arXiv subjects

J. M. Ibanez

Publications and source records attributed to J. M. Ibanez.

14 recordsLinked to original sources

arXiv2017

Tracing accretion variability of high-mass YSOs via light echoes

There is growing evidence for disk-mediated accretion being the dominant mode of star formation across nearly the whole stellar mass spectrum. The stochastic nature of this process has been realized which implies an inherent source variability. It can be traced more easily for low-mass YSOs (LMYSOs) since high-mass YSOs (HMYSOs) are still embedded even when reaching the ZAMS. While variable reflection nebulae around LMYSOs were among the earliest signs of star formation, little is known on the variability of scattered light from embedded clusters, the birthplaces of HMYSOs. Since the few most massive stars dominate this emission, their variability is literally reflected in scattered light. Moreover, because of their high luminosity, for a given ambient dust density and source distance, the associated nebulosities are much larger than those of LMYSOs. In this case, the light travel time becomes substantial. So the apparent brightness distribution constitutes a light echo, shaped by both the HMYSO variability history and the spatial distribution of the scattering medium. We report on early results of a NIR variability study of HMYSOs associated with Class II methanol masers which aims at revealing a possible correlation between maser flux density and infrared brightness. Additionally, relevant findings for the eruptive HMYSO S255IR-NIRS3 are presented.

astro-ph.GA↗

arXiv2009

Relativistic Magnetohydrodynamics: Renormalized eigenvectors and full wave decomposition Riemann solver

We obtain renormalized sets of right and left eigenvectors of the flux vector Jacobians of the relativistic MHD equations, which are regular and span a complete basis in any physical state including degenerate ones. The renormalization procedure relies on the characterization of the degeneracy types in terms of the normal and tangential components of the magnetic field to the wavefront in the fluid rest frame. Proper expressions of the renormalized eigenvectors in conserved variables are obtained through the corresponding matrix transformations. Our work completes previous analysis that present different sets of right eigenvectors for non-degenerate and degenerate states, and can be seen as a relativistic generalization of earlier work performed in classical MHD. Based on the full wave decomposition (FWD) provided by the the renormalized set of eigenvectors in conserved variables, we have also developed a linearized (Roe-type) Riemann solver. Extensive testing against one- and two-dimensional standard numerical problems allows us to conclude that our solver is very robust. When compared with a family of simpler solvers that avoid the knowledge of the full characteristic structure of the equations in the computation of the numerical fluxes, our solver turns out to be less diffusive than HLL and HLLC, and comparable in accuracy to the HLLD solver. The amount of operations needed by the FWD solver makes it less efficient computationally than those of the HLL family in one-dimensional problems. However its relative efficiency increases in multidimensional simulations.

astro-ph.IM↗

arXiv2007

Trapping Horizons as inner boundary conditions for black hole spacetimes

We present a set of inner boundary conditions for the numerical construction of dynamical black hole space-times, when employing a 3+1 constrained evolution scheme and an excision technique. These inner boundary conditions are heuristically motivated by the dynamical trapping horizon framework and are enforced in an elliptic subsystem of the full Einstein equation. In the stationary limit they reduce to existing isolated horizon boundary conditions. A characteristic analysis completes the discussion of inner boundary conditions for the radiative modes.

gr-qc↗

arXiv2005

Numerical 3+1 general relativistic magnetohydrodynamics: a local characteristic approach

We present a general procedure to solve numerically the general relativistic magnetohydrodynamics (GRMHD) equations within the framework of the 3+1 formalism. The work reported here extends our previous investigation in general relativistic hydrodynamics (Banyuls et al. 1997) where magnetic fields were not considered. The GRMHD equations are written in conservative form to exploit their hyperbolic character in the solution procedure. All theoretical ingredients necessary to build up high-resolution shock-capturing schemes based on the solution of local Riemann problems (i.e. Godunov-type schemes) are described. In particular, we use a renormalized set of regular eigenvectors of the flux Jacobians of the relativistic magnetohydrodynamics equations. In addition, the paper describes a procedure based on the equivalence principle of general relativity that allows the use of Riemann solvers designed for special relativistic magnetohydrodynamics in GRMHD. Our formulation and numerical methodology are assessed by performing various test simulations recently considered by different authors. These include magnetized shock tubes, spherical accretion onto a Schwarzschild black hole, equatorial accretion onto a Kerr black hole, and magnetized thick accretion disks around a black hole prone to the magnetorotational instability.

