Core collapse supernovae in the QCD phase diagram

We compare two classes of hybrid equations of state with a hadron-to-quark matter phase transition in their application to core collapse supernova simulations. The first one uses the quark bag model and describes the transition to three-flavor quark matter at low critical densities. The second one employs a Polyakov-loop extended Nambu-Jona-Lasinio (PNJL) model with parameters describing a phase transition to two-flavor quark matter at higher critical densities. These models possess a distinctly different temperature dependence of their transition densities which turns out to be crucial for the possible appearance of quark matter in supernova cores. During the early post-bounce accretion phase quark matter is found only if the phase transition takes place at sufficiently low densities as in the study based on the bag model. The increase critical density with increasing temperature, as obtained for our PNJL parametrization, prevents the formation of quark matter. The further evolution of the core collapse supernova as obtained applying the quark bag model leads to a structural reconfiguration of the central protoneutron star where, in addition to a massive pure quark matter core, a strong hydrodynamic shock wave forms and a second neutrino burst is released during the shock propagation across the neutrinospheres. We discuss the severe constraints in the freedom of choice of quark matter models and their parametrization due to the recently observed 2M _?? pulsar and their implications for further studies of core collapse supernovae in the QCD phase diagram.     

Numerical simulations of the aspherical collapse of laser and acoustically generated bubbles

The details of nonlinear axisymmetric oscillations and collapse of bubbles subject to large internal or external pressure disturbances, are studied via a boundary integral method. Weak viscous effects on the liquid side are accounted for by integrating the equations of motion across the boundary layer that is formed adjacent to the interface. Simulations of single-cavitation bubble luminescence (SCBL) and single-bubble sonoluminescence (SBSL) are performed under conditions similar to reported experimental observations, aiming at capturing the details of bubble collapse. It is shown that any small initial deviation from sphericity, modeled through a small initial elongation along the axis of symmetry, may result in the formation and impact of two counter-propagating jets during collapse of the bubble, provided the amplitude of the initial disturbance is large enough and the viscosity of the surrounding fluid is small enough. Comparison between simulations and experimental observations show that this is the case for bubbles induced via a nano-second laser pulse (SCBL) during a luminescence event. In a similar fashion, simulations show that loss of sphericity accompanied with jet formation and impact during collapse is also possible with acoustically trapped bubbles in a standing pressure wave (SBSL), due to the many afterbounces of the bubble during its collapse phase. In both cases jet impact occurs as a result of P(2) growth in the form of an afterbounce instability. When the sound amplitude is decreased or liquid viscosity is increased the intensity of the afterbounce is decreased and jet impact is suppressed. When the sound amplitude is increased jet formation is superceded by Rayleigh-Taylor instability. In the same context stable luminescence is quenched in experimental observations. In both SCBL and SBSL simulations the severity of jet impact during collapse is quite large, and its local nature quite distinct. This attests to the fact that it is an energy focusing mechanism whose importance in generating the conditions under which a luminescence event is observed should be further investigated.     

Collective dynamics of colloids at fluid interfaces

The evolution of an initially prepared distribution of micron-sized colloidal particles, trapped at a fluid interface and under the action of their mutual capillary attraction, is analyzed by using Brownian dynamics simulations. At a separation λ given by the capillary length of typically 1mm, the distance dependence of this attraction exhibits a crossover from a logarithmic decay, formally analogous to two-dimensional gravity, to an exponential decay. We discuss in detail the adaptation of a particle-mesh algorithm, as used in cosmological simulations to study structure formation due to gravitational collapse, to the present colloidal problem. These simulations confirm the predictions, as far as available, of a mean-field theory developed previously for this problem. The evolution is monitored by quantitative characteristics which are particularly sensitive to the formation of highly inhomogeneous structures. Upon increasing λ the dynamics shows a smooth transition from the spinodal decomposition expected for a simple fluid with short-ranged attraction to the self-gravitational collapse scenario.     

3D collapse of rotating stellar iron cores in general relativity including deleptonization and a nuclear equation of state

We present 2D and 3D simulations of the collapse of rotating stellar iron cores in general relativity employing a nuclear equation of state and an approximate treatment of deleptonization. We compare fully general relativistic and conformally flat evolutions and find that the latter treatment is sufficiently accurate for the core-collapse supernova problem. We focus on gravitational wave (GW) emission from rotating collapse, bounce, and early postbounce phases. Our results indicate that the GW signature of these phases is much more generic than previously estimated. We also track the growth of a nonaxisymmetric instability in one model, leading to strong narrow-band GW emission.     

Pulse collapse and blue-shifted enhanced supercontinuum in a photonic crystal fiber

The blue-shifted supercontinuum generation in a photonic crystal fiber pumped by high peak power femtosecond pulses with a wavelength located in the anomalous dispersion region is investigated experimentally and numerically.The formation of a blue-shifted enhanced supercontinuum due to the pulse collapse is demonstrated.The process of the pulse collapse is measured by using the grating-eliminated no-nonsense observation of ultrafast incident laser light e-fields technique(GRENOUILLE).Numerical simulations in spectral and temporal domains are conducted.The data from the numerical simulations are in good agreement with the experimental results.Our experimental results and numerical simulations show that pulse collapse is the determining factor in the generation of a blue-shifted supercontinuum.     

