leakage. Since flux-conserving GRMHD codes like Spritz depend upon a technical algorithm to recover the fundamental `primitive' variables from the evolved `conserved' ones, which is often error-prone, we propose a new robust, accurate and efficient conservative-to-primitive variable recovery scheme named `RePrimAnd', along with the proof of existence of a solution and its uniqueness. As a next natural step, we implemented this scheme in Spritz, and performed a number of demanding GRMHD tests including critical cases like a NS collapse to a black hole (BH) as well as the evolution of a BH-accretion disk system. The second part of the thesis focusses instead on the application of GRMHD codes to perform magnetized BNS merger simulations. In particular, using the WhiskyMHD code, we present a detailed study of BNS merger simulations forming a long-lived NS remnant and including long post-merger evolution. Exploring this `magnetar scenario' allows us to address some of the open questions in the context of the SGRB and accompanying kilonova of the GW170817 event. Finally, we also discuss the results of the first magnetized BNS merger simulation performed with Spritz and the RePrimAnd scheme, concluding with an outlook on the next steps

Numerical simulations of dynamical mass transfer in binaries

We present results from investigations of mass transfer instability in close binary star systems. By unstable mass transfer we mean the exchange of material where the response of the binary to the initial Roche lobe overflow causes the donor to loose even more material. Our work is guided by approximate arguments that dictate the stability boundaries for binary star systems. To proceed further one must explicitly treat extended mass and velocity distributions that are both nitially, and through their subsequent evolution in time, self-consistent. In this dissertation, we present the first three-dimensional, fully self-consistent treatment of mass transfer in close binary systems. To perform these calculations we have developed and tested a set of tools including a Self-Consistent Field code for generating polytropic binaries executing synchronous rotation upon circular orbits and a parallel, gravitational hydrodynamics code for evolving the binaries in time. We describe, in detail, these tools and their application to the evolution of binary star systems. We present extended simulations of two detached binaries that have been used to examine the accuracy of our computational techniques in addition to the simulations of interacting binaries

Massive Computation for Understanding Core-Collapse Supernova Explosions

How do massive stars explode? Progress toward the answer is driven by increases in compute power. Petascale supercomputers are enabling detailed 3D simulations of core-collapse supernovae that are elucidating the role of fluid instabilities, turbulence, and magnetic field amplification in supernova engines