13,840 research outputs found
Generalized Spinfoams
We reconsider the spinfoam dynamics that has been recently introduced, in the
generalized Kaminski-Kisielowski-Lewandowski (KKL) version where the foam is
not dual to a triangulation. We study the Euclidean as well as the Lorentzian
case. We show that this theory can still be obtained as a constrained BF theory
satisfying the simplicity constraint, now discretized on a general oriented
2-cell complex. This constraint implies that boundary states admit a (quantum)
geometrical interpretation in terms of polyhedra, generalizing the tetrahedral
geometry of the simplicial case. We also point out that the general solution to
this constraint (imposed weakly) depends on a quantum number r_f in addition to
those of loop quantum gravity. We compute the vertex amplitude and recover the
KKL amplitude in the Euclidean theory when r_f=0. We comment on the eventual
physical relevance of r_f, and the formal way to eliminate it.Comment: 16 pages, 3 figure
Formation of hot subdwarf B stars with neutron star components
Binary population synthesis predicts the existence of subdwarf B stars (sdBs)
with neutron star (NS) or black hole (BH) companions. We systematically
investigate the formation of sdB+NS binaries from binary evolution and aim to
obtain some clues for a search for such systems. We started from a series of
MS+NS systems and determined the parameter spaces for producing sdB+NS binaries
from the stable Roche-lobe overflow (RLOF) channel and from the common envelope
(CE) ejection channel. Various NS accretion efficiencies and NS masses were
examined to investigate the effects they have. We show the characteristics of
the produced sdB+NS systems, such as the mass of components, orbital period,
the semi-amplitude of the radial velocity (K), and the spin of the NS
component. In the stable RLOF channel, the orbital period of sdB+NS binaries
produced in this way ranges from several days to more than 1000 days and moves
toward the short-period (~ hr) side with increasing initial MS mass. the sdB+NS
systems that result from CE ejection have very short orbital periods and then
high values of K (up to 800km s^-1). Such systems are born in very young
populations (younger than 0.3 Gyr) and are potential gravitational wave sources
that might be resolved by the Laser Interferometer Space Antenna (LISA) in the
future. Gravitational wave radiation may again bring them into contact on a
timescale of only ~Myr. As a consequence, they are rare and hard to discover.
The pulsar signal is likely a feature of sdB+NS systems caused by stable RLOF,
and some NS components in sdB binaries may be millisecond pulsars.Comment: 12 pages, 6 figures, 4 tables. Accepted for publication in A&
Incompressible pairwise incompressible surfaces in almost alternating knot complements
AbstractIn this paper, we deal with incompressible pairwise incompressible surfaces in almost alternating knot complements. We show that the genus of a surface in an almost alternating knot exterior equals zero if there are two, four or six boundary components in the surface
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