3,052 research outputs found
The spinorial geometry of supersymmetric heterotic string backgrounds
We determine the geometry of supersymmetric heterotic string backgrounds for
which all parallel spinors with respect to the connection with
torsion , the NSNS three-form field strength, are Killing. We find
that there are two classes of such backgrounds, the null and the timelike. The
Killing spinors of the null backgrounds have stability subgroups
K\ltimes\bR^8 in , for , SU(4), , and , and the Killing spinors of the timelike backgrounds have
stability subgroups , SU(3), SU(2) and . The former admit a single
null -parallel vector field while the latter admit a timelike and
two, three, five and nine spacelike -parallel vector fields,
respectively. The spacetime of the null backgrounds is a Lorentzian
two-parameter family of Riemannian manifolds with skew-symmetric torsion.
If the rotation of the null vector field vanishes, the holonomy of the
connection with torsion of is contained in . The spacetime of time-like
backgrounds is a principal bundle with fibre a Lorentzian Lie group and
base space a suitable Riemannian manifold with skew-symmetric torsion. The
principal bundle is equipped with a connection which determines the
non-horizontal part of the spacetime metric and of . The curvature of
takes values in an appropriate Lie algebra constructed from that of
. In addition has only horizontal components and contains the
Pontrjagin class of . We have computed in all cases the Killing spinor
bilinears, expressed the fluxes in terms of the geometry and determine the
field equations that are implied by the Killing spinor equations.Comment: 73pp. v2: minor change
Perinatal Plasma Monocyte Chemotactic Protein-1 Concentrations in Intrauterine Growth Restriction
Monocyte-chemotactic-protein-1 (MCP-1) plays vital roles in immune response, angiogenesis, and pregnancy outcome. We investigated plasma MCP-1 concentrations in 40 mothers and their 20 intrauterine-growth-restricted (IUGR) and 20 appropriate-for-gestational-age (AGA) fetuses and neonates on postnatal days 1 (N1) and 4 (N4). Maternal and fetal MCP-1
concentrations were decreased (P<001 and P = .018, resp.), whereas N1 MCP-1 concentrations were elevated in
IUGR group (P = .012). In both groups, fetal MCP-1 concentrations were lower compared to N1 and N4 ones
(P = .045, P = .012, resp., for AGA, P
< .001 in each case for IUGR). Reduced maternal and fetal MCP-1
concentrations in IUGR may reflect failure of trophoblast invasion, suggesting that down-regulation of MCP-1 may be involved in the pathogenesis of IUGR. Increased MCP-1 concentrations in IUGR neonates and higher postnatal ones in all infants may be attributed to gradual initiation of ex utero angiogenesis, which is possibly enhanced in IUGR
Penrose limits, supergravity and brane dynamics
We investigate the Penrose limits of classical string and M-theory
backgrounds. We prove that the number of (super)symmetries of a supergravity
background never decreases in the limit. We classify all the possible Penrose
limits of AdS x S spacetimes and of supergravity brane solutions. We also
present the Penrose limits of various other solutions: intersecting branes,
supersymmetric black holes and strings in diverse dimensions, and cosmological
models. We explore the Penrose limit of an isometrically embedded spacetime and
find a generalisation to spaces with more than one time. Finally, we show that
the Penrose limit is a large tension limit for all branes including those with
fields of Born--Infeld type.Comment: 67 page
Radio Astronomy
Contains reports on five research projects.National Aeronautics and Space Administration (Grant NGL 22-009-016)National Science Foundation (Grant GP-14854)Joint Services Electronics Programs (U. S. Army, U. S. Navy, and U. S. Air Force) under Contract DA 28-043-AMC-02536(E)National Science Foundation (Grant GP-13056
Radio Astronomy
Contains reports on six research projects.National Aeronautics and Space Administration (Grant NGL-22-009-016)National Science Foundation (Grant GP-8415
Impulsive electron acceleration by Gravitational Waves
We investigate the non-linear interaction of a strong Gravitational Wave with
the plasma during the collapse of a massive magnetized star to form a black
hole, or during the merging of neutron star binaries (central engine). We found
that under certain conditions this coupling may result in an efficient energy
space diffusion of particles. We suggest that the atmosphere created around the
central engine is filled with 3-D magnetic neutral sheets (magnetic nulls). We
demonstrate that the passage of strong pulses of Gravitational Waves through
the magnetic neutral sheets accelerates electrons to very high energies.
Superposition of many such short lived accelerators, embedded inside a
turbulent plasma, may be the source for the observed impulsive short lived
bursts. We conclude that in several astrophysical events, gravitational pulses
may accelerate the tail of the ambient plasma to very high energies and become
the driver for many types of astrophysical bursts.Comment: 13 pages, 8 figures, accepted to The Astrophysical Journa
Radio Astronomy
Contains reports on isx research projects.National Aeronautics and Space Administration, Langley Research Center (Contract NAS1-10693)National Science Foundation (Grant GP-21348)National Science Foundation (Grant GP-14589)California Institute of Technology Contract 952568Joint Services Electronics Programs (U. S. Army, U. S. Navy, and U. S. Air Force) under Contract DAAB07-71-C-030
Radio Astronomy
Contains research objectives, summary of research and reports on six research projects.National Aeronautics and Space Administration (Grant NGL 22-009-016)National Aeronautics and Space Administration (Grant NGR 22-009-421)National Aeronautics and Space Administration (Langley Research Center Contract NAS1-10693)National Science Foundation (Grant GP-20769A#2)National Science Foundation (Grant GP-21348A#2)Joint Services Electronics Programs (U. S. Army, U. S. Navy, and U. S. Air Force) under Contract DAAB07-71-C-0300U. S. Air Force -- Wright-Patterson (Contract F33615-72-C-2129)California Institute of Technology Contract 952568M.I.T. Sloan Fund for Basic Research (Grant 241)M.I.T. Thomas C. Desmond Fun
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