B. Müller

Effects of inelastic neutrino-nucleus scattering on supernova dynamics and radiated neutrino spectra.

Based on the shell model for Gamow-Teller and the random phase approximation for forbidden transitions, we calculate cross sections for inelastic neutrino-nucleus scattering (INNS) under supernova (SN) conditions, assuming a matter composition given by nuclear statistical equilibrium. The cross sections are incorporated into state-of-the-art stellar core-collapse simulations with detailed energy-dependent neutrino transport. While no significant effect on the SN dynamics is observed, INNS increases the neutrino opacities noticeably and strongly reduces the high-energy tail of the neutrino spectrum emitted in the neutrino burst at shock breakout. Relatedly the expected event rates for the observation of such neutrinos by earthbound detectors are reduced by up to about 60%.

Neutrino oscillation signatures of oxygen-neon-magnesium supernovae

We discuss the flavor conversion of neutrinos from core-collapse supernovae that have oxygen-neon-magnesium (ONeMg) cores. Using the numerically calculated evolution of the star up to 650 ms post bounce, we find that, for the normal mass hierarchy, the electron neutrino flux in a detector shows signatures of two typical features of an ONeMg-core supernova: a sharp step in the density profile at the base of the He shell and a faster shock wave propagation compared to iron core supernovae. Before the shock hits the density step (t < or approx. 150 ms), the survival probability of electron neutrinos above {approx}20 MeV of energy is about {approx}0.68, in contrast to values of {approx}0.32 or less for an iron core supernova. The passage of the shock through the step and its subsequent propagation cause a decrease of the survival probability and a decrease of the amplitude of oscillations in the Earth, reflecting the transition to a more adiabatic propagation inside the star. These changes affect the lower energy neutrinos first; they are faster and more sizable for larger {theta}{sub 13}. They are unique of ONeMg-core supernovae, and give the possibility to test the speed of the shock wave. The time modulation ofmorexa0» the Earth effect and its negative sign at the neutronization peak are the most robust signatures in a detector.«xa0less

The SuperN-Project: Understanding Core Collapse Supernovae

We give an overview of the problems and the current status of (core collapse) supernova modeling, and discuss the system of equations and the algorithm for its solution that are employed in our code. We also report on our recent progress, and focus on the ongoing calculations that are performed on the SX-8 at the HLRS Stuttgart.

Oscillating modes of driven colloids in overdamped systems

Microscopic colloidal particles suspended in liquids are a prominent example of an overdamped system where viscous forces dominate over inertial effects. Frequently, colloids are used as sensitive probes, e.g., in biophysical applications from which molecular forces are inferred. The interpretation of such experiments rests on the assumption that, even when the particles are driven, the liquid remains in equilibrium. Here we experimentally demonstrate that this is not valid for particles in viscoelastic fluids. Even at small driving forces, we observe particle oscillations with several tens of seconds. They are attributed to non-equilibrium fluctuations of the fluid being excited by the particle’s motion. The oscillatory dynamics is in quantitative agreement with an overdamped Langevin equation with negative friction-memory term being equivalent to a stochastically driven underdamped oscillator. Such oscillatory modes are expected to widen the use of colloids as model systems but must also be considered in colloidal probe experiments.The motion of microparticles suspended in liquids is assumed to be dominated by viscous forces. Here, Berner et al. challenge this consensus by observing underdamped particle oscillations in a viscoelastic fluid and attributing it to the non-equilibrium fluctuations of liquid excited by particles.

Heat radiation and transfer for point particles in arbitrary geometries

We study heat radiation and heat transfer for pointlike particles in a system of other objects. Starting from exact many-body expressions found from scattering theory and fluctuational electrodynamics, we find that transfer and radiation for point particles are given in terms of the Greens function of the system in the absence of the point particles. These general expressions contain no approximation for the surrounding objects. As an application, we compute the heat transfer between two point particles in the presence of a sphere of arbitrary size and show that the transfer is enhanced by several orders of magnitude through the presence of the sphere, depending on the materials. Furthermore, we compute the heat emission of a point particle in front of a planar mirror. Finally, we show that a particle placed inside a spherical mirror cavity does not radiate energy.

The SuperN-Project: Current Progress in Modelling Core Collapse Supernovae

We give an overview of the problems and the current status of our twodimensional (core collapse) supernova modelling, and discuss the system of equations and the algorithm for its solution that are employed in our code. In particular we report our recent progress, and focus on the ongoing calculations that are performed on the NEC SX-8 at the HLRS Stuttgart. Especially, we will argue that it might be possible that neutrino-driven supernova explosions set in at much later times than previously considered. This, of course, enhances the need of a code that can make efficient use of the multi-node capability of the NEC SX-8 for long-time simulations of the postbounce evolution of collapsing stellar cores.

Supernova simulations with the radiation hydrodynamics code PROMETHEUS/VERTEX

We give an overview of the problems and the current status of our two dimensional (core collapse) supernova modelling, and discuss the system of equations and the algorithm for its solution that are employed in our code. In particular we report our recent progress, and focus on the ongoing calculations that are performed on the NEC SX-8 at the HLRS Stuttgart. We also discuss recent optimizations carried out within the framework of the Teraflop Workbench, and comment on the parallel performance of the code, stressing the importance of developing a MPI version of the employed hydrodynamics module.

Anisotropic particles near surfaces: Propulsion force and friction

We theoretically study the phenomenon of self-propulsion through Casimir forces in thermal non-equilibrium. Using fluctuational electrodynamics, we derive a formula for the self-propulsion force for an arbitrary small object in two scenarios, i) for the object being isolated, and ii) for the object being close to a planar surface. In the latter case, the self-propulsion force (i.e., the force parallel to the surface) increases with decreasing distance, i.e., it couples to the near-field. We numerically calculate the lateral force acting on a hot spheroid near a surface and show that it can be as large as the gravitational force, thus being potentially measurable in fly-by experiments. We close by linking our results to well-known relations of linear response theory in fluctuational electrodynamics: Looking at the friction of the anisotropic object for constant velocity, we identify a correction term that is additional to the typically used approach.