Wolfgang Keil

Ledoux Convection in Protoneutron Stars?A Clue to Supernova Nucleosynthesis?

Two-dimensional hydrodynamical simulations of the deleptonization of a newly formed neutron star (NS) were performed. Driven by negative lepton fraction and entropy gradients, convection starts near the neutrinosphere about 20-30 ms after core bounce but moves deeper into the protoneutron star (PNS), and after about 1 s the whole PNS is convective. The deleptonization of the star proceeds much faster than in the corresponding spherically symmetrical model because the lepton flux and the neutrino (ν) luminosities increase by up to a factor of 2. The convection below the neutrinosphere raises the neutrinospheric temperatures and mean energies of the emitted νs by 10%-20%. This can have important implications for the supernova (SN) explosion mechanism, and it changes the detectable ν signal from the Kelvin-Helmholtz cooling of the PNS. In particular, the enhanced νe flux relative to the e flux during the early post-bounce evolution might solve the overproduction problem of certain elements in the neutrino-heated ejecta in models of Type II SN explosions.

NUCLEON SPIN FLUCTUATIONS AND THE SUPERNOVA EMISSION OF NEUTRINOS AND AXIONS

In the hot and dense medium of a supernova (SN) core, the nucleon spins fluctuate so fast that the axial-vector neutrino opacity and the axion emissivity are expected to be significantly modified. Axions with

A Fresh Look at Axions and SN 1987A

m_a\alt10^{-2}\,{\rm eV}

Coverage, continuity, and visual cortical architecture

are not excluded by SN~1987A. A substantial transfer of energy in neutrino-nucleon (

Reorganization of columnar architecture in the growing visual cortex

\nu N

Can retinal ganglion cell dipoles seed iso-orientation domains in the visual cortex?

) collisions is enabled which may alter the spectra of SN neutrinos relative to calculations where energy-conserving

Random Wiring, Ganglion Cell Mosaics, and the Functional Architecture of the Visual Cortex

\nu N

Neutrinos and the evolution of newly born neutron stars

collisions had been assumed near the neutrinosphere.

Pinwheel crystallization in a dimension reduction model of visual cortical development

We re-examine the very stringent limits on the axion mass based on the strength and duration of the neutrino signal from SN 1987A, in the light of new measurements of the axial-vector coupling strength of nucleons, possible suppression of axion emission due to many-body effects, and additional emission processes involving pions. The suppression of axion emission due to nucleon spin fluctuations induced by many-body effects degrades previous limits by a factor of about 2. Emission processes involving thermal pions can strengthen the limits by a factor of 3-4 within a perturbative treatment that neglects saturation of nucleon spin fluctuations. Inclusion of saturation effects, however, tends to make the limits less dependent on pion abundances. The resulting axion mass limit also depends on the precise couplings of the axion and ranges from 0.5x10**(-3) eV to 6x10**(-3) eV.

Convection in newly born neutron stars

BackgroundThe primary visual cortex of many mammals contains a continuous representation of visual space, with a roughly repetitive aperiodic map of orientation preferences superimposed. It was recently found that orientation preference maps (OPMs) obey statistical laws which are apparently invariant among species widely separated in eutherian evolution. Here, we examine whether one of the most prominent models for the optimization of cortical maps, the elastic net (EN) model, can reproduce this common design. The EN model generates representations which optimally trade of stimulus space coverage and map continuity. While this model has been used in numerous studies, no analytical results about the precise layout of the predicted OPMs have been obtained so far.ResultsWe present a mathematical approach to analytically calculate the cortical representations predicted by the EN model for the joint mapping of stimulus position and orientation. We find that in all the previously studied regimes, predicted OPM layouts are perfectly periodic. An unbiased search through the EN parameter space identifies a novel regime of aperiodic OPMs with pinwheel densities lower than found in experiments. In an extreme limit, aperiodic OPMs quantitatively resembling experimental observations emerge. Stabilization of these layouts results from strong nonlocal interactions rather than from a coverage-continuity-compromise.ConclusionsOur results demonstrate that optimization models for stimulus representations dominated by nonlocal suppressive interactions are in principle capable of correctly predicting the common OPM design. They question that visual cortical feature representations can be explained by a coverage-continuity-compromise.