astro-ph↗

arXiv2004

CFC+: Improved dynamics and gravitational waveforms from relativistic core collapse simulations

Core collapse supernovae are a promising source of detectable gravitational waves. Most of the existing (multidimensional) numerical simulations of core collapse in general relativity have been done using approximations of the Einstein field equations. As recently shown by Dimmelmeier et al (2002a,b), one of the most interesting such approximation is the so-called conformal flatness condition (CFC) of Isenberg, Wilson and Mathews. Building on this previous work we present here new results from numerical simulations of relativistic rotational core collapse in axisymmetry, aiming at improving the dynamics and the gravitational waveforms. The computer code used for these simulations evolves the coupled system of metric and fluid equations using the 3+1 formalism, specialized to a new framework for the gravitational field equations which we call CFC+. In this approach we add new degrees of freedom to the original CFC equations, which extend them by terms of second post-Newtonian order. The corrections for CFC+ are computed solving a system of elliptic linear equations. The new formalism is assessed with time evolutions of both rotating neutron stars in equilibrium and gravitational core collapse of rotating polytropes. Gravitational wave signals for a comprehensive sample of collapse models are extracted using either the quadrupole formula or directly from the metric. We discuss our results on the dynamics and the gravitational wave emission through a detailed comparison between CFC and CFC+ simulations. The main conclusion is that, for the neutron star spacetimes analyzed in the present work, no significant differences are found among CFC, CFC+, and full general relativity, which highlights the suitability of the former.

astro-ph↗

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↗

arXiv1999

Riemann Solvers in General Relativistic Hydrodynamics

Our contribution concerns with the numerical solution of the 3D general relativistic hydrodynamical system of equations within the framework of the 3+1 formalism. We summarize the theoretical ingredients which are necessary in order to build up a numerical scheme based on the solution of local Riemann problems. Hence, the full spectral decomposition of the Jacobian matrices of the system, i.e., the eigenvalues and the right and left eigenvectors, is explicitly shown. An alternative approach consists in using any of the special relativistic Riemann solvers recently developed for describing the evolution of special relativistic flows. Our proposal relies on a local change of coordinates in terms of which the spacetime metric is locally Minkowskian and permits an accurate description of numerical general relativistic hydrodynamics.

astro-ph↗

arXiv1999

High--Resolution 3D Simulations of Relativistic Jets

We have performed high-resolution 3D simulations of relativistic jets with beam flow Lorentz factors up to 7, a spatial resolution of 8 cells per beam radius, and for up to 75 normalized time units to study the morphology and dynamics of 3D relativistic jets. Our simulations show that the coherent fast backflows found in axisymmetric models are not present in 3D models. We further find that when the jet is exposed to non-axisymmetric perturbations, (i) it does not display the strong perturbations found for 3D classical hydrodynamic and MHD jets (at least during the period of time covered by our simulations), and (ii) it does propagate according to the 1D estimate. Small 3D effects in the relativistic beam give rise to a lumpy distribution of apparent speeds like that observed in M87. The beam is surrounded by a boundary layer of high specific internal energy. The properties of this layer are briefly discussed.

astro-ph↗

arXiv1999

An Efficient Implementation of Flux Formulae in Multidimensional Relativistic Hydrodynamical Codes

We derive and analyze a simplified formulation of the numerical viscosity terms appearing in the expression of the numerical fluxes associated to several High-Resolution Shock-Capturing schemes. After some algebraic pre-processing, we give explicit expressions for the numerical viscosity terms of two of the most widely used flux formulae, which implementation saves computational time in multidimensional simulations of relativistic flows. Additionally, such treatment explicitely cancells and factorizes a number of terms helping to amortiguate the growing of round-off errors. We have checked the performance of our formulation running a 3D relativistic hydrodynamical code to solve a standard test-bed problem and found that the improvement in efficiency is of high practical interest in numerical simulations of relativistic flows in Astrophysics.

