Ruben Cabezon | University of Basel (original) (raw)

Papers by Ruben Cabezon

Astronomy & Astrophysics, 2012

2019 19th IEEE/ACM International Symposium on Cluster, Cloud and Grid Computing (CCGRID)

Journal of Physics G: Nuclear and Particle Physics

Astronomy & Astrophysics, 2014

Nuclei in the Cosmos, 2010

ABSTRACT Classical novae are cataclysmic binary star systems in which matter from an unevolved st... more ABSTRACT Classical novae are cataclysmic binary star systems in which matter from an unevolved star is slowly accreted by a white dwarf companion. When enough mass has been accreted the pressure at the base of its envelope becomes high enough for the ignition of hydrogen, and a thermonuclear runaway occurs. Apart from releasing a large amount of energy in various parts of the electromagnetic spectrum, the nova explosion also results in the ejection of matter, forming an expanding shell around the system. In this study we aim to investigate the evolution of this nova shell to ascertain whether or not the secondary star can be significantly polluted by the ejecta material. To model the expanding shell we use a smooth particle hydrodynamics (SPH) code. The simulation is fully 3D and includes the nova ejecta, the main sequence companion, and the white dwarf (as a gravitational potential). The initial conditions for the nova ejecta are taken from the late stages of a detailed 1D hydrodynamical-nucleosynthetic simulation of the nova outburst on the white dwarf surface. Our very preliminary results show that some matter is accreted by the companion star, as expected, however it remains to be seen if the amount accreted is significant. We also find that the impact of the shell material on the main sequence star envelope enhances mass transfer in the system, so that some ejecta is probably also re-accreted by the white dwarf itself. This will affect the next nova explosion, as the white dwarf surface composition will be altered, especially at low metallicity. We also discuss necessary improvements for our future simulations.

We describe and check a novel formulation of Smoothed Particle Hydrodynamics (SPH) based on an In... more We describe and check a novel formulation of Smoothed Particle Hydrodynamics (SPH) based on an Integral Approach to the Derivatives, called IAD_0, that can be applied to simulate astrophysical systems. The method relies in a tensor approach to calculating gradients, which is more accurate than the standard procedure (STD), due to its better renormalization properties. The proposed scheme fully conserves momentum and energy in isentropic flows, and is less susceptible to the pairing instability. The resulting algorithm is verified using two tests: a two-dimensional simulation of the Kelvin-Helmholtz instability and the three-dimensional simulation of the merging of two polytropes. The analysis of these test cases suggests that the method is able to improve the results of the standard technique with only a moderate computational overload.

Classical novae are cataclysmic binary star systems in which matter from an unevolved star is slo... more Classical novae are cataclysmic binary star systems in which matter from an unevolved star is slowly accreted by a white dwarf companion. When enough mass has been accreted the pressure at the base of its envelope becomes high enough for the ignition of hydrogen, and a thermonuclear runaway occurs. Apart from releasing a large amount of energy in various parts of the electromagnetic spectrum, the nova explosion also results in the ejection of matter, forming an expanding shell around the system. In this study we aim to investigate the evolution of this nova shell to ascertain whether or not the secondary star can be significantly polluted by the ejecta material. To model the expanding shell we use a smooth particle hydrodynamics (SPH) code. The simulation is fully 3D and includes the nova ejecta, the main sequence companion, and the white dwarf (as a gravitational potential). The initial conditions for the nova ejecta are taken from the late stages of a detailed 1D hydrodynamical-nu...

Astronomy & Astrophysics, 2010

... (2007) produces ten times more stars than required to match the observed early-R to red clump... more ... (2007) produces ten times more stars than required to match the observed early-R to red clump star ratio. This discrepancy becomes more severe after the analysis by Zamora et al. ... The scale along the sequence represents the logarithm of temperature in Kelvin degrees. ...

The smoothed particle hydrodynamics (SPH) technique is a numerical method for solving gas-dynamic... more The smoothed particle hydrodynamics (SPH) technique is a numerical method for solving gas-dynamical problems that has been applied to simulate the evolution of a wide variety of astrophysical systems. The method has a second-order accuracy, with a resolution that is usually much larger in the compressed regions than in the diluted zones of the fluid. In this work, we propose and check a scheme to balance and equalize the resolution of SPH between high and low density regions. This method relies in the versatility of a family of interpolators called Sinc kernels, which allows to increase the quality of interpolations just varying a single parameter (the exponent of the Sinc function). The scheme is checked and validated through a number of numerical tests, going from standard one-dimensional Riemann problems in shock tubes, to multidimensional simulations of explosions, hydrodynamic instabilities and the collapse of a sun-like polytrope. The analysis of the hydrodynamical simulations suggests that the scheme devised to equalizing accuracy improves the treatment of the post-shock regions and, in general, of the rarefacted zones of fluids while causing no harm to the growth of hydrodynamic instabilities. The method is robust and easy to implement with a low computational overload. It conserves mass, energy, and momentum and reduces to the standard SPH scheme in those regions of the fluid with smooth density gradients.