Pulse collapse and blue-shifted enhanced supercontinuumin photonic crystal fiber

The blue-shifted supercontinuum generation in a photonic crystal fiber pumped by high peak power femtosecond pulses with wavelength located in the anomalous dispersion region is investigated experimentally and numerically. The formation of a blue-shifted enhanced supercontinuum due to the pulse collapse is demonstrated. The process of the pulse collapse is measured by using the grating-eliminated no-nonsense observation of ultrafast incident laser light e-fields technique (GRENOUILLE). Numerical simulations in spectral and temporal domains are conducted. The data from numerical simulations are in good agreement with the experimental results. Our experimental results and numerical simulations show that the pulse collapse is the determining factor in the generation of blue-shifted supercontinuum.     

Tuning oxygen packing in silica by nonhydrostatic pressure

The transformation of SiO2 from low pressure tetrahedral phases into denser octahedral phases takes place via the collapse of the oxygen sublattice into a close-packed arrangement. The transition paths and the resulting products are known to be affected by the presence of anisotropic stresses, which are difficult to control, so interpretation of the experimental results is problematic. Based on nonhydrostatic molecular dynamics simulations, we show that the collapse of the oxygen sublattice in the specific case of cristobalite is concomitant with the disappearance of tetrahedral units and that non hydrostatic stresses can be tuned to yield phases with different oxygen close-packed sublattices, including the alpha-PbO2-like phase, for which we provide a microscopic formation path, and phases with a cubic close packing, like anatase, not seen in experiments yet.     

Kitchen salt as a target for supernova neutrinos

According to the model of rotating collapsar, the gravitational stellar collapse occurs in two stages. During the first stage, electron neutrinos with average energies from 30 to 40 MeV, formed in the neutronization reaction (p + e ^? → n + ν_e), are mainly emitted. Previously iron was considered as a target for detecting neutrinos of such energies. It is shown in this study that addition of kitchen salt to the structure of existing detectors can both significantly improve the neutrino type identification and increase the active mass of existing detectors.     

Formation of fundamental solitons in the two-dimensional nonlinear Schrödinger equation with a lattice potential

We consider self-trapping of 2D solitons in the model based on theGross-Pitaevskii/nonlinear Schrödinger equation with the self-attractivecubic nonlinearity and a periodic potential of the optical-lattice (OL)type. It is known that this model may suppress the collapse, giving rise toa family of stable fundamental solitons. Here, we report essential dynamical featuresof self-trapping of the fundamental solitons from input configurations oftwo types, with vorticity 0 or 1. We identify regions in the respectiveparameter spaces corresponding to the formation of the soliton, collapse,and decay. A noteworthy result is the self-trapping of stable fundamentalsolitons in cases when the input norm essentially exceeds the collapsethreshold. We also compare predictions of the dynamical variationalapproximation with direct numerical simulations.     

Critical collapse of a complex scalar field with angular momentum

We report a new critical solution found at the threshold of axisymmetric gravitational collapse of a complex scalar field with angular momentum. To carry angular momentum the scalar field cannot be axisymmetric; however, its azimuthal dependence is defined so that the resulting stress-energy tensor and spacetime metric are axisymmetric. The critical solution found is nonspherical, discretely self-similar with an echoing exponent Delta=0.42(+/-4%), and exhibits a scaling exponent gamma=0.11(+/-10%) in near-critical collapse. Our simulations suggest that the solution is universal (within the imposed symmetry class), modulo a family-dependent constant, complex phase.     

Topological defects in contracting universes

We study the behavior and consequences of cosmic string networks in contracting universes. They approximately behave during the collapse phase as radiation fluids. Scaling solutions describing this are derived and tested against high-resolution numerical simulations. A string network in a contracting universe, together with the gravitational radiation it generates, can affect the dynamics of the universe both locally and globally and be an important source of radiation, entropy, and inhomogeneity. We discuss possible implications for bouncing and cyclic models.     

Electron capture rates on nuclei and implications for stellar core collapse

Supernova simulations to date have assumed that during core collapse electron captures occur dominantly on free protons, while captures on heavy nuclei are Pauli blocked and are ignored. We have calculated rates for electron capture on nuclei with mass numbers A=65-112 for the temperatures and densities appropriate for core collapse. We find that these rates are large enough so that, in contrast to previous assumptions, electron capture on nuclei dominates over capture on free protons. This leads to significant changes in core collapse simulations.     

Solvation of halogen ions in aqueous solutions at 500 K–600 K under 100 atm

Structural properties of the pure water and halogen solutions at high temperatures and pressures are studied by using the molecular dynamics simulations and quantum molecular simulations. The related characters are calculated as functions of temperature and pressure. The results show that the hydrogen bonded networks become looser as temperature increases,with the collapse of the traditional tetrahedral structure. It is similar to the concentration-dependent collapse in the Na Cl solutions. However, adding other halogen elements has no further effects on the already weakly bonded water molecules.At the phase changing points, the process of hydration is evident for the bigger ions, so that the bigger the ion is, the smaller a cluster is formed.     