astro-ph↗

arXiv1999

GENESIS: A high-resolution code for 3D relativistic hydrodynamics

The main features of a three dimensional, high-resolution special relativistic hydro code based on relativistic Riemann solvers are described. The capabilities and performance of the code are discussed. In particular, we present the results of extensive test calculations which demonstrate that the code can accurately and efficiently handle strong shocks in three spatial dimensions. Results of the performance of the code on single and multi-processor machines are given. Simulations (in double precision) with 7x10^6 computational cells require less than 1 Gb of RAM memory and ~7x10^-5 CPU seconds per zone and time step (on a SCI Cray-Origin 2000 with a R10000 processor). Currently, a version of the numerical code is under development, which is suited for massively parallel computers with distributed memory architecture (like, e.g., Cray T3E).

astro-ph↗

arXiv1998

Numerical evolution of matter in dynamical axisymmetric black hole spacetimes. I. Methods and tests

We have developed a numerical code to study the evolution of self-gravitating matter in dynamic black hole axisymmetric spacetimes in general relativity. The matter fields are evolved with a high-resolution shock-capturing scheme that uses the characteristic information of the general relativistic hydrodynamic equations to build up a linearized Riemann solver. The spacetime is evolved with an axisymmetric ADM code designed to evolve a wormhole in full general relativity. We discuss the numerical and algorithmic issues related to the effective coupling of the hydrodynamical and spacetime pieces of the code, as well as the numerical methods and gauge conditions we use to evolve such spacetimes. The code has been put through a series of tests that verify that it functions correctly. Particularly, we develop and describe a new set of testbed calculations and techniques designed to handle dynamically sliced, self-gravitating matter flows on black holes, and subject the code to these tests. We make some studies of the spherical and axisymmetric accretion onto a dynamic black hole, the fully dynamical evolution of imploding shells of dust with a black hole, the evolution of matter in rotating spacetimes, the gravitational radiation induced by the presence of the matter fields and the behavior of apparent horizons through the evolution.

gr-qc↗

arXiv1998

Legendre expansion of the neutrino-antineutrino annihilation kernel: Influence of high order terms

We calculate the Legendre expansion of the rate of the process $ν+ \barν \leftrightarrow e^+ + e^-$ up to 3rd order extending previous results of other authors which only consider the 0th and 1st order terms. Using different closure relations for the moment equations of the radiative transfer equation we discuss the physical implications of taking into account quadratic and cubic terms on the energy deposition outside the neutrinosphere in a simplified model. The main conclusion is that 2nd order is necessary in the semi-transparent region and gives good results if an appropriate closure relation is used.

astro-ph↗

arXiv1995

Neutrino pair synchrotron radiation from relativistic electrons in strong magnetic fields

The emissivity for the neutrino pair synchrotron radiation in strong magnetic fields has been calculated both analytically and numerically for high densities and moderate temperatures, as can be found in neutron stars. Under these conditions, the electrons are relativistic and degenerate. We give here our results in terms of an universal function of a single variable. For two different regimes of the electron gas we present a simplified calculation and compare our results to those of Kaminker et al. Agreement is found for the classical region, where many Landau levels contribute to the emissivity , but some differences arise in the quantum regime. One finds that the emissivity for neutrino pair synchrotron radiation is competitive, and can be dominant, with other neutrino processes for magnetic fields of the order $B \sim 10^{14} - 10^{15} G $.This indicates the relevance of this process for some astrophysical scenarios, such as neutron stars and supernovae.

astro-ph↗

arXiv1995

Galaxy clusters and microwave background anisotropy

Previous estimates of the microwave background anisotropies produced by freely falling spherical clusters are discussed. These estimates are based on the Swiss-Cheese and Tolman-Bondi models. It is proved that these models give only upper limits to the anisotropies produced by the observed galaxy clusters. By using spherically symmetric codes including pressureless matter and a hot baryonic gas, new upper limits are obtained. The contributions of the hot gas and the pressureless component to the total anisotropy are compared. The effects produced by the pressure are proved to be negligible; hence, estimations of the cluster anisotropies based on N-body simulations are hereafter justified. After the phenomenon of violent relaxation, any realistic rich cluster can only produce small anisotropies with amplitudes of order $10^{-7}$. During the rapid process of violent relaxation, the anisotropies produced by nonlinear clusters are expected to range in the interval $(10^{-6},10^{-5})$. The angular scales of these anisotropies are discussed.

astro-ph↗