We present a detailed, 3D hydrodynamics study of the neutrino-driven winds that emerge from the r... more We present a detailed, 3D hydrodynamics study of the neutrino-driven winds that emerge from the remnant of a NS merger. Our simulations are performed with the Newtonian, Eulerian code FISH, augmented by a detailed, spectral neutrino leakage scheme that accounts for heating due to neutrino absorption in optically thin conditions. Consistent with the 2D study of Dessart et al. (2009), we find that a strong baryonic wind is blown out along the original binary rotation axis within 100 ms after the merger. We compute a lower limit on the expelled mass of 3.5×10−3M⊙, large enough to be relevant for heavy element nucleosynthesis. The physical properties vary significantly between different wind regions. For example, due to stronger neutrino irradiation, the polar regions show substantially larger Ye than those at lower latitudes. This has its bearings on the nucleosynthesis: the polar ejecta produce interesting r-process contributions from A∼80 to about 130, while the more neutron-rich, lower-latitude parts produce also elements up to the third r-process peak near A∼195. We also calculate the properties of electromagnetic transients that are powered by the radioactivity in the wind, in addition to the macronova transient that stems from the dynamic ejecta. The high-latitude (polar) regions produce UV/optical transients reaching luminosities up to 1e41erg/s, which peak around 1 day in optical and 0.3 days in bolometric luminosity. The lower-latitude regions, due to their contamination with high-opacity heavy elements, produce dimmer and more red signals, peaking after ∼2 days in optical and infrared. Our numerical experiments indicate that it will be difficult to infer the collapse time-scale of the HMNS to a BH based on the wind electromagnetic transient, at least for collapse time-scales larger than the wind production time-scale.

We introduce a new algorithm for the calculation of multidimensional optical depths in approximat... more We introduce a new algorithm for the calculation of multidimensional optical depths in approximate radiative transport schemes, equally applicable to neutrinos and photons. Motivated by (but not limited to) neutrino transport in three-dimensional simulations of core-collapse supernovae and neutron star mergers, our method makes no assumptions about the geometry of the matter distribution, apart from expecting optically transparent boundaries. Based on local information about opacities, the algorithm figures out an escape route that tends to minimize the optical depth without assuming any pre-defined paths for radiation. Its adaptivity makes it suitable for a variety of astrophysical settings with complicated geometry (e.g., core-collapse supernovae, compact binary mergers, tidal disruptions, star formation, etc.). We implement the MODA algorithm into both a Eulerian hydrodynamics code with a fixed, uniform grid and into an SPH code where we make use a tree structure that is otherwise used for searching neighbours and calculating gravity. In a series of numerical experiments, we compare the MODA results with analytically known solutions. We also use snapshots from actual 3D simulations and compare the results of MODA with those obtained with other methods such as the global and local ray-by-ray method. It turns out that MODA achieves excellent accuracy at a moderate computational cost. In an appendix we also discuss implementation details and parallelization strategies.

The direct impact of white dwarfs has been suggested as a plausible channel for type Ia supernova... more The direct impact of white dwarfs has been suggested as a plausible channel for type Ia supernovae. In spite of their (a priori) rareness, in highly populated globular clusters and in galactic centers, where the amount of white dwarfs is considerable, the rate of violent collisions between two of them might be non- negligible. Even more, there are indications that binary white dwarf systems orbited by a third stellar-mass body have an important chance to induce a clean head-on collision. Therefore, this scenario represents a source of contamination for the supernova light-curves sample that it is used as standard candles in cosmology, and it deserves further investigation. Some groups have conducted numerical simulations of this scenario, but their results show several differences. In this paper we address some of the possible sources of these differences, presenting the results of high resolution hydrodynamical simulations jointly with a detailed nuclear post-processing of the nuclear abundances, to check the viability of white dwarf collisions to produce significant amounts of 56Ni. To that purpose, we use a 2D-axial symmetric smoothed particle hydrodynamic code to obtain a resolution considerably higher than in previous studies. In this work, we also study how the initial mass and nuclear composition affect the results. The gravitational wave emission is also calculated, as this is a unique signature of this kind of events. All calculated models produce a significant amount of 56Ni, ranging from 0.1 M⊙ to 1.1 M⊙, compatible not only with normal-Branch type Ia supernova but also with the subluminous and super-Chandrasekhar subset. Nevertheless, the distribution mass-function of white dwarfs favors collisions among 0.6 − 0.7 M⊙ objects, leading to subluminous events.

