arXiv subjects

J. Novak

Publications and source records attributed to J. Novak.

26 records · Page 2Linked to original sources

arXiv2012

An extended equation of state for core-collapse simulations

In stellar core-collapse events matter is heated and compressed to densities above nuclear matter saturation density. For progenitors stars with masses above about 25 solar masses, which eventually form a black hole, the temperatures and densities reached during the collapse are so high that a traditional description in terms of electrons, nuclei, and nucleons is no longer adequate. We present here an improved equation of state which contains in addition pions and hyperons. They become abundant in the high temperature and density regime. We study the different constraints on such an equation of state, coming from both hyperonic data and observations of neutron star properties. In order to test the zero-temperature versions, we perform numerical simulations of the collapse of a neutron star with such additional particles to a black hole. We discuss the influence of the additional particles on the thermodynamic properties within the hot versions of the equation of state and we show that in regimes relevant to core-collapse and black hole formation, the effects of pions and hyperons on pressure, internal energy and sound speed are not negligible.

nucl-th↗

arXiv2008

Mathematical Issues in a Fully-Constrained Formulation of Einstein Equations

Bonazzola, Gourgoulhon, Grandclément, and Novak [Phys. Rev. D {\bf 70}, 104007 (2004)] proposed a new formulation for 3+1 numerical relativity. Einstein equations result, according to that formalism, in a coupled elliptic-hyperbolic system. We have carried out a preliminary analysis of the mathematical structure of that system, in particular focusing on the equations governing the evolution for the deviation of a conformal metric from a flat fiducial one. The choice of a Dirac's gauge for the spatial coordinates guarantees the mathematical characterization of that system as a (strongly) hyperbolic system of conservation laws. In the presence of boundaries, this characterization also depends on the boundary conditions for the shift vector in the elliptic subsystem. This interplay between the hyperbolic and elliptic parts of the complete evolution system is used to assess the prescription of inner boundary conditions for the hyperbolic part when using an excision approach to black hole spacetime evolutions.

gr-qc↗

arXiv2004

A constrained scheme for Einstein equations based on Dirac gauge and spherical coordinates

We propose a new formulation for 3+1 numerical relativity, based on a constrained scheme and a generalization of Dirac gauge to spherical coordinates. This is made possible thanks to the introduction of a flat 3-metric on the spatial hypersurfaces t=const, which corresponds to the asymptotic structure of the physical 3-metric induced by the spacetime metric. Thanks to the joint use of Dirac gauge, maximal slicing and spherical components of tensor fields, the ten Einstein equations are reduced to a system of five quasi-linear elliptic equations (including the Hamiltonian and momentum constraints) coupled to two quasi-linear scalar wave equations. The remaining three degrees of freedom are fixed by the Dirac gauge. Indeed this gauge allows a direct computation of the spherical components of the conformal metric from the two scalar potentials which obey the wave equations. We present some numerical evolution of 3-D gravitational wave spacetimes which demonstrates the stability of the proposed scheme.

gr-qc↗

arXiv2002

Stationary structure of relativistic superfluid neutron stars

We describe recent progress in the numerical study of the structure of rapidly rotating superfluid neutron star models in full general relativity. The superfluid neutron star is described by a model of two interpenetrating and interacting fluids, one representing the superfluid neutrons and the second consisting of the remaining charged particles (protons, electrons, muons). We consider general stationary configurations where the two fluids can have different rotation rates around a common rotation axis. The previously discovered existence of configurations with one fluid in a prolate shape is confirmed.

gr-qc↗

arXiv1999

Gravitational waves from the collapse and bounce of a stellar core in tensor-scalar gravity

