438 research outputs found
Killing spinors in supergravity with 4-fluxes
We study the spinorial Killing equation of supergravity involving a torsion
3-form \T as well as a flux 4-form \F. In dimension seven, we construct
explicit families of compact solutions out of 3-Sasakian geometries, nearly
parallel \G_2-geometries and on the homogeneous Aloff-Wallach space. The
constraint \F \cdot \Psi = 0 defines a non empty subfamily of solutions. We
investigate the constraint \T \cdot \Psi = 0, too, and show that it singles
out a very special choice of numerical parameters in the Killing equation,
which can also be justified geometrically
Unit Killing Vector Fields on Nearly Kahler Manifolds
We study 6-dimensional nearly Kahler manifolds admitting a Killing vector
field of unit length. In the compact case it is shown that up to a finite cover
there is only one geometry possible, that of the 3--symmetric space
Pseudo-Riemannian manifolds with recurrent spinor fields
The existence of a recurrent spinor field on a pseudo-Riemannian spin
manifold is closely related to the existence of a parallel
1-dimensional complex subbundle of the spinor bundle of . We
characterize the following simply connected pseudo-Riemannian manifolds
admitting such subbundles in terms of their holonomy algebras: Riemannian
manifolds; Lorentzian manifolds; pseudo-Riemannian manifolds with irreducible
holonomy algebras; pseudo-Riemannian manifolds of neutral signature admitting
two complementary parallel isotropic distributions.Comment: 13 pages, the final versio
Kinetics of the Multiferroic Switching in MnWO
The time dependence of switching multiferroic domains in MnWO has been
studied by time-resolved polarized neutron diffraction. Inverting an external
electric field inverts the chiral magnetic component within rise times ranging
between a few and some tens of milliseconds in perfect agreement with
macroscopic techniques. There is no evidence for any faster process in the
inversion of the chiral magnetic structure. The time dependence is well
described by a temperature-dependent rise time suggesting a well-defined
process of domain reversion. As expected, the rise times decrease when heating
towards the upper boundary of the ferroelectric phase. However, switching also
becomes faster upon cooling towards the lower boundary, which is associated
with a first-order phase transition
On the characteristic connection of gwistor space
We give a brief presentation of gwistor space, which is a new concept from
G_2 geometry. Then we compute the characteristic torsion T^c of the gwistor
space of an oriented Riemannian 4-manifold with constant sectional curvature k
and deduce the condition under which T^c is \nabla^c-parallel; this allows for
the classification of the G_2 structure with torsion and the characteristic
holonomy according to known references. The case with the Einstein base
manifold is envisaged.Comment: Many changes since first version, including title; Central European
Journal of Mathematics, 201
Large-Scale Outflows in Edge-on Seyfert Galaxies. III. Kiloparsec-Scale Soft X-ray Emission
We present ROSAT PSPC and HRI images of eight galaxies selected from a
distance-limited sample of 22 edge-on Seyfert galaxies. Kiloparsec-scale soft
X-ray nebulae extend along the galaxy minor axes in three galaxies (NGC 2992,
NGC 4388 and NGC 5506). The extended X-ray emission has 0.2-2.4 keV X-ray
luminosities of . The X-ray nebulae are
roughly co-spatial with the large-scale radio emission, suggesting that both
are produced by large-scale galactic outflows. Assuming pressure balance
between the radio and X-ray plasmas, the X-ray filling factor is \gapprox
10^4 times larger than the radio plasma filling factor, suggesting that
large-scale outflows in Seyfert galaxies are predominantly winds of thermal
X-ray emitting gas. We favor an interpretation in which large-scale outflows
originate as AGN-driven jets that entrain and heat gas on kpc scales as they
make their way out of the galaxy. AGN- and starburst-driven winds are also
possible explanations in cases where the winds are oriented along the rotation
axis of the galaxy disk.Comment: 24 pages, 7 ps figures, AASTEX 4.0, accepted for ApJ April 1, 199
K\"ahlerian Twistor Spinors
On a K\"ahler spin manifold K\"ahlerian twistor spinors are a natural
analogue of twistor spinors on Riemannian spin manifolds. They are defined as
sections in the kernel of a first order differential operator adapted to the
K\"ahler structure, called K\"ahlerian twistor (Penrose) operator. We study
K\"ahlerian twistor spinors and give a complete description of compact K\"ahler
manifolds of constant scalar curvature admitting such spinors. As in the
Riemannian case, the existence of K\"ahlerian twistor spinors is related to the
lower bound of the spectrum of the Dirac operator.Comment: shorter version; to appear in Math.
Formation of PAHs and Carbonaceous Solids in Gas-Phase Condensation Experiments
Carbonaceous grains represent a major component of cosmic dust. In order to
understand their formation pathways, they have been prepared in the laboratory
by gas-phase condensation reactions such as laser pyrolysis and laser ablation.
Our studies demonstrate that the temperature in the condensation zone
determines the formation pathway of carbonaceous particles. At temperatures
lower than 1700 K, the condensation by-products are mainly polycyclic aromatic
hydrocarbons (PAHs), that are also the precursors or building blocks for the
condensing soot grains. The low-temperature condensates contain PAH mixtures
that are mainly composed of volatile 3-5 ring systems. At condensation
temperatures higher than 3500 K, fullerene-like carbon grains and fullerene
compounds are formed. Fullerene fragments or complete fullerenes equip the
nucleating particles. Fullerenes can be identified as soluble components.
Consequently, condensation products in cool and hot astrophysical environments
such as cool and hot AGB stars or Wolf Rayet stars should be different and
should have distinct spectral properties.Comment: 7 pages, 5 figure
Z_2-Bi-Gradings, Majorana Modules and the Standard Model Action
The action functional of the Standard Model of particle physics is intimately
related to a specific class of first order differential operators called Dirac
operators of Pauli type ("Pauli-Dirac operators"). The aim of this article is
to carefully analyze the geometrical structure of this class of Dirac operators
on the basis of real Dirac operators of simple type. On the basis of simple
type Dirac operators, it is shown how the Standard Model action (STM action)
may be viewed as generalizing the Einstein-Hilbert action in a similar way the
Einstein-Hilbert action is generalized by a cosmological constant. Furthermore,
we demonstrate how the geometrical scheme presented allows to naturally
incorporate also Majorana mass terms within the Standard Model. For reasons of
consistency these Majorana mass terms are shown to dynamically contribute to
the Einstein-Hilbert action by a "true" cosmological constant. Due to its
specific form, this cosmological constant can be very small. Nonetheless, this
cosmological constant may provide a significant contribution to dark
matter/energy. In the geometrical description presented this possibility arises
from a subtle interplay between Dirac and Majorana masses
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