The behavior of IAD_0 scheme, a fully conservative SPH scheme based on a tensor formulation, is a... more The behavior of IAD_0 scheme, a fully conservative SPH scheme based on a tensor formulation, is analyzed in connection with several astrophysical scenarios, and compared to the same simulations carried out with the standard SPH technique. The proposed hydrodynamic scheme is validated using a variety of numerical tests that cover important topics in astrophysics, such as the evolution of supernova remnants, the stability of self-gravitating bodies and the coalescence of compact objects. The results suggest that the SPH scheme built with the integral approach to the derivatives premise improves the results of the standard SPH technique. In particular, it is observed a better development of hydrodynamic instabilities, an improved description of self-gravitant structures in equilibrium and a reasonable description of the process of coalescence of two white dwarfs. A good energy, and linear and angular momentum conservation, generally better than that of standard SPH, was also obtained. In addition the new scheme is less susceptible to suffer pairing instability.

In this paper we develop and check a fully conservative SPH scheme based on a tensor formulation ... more In this paper we develop and check a fully conservative SPH scheme based on a tensor formulation which can be applied to simulate astrophysical systems. In the proposed scheme derivatives are calculated from an integral expression which leads to a tensor, rather than vectorial, estimation of gradients and reduces to the standard formulation in the continuum limit. The new formulation improves the interpolation of physical magnitudes, leading to a set of conservative equations which looks similar to those of standard SPH. The resulting scheme was checked using a variety of well known tests, all of them simulated in two dimensions. An application of the proposed tensor method to astrophysics was also discussed by simulating the stability of a sun-like polytrope calculated in three dimensions.

Observational evidences point to a common explosion mechanism of Type Ia supernovae based on a de... more Observational evidences point to a common explosion mechanism of Type Ia supernovae based on a delayed detonation of a white dwarf. However, all attempts to find a convincing ignition mechanism based on a delayed detonation in a destabilized, expanding, white dwarf have been elusive so far. One of the possibilities that has been invoked is that an inefficient deflagration leads to pulsation of a Chandrasekhar-mass white dwarf, followed by formation of an accretion shock that confines a carbon-oxygen rich core, while transforming the kinetic energy of the collapsing halo into thermal energy of the core, until an inward moving detonation is formed. This chain of events has been termed Pulsating Reverse Detonation (PRD). In this work we present three dimensional numerical simulations of PRD models from the time of detonation initiation up to homologous expansion. Different models characterized by the amount of mass burned during the deflagration phase, M_defl, give explosions spanning a range of kinetic energies, K ~ (1.0-1.2) foes, and 56Ni masses, M(56Ni) ~ 0.6-0.8 M_sun, which are compatible with what is expected for typical Type Ia supernovae. Spectra and light curves of angle-averaged spherically symmetric versions of the PRD models are discussed. Type Ia supernova spectra pose the most stringent requirements on PRD models.

The axisymmetric form of the hydrodynamic equations within the smoothed particle hydrodynamics (S... more The axisymmetric form of the hydrodynamic equations within the smoothed particle hydrodynamics (SPH) formalism is presented and checked using idealized scenarios taken from astrophysics (free fall collapse, implosion and further pulsation of a sun-like star), gas dynamics (wall heating problem, collision of two streams of gas) and inertial confinement fusion (ICF, -ablative implosion of a small capsule-). New material concerning the standard SPH formalism is given. That includes the numerical handling of those mass points which move close to the singularity axis, more accurate expressions for the artificial viscosity and the heat conduction term and an easy way to incorporate self-gravity in the simulations. The algorithm developed to compute gravity does not rely in any sort of grid, leading to a numerical scheme totally compatible with the lagrangian nature of the SPH equations.

A set of interpolating functions of the type f(v)={(sin[v pi/2])/(v pi/2)}^n is analyzed in the c... more A set of interpolating functions of the type f(v)={(sin[v pi/2])/(v pi/2)}^n is analyzed in the context of the smoothed-particle hydrodynamics (SPH) technique. The behaviour of these kernels for several values of the parameter n has been studied either analytically as well as numerically in connection with several tests carried out in two dimensions. The main advantage of this kernel relies in its flexibility because for n=3 it is similar to the standard widely used cubic-spline, whereas for n>3 the interpolating function becomes more centrally condensed, being well suited to track discontinuities such as shock fronts and thermal waves.

Astronomy & Astrophysics, 2012

2019 19th IEEE/ACM International Symposium on Cluster, Cloud and Grid Computing (CCGRID)

Journal of Physics G: Nuclear and Particle Physics

Astronomy & Astrophysics, 2014

Nuclei in the Cosmos, 2010

Astronomy & Astrophysics, 2010