Axisymmetric magneto-hydrodynamics with SPH

Many interesting terrestrial and astrophysical scenarios involving magnetic fields can be approached in axial geometry. Even though the Lagrangian smoothed particle hydrodynamics (SPH) technique has been successfully extended to handle magneto-hydrodynamic (MHD) problems, a well-verified, axisymmetric MHD scheme based on the SPH technique does not exist. In this work, we propose and check a new axisymmetric MHD hydrodynamic code that can be applied to astrophysical and engineering problems which display an adequate geometry. We show that a hydrodynamic code built on these axisymmetric premises is able to produce similar results to standard 3D-SPHMHD codes but with much lesser computational effort.Peer ReviewedPostprint (published version

Nucleosynthesis as a result of multiple delayed detonations in Type Ia Supernovae

The explosion of a white dwarf of mass 1.36 M$_\odot$ has been simulated in three dimensions with the aid of a SPH code. The explosion follows the delayed detonation paradigma. In this case the deflagration-detonation transition is induced by the large corrugation of the flame front resulting from Rayleigh-Taylor instability and turbulence. The nucleosynthetic yields have been calculated, showing that some neutronized isotopes such as 54^Fe or 58^Ni are not overproduced with respect to the solar system ratios. The distribution of intermediate-mass elements is also compatible with the spectra of normal SNIa. The excepcion is, however, the abundance of carbon and oxygen, which are overproduced.Comment: 3 pages, 2 figures. Proceedings "Nuclei in the Cosmos VII" 2002. To appear in Nucl. Phys.

Coulomb corrections to the equation of state of nuclear statistical equilibrium matter: implications for SNIa nucleosynthesis and the accretion-induced collapse of white dwarfs

Coulomb corrections to the equation of state of degenerate matter are usually neglected in high-temperature regimes, owing to the inverse dependence of the plasma coupling constant, G, on temperature. However, nuclear statistical equilibrium matter is characterized by a large abundance by mass of large-Z (iron group) nuclei. It is found that Coulomb corrections to the ion ideal gas equation of state of matter in nuclear statistical equilibrium are important at temperatures T≲5–10×109 K and densities ρ≳108 g cm−3. At a temperature T=8.5×109 K and a density ρ=8×109 g cm−3, the neutronization rate is larger by ≳28 per cent when Coulomb corrections are included. However, the conductive velocity of a thermonuclear deflagration wave in C-O drops by ∼16 per cent when Coulomb corrections to the heat capacity are taken into account. The implications for SNIa models and nucleosynthesis, and also for the accretion-induced collapse of white dwarfs, are discussed. Particularly relevant is the result that the minimum density for collapse of a white dwarf to a neutron star is shifted down to 5.5–6×109 g cm−3, a value substantially lower than previously thought.Peer ReviewedPostprint (published version

Mixing Sinc kernels to improve interpolations in smoothed particle hydrodynamics without pairing instability

The smoothed particle hydrodynamics technique strongly relies on the proper choice of interpolating functions. In this work, we revisit and extend the main properties of a family of interpolators called $Sinc~kernels$ and compare them with those of the widely used family of Wendland kernels. We show that a linear combination of low and high-order Sinc kernels generates good quality interpolators, which are resistant to the pairing instability while keeping good sampling properties in a wide range of neighbor interpolating points, $60\le n_b\le 400$. We show that a particular case of this linear mix of Sincs produces a well-balanced and robust kernel, that improves previous results in the Gresho-Chan vortex experiment even when the number of neighbors is not large, while yielding a good convergence rate. Although such a mixing technique is ideally suited for Sinc kernels owing to their excellent flexibility, it can be easily applied to other interpolating families such as the B-splines and Wendland kernels.Comment: 13 pages, 14 figures, 4 tables, submitted to MNRA