Ensemble properties and molecular dynamics of unstable systems

The Hertel-Thirring cell model for unstable systems (of purely attractive particles) is solved in the canonical ensemble for arbitrary dimensions. The differences between the phase transitions found in the canonical and in the microcanonical ensemble are discussed. The cluster phase (with a complete collapse in the ground state) exhibits the nonextensive character of the cell model. The results of the cell model are compared with molecular-dynamics simulations of a one-dimensional model with a rectangular-well pair potential. The simulations support the relevance of the cell model to characterize basic properties of gravitational systems.     

Phase transition,formation and fragmentation of fullerenes

We present a statistical mechanics model treating the formation and the fragmentation of fullerenes as a phase transition. Based on this model, we investigate the formation and fragmentation of C₆₀ and C₂₄₀ fullerenes from and to a gas of carbon dimers by means of molecular dynamics (MD) simulations. These simulations were conducted for 500 ns using a topologically-constrained forcefield. At the phase transition temperature, both the cage and gaseous phases were found to coexist and the system continuously oscillates between the two phases. Combining the results of the MD simulations and the statistical mechanics approach, we obtain the dependence of the phase transition temperature on pressure and compare the results of our model with arc-discharge experiments.     

Non-singular Brans–Dicke collapse in deformed phase space

We study the collapse process of a homogeneous perfect fluid (in FLRW background) with a barotropic equation of state in Brans–Dicke (BD) theory in the presence of phase space deformation effects. Such a deformation is introduced as a particular type of non-commutativity between phase space coordinates. For the commutative case, it has been shown in the literature (Scheel, 1995), that the dust collapse in BD theory leads to the formation of a spacetime singularity which is covered by an event horizon. In comparison to general relativity (GR), the authors concluded that the final state of black holes in BD theory is identical to the GR case but differs from GR during the dynamical evolution of the collapse process. However, the presence of non-commutative effects influences the dynamics of the collapse scenario and consequently a non-singular evolution is developed in the sense that a bounce emerges at a minimum radius, after which an expanding phase begins. Such a behavior is observed for positive values of the BD coupling parameter. For large positive values of the BD coupling parameter, when non-commutative effects are present, the dynamics of collapse process differs from the GR case. Finally, we show that for negative values of the BD coupling parameter, the singularity is replaced by an oscillatory bounce occurring at a finite time, with the frequency of oscillation and amplitude being damped at late times.     

Bright solitary waves of trapped atomic Bose-Einstein condensates

Motivated by recent experimental observations, we study theoretically multiple bright solitary waves of trapped Bose-Einstein condensates. Through variational and numerical analyses, we determine the threshold for collapse of these states. Under π-phase differences between adjacent waves, we show that the experimental states lie consistently at the threshold for collapse, where the corresponding in-phase states are highly unstable. Following the observation of two long-lived solitary waves in a trap, we perform detailed three-dimensional simulations which confirm that in-phase waves undergo collapse while a π-phase difference preserves the long-lived dynamics and gives excellent quantitative agreement with experiment. Furthermore, intermediate phase differences lead to the growth of population asymmetries between the waves, which ultimately triggers collapse.     

Pair superfluidity of three-body constrained bosons in two dimensions

We examine the equilibrium properties of lattice bosons with attractive on-site interactions in the presence of a three-body hard-core constraint that stabilizes the system against collapse and gives rise to a dimer superfluid phase. Employing quantum Monte Carlo simulations, the ground state phase diagram of this system on the square lattice is analyzed. In particular, we study the quantum phase transition between the atomic and dimer superfluid regime and analyze the nature of the superfluid-insulator transitions. Evidence is provided for the existence of a tricritical point along the saturation transition line, where the transition changes from being first order to a continuous transition of the dilute Bose gas of holes. The Berzinskii-Kosterlitz-Thouless transition from the dimer superfluid to the normal fluid is found to be consistent with an anomalous stiffness jump, as expected from the unbinding of half-vortices.     

Entropic collapse transition of a polymer in a solvent with a nonadditive potential

We use molecular dynamics simulations to study an entropy-driven collapse transition of a flexible polymer in a solvent. Monomers and solvent particles interact with a steeply repulsive soft-sphere potential. We consider a nonadditive potential system in which the effective diameter describing the solvent-monomer interaction is greater than or equal to the diameters corresponding to the solvent-solvent and monomer-monomer interactions, which are set equal. We examine the effects of nonadditivity of the solvent-monomer potential and solvent density on the collapse transition. We find that a small degree of nonadditivity will drive the transition at sufficiently high solvent density. Increasing the density leads to a collapse transition at lower values of nonadditivity.     

Consequences of nuclear electron capture in core collapse supernovae

The most important weak nuclear interaction to the dynamics of stellar core collapse is electron capture, primarily on nuclei with masses larger than 60. In prior simulations of core collapse, electron capture on these nuclei has been treated in a highly parametrized fashion, if not ignored. With realistic treatment of electron capture on heavy nuclei come significant changes in the hydrodynamics of core collapse and bounce. We discuss these as well as the ramifications for the postbounce evolution in core collapse supernovae.