Tensor-scalar theory of gravity allows the generation of gravitational waves from astrophysical sources, like Supernovæ, even in the spherical case. That motivated us to study the collapse of a degenerate stellar core, within tensor-scalar gravity, leading to the formation of a neutron star through a bounce and the formation of a shock. We discuss in this paper the effects of the scalar field on the evolution of the system, as well as the appearance of strong non-perturbative effects of this scalar field (the so-called ``spontaneous scalarization''). As a main result, we describe the resulting gravitational monopolar radiation (form and amplitude) and discuss the possibility of its detection by the gravitational detectors currently under construction, taking into account the existing constraints on the scalar field. From the numerical point of view it is worthy to point out that we have developed a combined code which uses pseudo-spectral methods, for the evolution of the scalar field, and High Resolution Shock-Capturing schemes, for the evolution of the hydrodynamical system. Although this code has been used to integrate the field equations of that theory of gravity, in the spherically symmetric case, a by-product of the present work is to gain experience for an ulterior extension to multidimensional problems in Numerical Relativity of such numerical strategy.

astro-ph↗

arXiv1998

Neutron star transition to strong-scalar-field state in tensor scalar gravity

Spherical neutron star models are studied within tensor-scalar theories of gravity. Particularly, it is shown that, under some conditions on the second derivative of the coupling function and on star's mass, for a given star there exist two strong-scalar-field solutions as well as the usual weak-field one. This last solution happens to be unstable and a star, becoming massive enough to allow for all three solutions, evolves to reach one of the strong field configurations. This transition is dynamically computed and it appears that the star radiates away the difference in energy between both states (a few $10^{-3} M_\odot c^2$) as gravitational radiation. Since part of the energy ($\sim 10^{-5} M_\odot c^2$) is injected into the star as kinetic energy, the velocity of star's surface can reach up to $10^{-2} c$. The waveform of this monopolar radiation is shown as well as the oscillations undergone by the star. These oscillations are also studied within the slowly-rotating approximation, in order to estimate an order of magnitude of the resulting quadrupolar radiation.

gr-qc↗

arXiv1998

Spherical neutron star collapse toward a black hole in tensor-scalar theory of gravity

Complete tensor-scalar and hydrodynamic equations are presented and integrated, for a self-gravitating perfect fluid. The initial conditions describe unstable-equilibrium neutron star configuration, with a polytropic equation of state. They are necessary in order to follow the gravitational collapse (including full hydrodynamics) of this star toward a black hole and to study the resulting scalar gravitational wave. The amplitude of this wave, as well as the radiated energy dramatically increase above some critical value of the parameter of the coupling function, due to the spontaneous scalarization, an effect not present in Brans-Dicke theory. In most cases, the pressure of the collapsing fluid does not have a significant impact on the resulting signal. These kind of sources are not likely to be observed by future laser interferometric detectors (such as VIRGO or LIGO) of gravitational waves, if they are located at more than a few 100 kpc. However, spontaneous scalarization could be constrained if such a gravitational collapse is detected by its quadrupolar gravitational signal, since this latter is quite lower than the monopolar one.

gr-qc↗

arXiv1995

ROTATING NEUTRON STAR MODELS WITH MAGNETIC FIELD

We present the first numerical solutions of the coupled Einstein-Maxwell equations describing rapidly rotating neutron stars endowed with a magnetic field. These solutions are fully relativistic and self-consistent, all the effects of the electromagnetic field on the star's equilibrium (Lorentz force, spacetime curvature generated by the electromagnetic stress-energy) being taken into account. The magnetic field is axisymmetric and poloidal. Five dense matter equations of state are employed. The partial differential equation system is integrated by means of a pseudo-spectral method. Various tests passed by the numerical code are presented. The effects of the magnetic field on neutron stars structure are then investigated, especially by comparing magnetized and non-magnetized configurations with the same baryon number. The deformation of the star induced by the magnetic field is important only for huge values of B (B>10^{10} T). The maximum mass as well as the maximum rotational velocity are found to increase with the magnetic field. The maximum allowable poloidal magnetic field is of the order of 10^{14} T (10^{18} G) and is reached when the magnetic pressure is comparable to the fluid pressure at the centre of the star. For such values, the maximum mass of neutron stars is found to increase by 13 to 29 % (depending upon the EOS) with respect to the maximum mass of non-magnetized stars.

gr-qc↗