Building initial models of rotating white dwarfs with SPH

A general procedure to build self-gravitational, rotating equilibrium structures with the Smoothed Particle Hydrodynamics (SPH) technique does not exist. In particular, obtaining stable rotating configurations for white dwarf (WD) stars is currently a major drawback of many astrophysical simulations. Rotating WDs with low internal temperatures are connected with both, explosive and implosive scenarios such as type Ia supernova explosions or neutron stars formation. Simulations of these events with SPH codes demand stable enough particle configurations as initial models. In this work we have developed and tested a relaxation method to obtain equilibrium configurations of rotating WDs. This method is straightforward and takes advantage of the excellent mass and angular momentum conservation properties of the SPH technique. Although we focus on rigid rotation and its potential applications to several Type Ia supernova scenarios, we also show that our proposal is also able to provide good initial models in differential rotation, which has the potential to benefit many other types of simulations where rotation plays a capital role, like disk evolution and stellar formation.Peer ReviewedPostprint (published version

Integral SPH: Connecting the partition of unit to accurate gradient estimation

AYA2017-86274-P Del enfriamiento a las explosiones: la física de los objetos compactosPostprint (published version

Surface and core detonations in rotating white dwarfs

The feasibility of the double detonation mechanism—surface helium detonation followed by complete carbon detonation of the core—in a rotating white dwarf with mass ;1Me is studied using three-dimensional hydrodynamic simulations. A rapid rigid rotation of the white dwarf was assumed, so that its initial spherical geometry is considerably distorted. Unlike spherically symmetric models, we found that when helium ignition is located far from the spinning axis, the detonation fronts converge asynchronically at the antipodes of the ignition point. Nevertheless, the detonation of the carbon core still remains as the most probable outcome. The detonation of the core gives rise to a strong explosion, matching many of the basic observational constraints of Type Ia supernovae (SNe Ia). We conclude that the double detonation mechanism also works when the white dwarf is rapidly rotating. These results provide further evidence for the viability of sub-Chandrasekhar-mass models as well as some double degenerate models (those having some helium fuel at the merging moment), making them appealing channels for the production of SN Ia events.Peer ReviewedPostprint (published version

Integral smoothed particle hydrodynamics with an improved partition of unit and a better track of contact discontinuities

The correct evaluation of gradients is at the cornerstone of the smoothed particle hydrodynamics (SPH) technique. Using an integral approach to estimate gradients has proven to enhance accuracy substantially. Such approach retains the Lagrangian structure of SPH equations and is fully conservative. In this paper we study, among other things, the connection between the choice of the volume elements (VEs), which enters in the SPH summations, and the accuracy in the gradient estimation within the integral approach scheme (ISPH). A new kind of VEs are proposed which improve the partition of unit and are fully compatible with the Lagrangian formulation of SPH, including the grad-h corrections. Using analytic considerations, simple static toy models in 1D, and a few full 3D test cases, we show that any improvement in the partition of unit also leads to a better calculation of gradients when the integral approach is used jointly. Additionally, we propose a simple-to-implement variant of the ISPH scheme which is more adequate to handle sharp density contrasts.Comment: 29 pages, 17 figures, submitted to Journal of Computational Physic

A moderately-sized nuclear network to assist multi-D hydrodynamic simulations of supernova explosions

A key ingredient in any numerical study of supernova explosions is the nuclear network routine that is coupled with the hydrodynamic simulation code. When these studies are performed in more than one dimension, the size of the network is severely limited by computational issues. In this work, we propose a nuclear network (net87) which is close to one hundred nuclei and could be appropriate to simulate supernova explosions in multidimensional studies. One relevant feature is that electron and positron captures on free protons and neutrons have been incorporated to the network. Such addition allows for a better track of both, the neutronized species and the gas pressure. A second important feature is that the reactions are implicitly coupled with the temperature, which enhances the stability in the nuclear statistical equilibrium (NSE) regime. Here we analyze the performance of net87 in light of both: the computational overhead of the algorithm and the outcome in terms of the released nuclear energy and produced yields in typical Type Ia Supernova conditions.Postprint (published version