High resolution simulations of the head-on collision of white dwarfs

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 Msun to 1.1 Msun, 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 Msun objects, leading to subluminous events.Comment: 24 pages, 12 figures, accepted for publication in MNRA

Numerical aspects of low Mach number flows in astrophysics: preconditioning techniques

Internal flows inside gravitationally stable astrophysical objects, such as the Sun, normal and compact stars, are rotating, highly compressed and extremely subsonic. Such low Mach number flows are usually encountered when studying, for example, the dynamo action in stars and planets or the nuclear burst on neutron stars and white dwarfs. Handling of such flows numerically on time-scales longer than the dynamical one is complicated and challenging. The aim of this paper is to address the numerical problems associated with the modelling of internal quasi-stationary, rotating low Mach number flows in stars and to discuss possible solution scenarios. It is shown that the quasi-symmetric approximate factorization method (AFM) as a pre-conditioner within a non-linear Newton-type defect-correction solution procedure is best suited for modelling quasi-stationary weakly compressible flows with moderate low Mach numbers. This method is robust as it can be applied to model time-dependent compressible flows without further modifications. The AFM-pre-conditioning techniques are shown to be extendable into three dimensions with an arbitrary equation of state. Classical dimensional splitting techniques, however, such as the alternating direction implicit or line-Gauss-Seidel methods are not suited for modelling compressible low Mach number flows. It is also argued that hot and low Mach number astrophysical flows cannot be considered as an asymptotic limit of incompressible flows, but rather as highly compressed flows with extremely stiff pressure terms. We show that, unlike the pseudo-pressure in incompressible fluids, a Poisson-like treatment for the pressure would smooth unnecessarily physically induced acoustic perturbations, thereby violating the conservation of the total energy. Results of several hydrodynamical calculations are presented, which demonstrate the capability of the solver to search for solutions, that correspond to stationary, viscous and rotating flows with a Mach number as small as as well as to fluid flows that are subject to ultra-strong Newtonian and general relativistic gravitational field

Pushing 1D CCSNe to explosions: model and SN 1987A

We report on a method, PUSH, for triggering core-collapse supernova explosions of massive stars in spherical symmetry. We explore basic explosion properties and calibrate PUSH such that the observables of SN1987A are reproduced. Our simulations are based on the general relativistic hydrodynamics code AGILE combined with the detailed neutrino transport scheme IDSA for electron neutrinos and ALS for the muon and tau neutrinos. To trigger explosions in the otherwise non-exploding simulations, we rely on the neutrino-driven mechanism. The PUSH method locally increases the energy deposition in the gain region through energy deposition by the heavy neutrino flavors. Our setup allows us to model the explosion for several seconds after core bounce. We explore the progenitor range 18-21M$_{\odot}$. Our studies reveal a distinction between high compactness (HC) and low compactness (LC) progenitor models, where LC models tend to explore earlier, with a lower explosion energy, and with a lower remnant mass. HC models are needed to obtain explosion energies around 1 Bethe, as observed for SN1987A. However, all the models with sufficiently high explosion energy overproduce $^{56}$Ni. We conclude that fallback is needed to reproduce the observed nucleosynthesis yields. The nucleosynthesis yields of $^{57-58}$Ni depend sensitively on the electron fraction and on the location of the mass cut with respect to the initial shell structure of the progenitor star. We identify a progenitor and a suitable set of PUSH parameters that fit the explosion properties of SN1987A when assuming 0.1M$_{\odot}$ of fallback. We predict a neutron star with a gravitational mass of 1.50M$_{\odot}$. We find correlations between explosion properties and the compactness of the progenitor model in the explored progenitors. However, a more complete analysis will require the exploration of a larger set of progenitors with PUSH.Comment: revised version as accepted by ApJ (results unchanged, text modified for clarification, a few references added); 26 pages, 20 figure

Nucleosynthesis in O-Ne-Mg Supernovae

We have studied detailed nucleosynthesis in the shocked surface layers of an Oxygen-Neon-Magnesium core collapse supernova with an eye to determining if the conditions are suitable for r process nucleosynthesis. We find no such conditions in an unmodified model, but do find overproduction of N=50 nuclei (previously seen in early neutron-rich neutrino winds) in amounts that, if ejected, would pose serious problems for galactic chemical evolution.Comment: 12 pages, 1 figure, to be published in Astrophysical Journal Letter

Spinon localization in the heat transport of the spin-1/2 ladder compound (C5_5H12_{12}N)2_2CuBr4_4

We present experiments on the magnetic field-dependent thermal transport in the spin-1/2 ladder system (C$_5$H$_{12}$N)$_2$CuBr$_4$. The thermal conductivity $\kappa(B)$ is only weakly affected by the field-induced transitions between the gapless Luttinger-liquid state realized for $B_{c1}< B < B_{c2}$ and the gapped states, suggesting the absence of a direct contribution of the spin excitations to the heat transport. We observe, however, that the thermal conductivity is strongly suppressed by the magnetic field deeply within the Luttinger-liquid state. These surprising observations are discussed in terms of localization of spinons within finite ladder segments and spinon-phonon umklapp scattering of the predominantly phononic heat transport.Comment: 4 pages, 3 figure

Instabilities and Clumping in Type Ia Supernova Remnants

We present two-dimensional high-resolution hydrodynamical simulations in spherical polar coordinates of a Type Ia supernova interacting with a constant density interstellar medium. The ejecta are assumed to be freely expanding with an exponential density profile. The interaction gives rise to a double-shocked structure susceptible to hydrodynamic instabilities. The Rayleigh-Taylor instability initially grows, but the Kelvin-Helmholtz instability takes over, producing vortex rings. The nonlinear instability initially evolves toward longer wavelengths and eventually fades away when the reverse shock front is in the flatter part of the supernova density distribution. Based on observations of X-ray knots and the protrusion in the southeast outlin of Tycho's supernova remnant, we include clumping in the ejecta. The clump interaction with the reverse shock induces Rayleigh-Taylor and Kelvin-Helmholtz instabilities on the clump surface that facilitate fragmentation. In order to survive crushing and to have a bulging effect on the forward shock, the clump's initial density ratio to the surrounding ejecta must be at least 100 for the conditions in Tycho's remnant. The 56Ni bubble effect may be important for the development of clumpiness in the ejecta. The observed presence of an Fe clump would then require a non-radioactive origin for this Fe, possibly 54Fe. The large radial distance of the X-ray emitting Si and S ejecta from the remnant center indicates that they were initially in clumps.Comment: 27 pages, 4 postscript figures, 5 GIF figures submitted to Astrophysical Journa