36,517 research outputs found
Keck IR Spectroscopy of WZ Sge: Detection of Molecular Emission from the Accretion Disk
Time-resolved IR spectroscopy of WZ Sge was obtained using NIRSPEC on Keck
II. We detect CO and H emission from the accretion disk placing WZ
Sge into a rarefied class of astronomical objects including YSOs and high
luminosity early-type stars. During the eclipse phase, the molecular emission
greatly weakens but no firm evidence for the secondary star is seen allowing
new limits on its luminosity to be determined. The detection of molecular
emission provides physical properties within the outer disk of T=3000K and
N cm. Such a cool, dense region, not associated with areas
of H I and He I emission, provides the first observational confirmation of
predictions made by accretion disk models.Comment: 10 pages, 3 figures. Accepted for publication in ApJ Letter
Polymeric foams from cross-linkable poly-N-ary lenebenzimidazoles
Foamed cross-linked poly-N-arylenebinzimidazoles are prepared by mixing an organic tetraamine and an ortho substituted aromatic dicarboxylic acid anhydride in the presence of a blowing agent, and then heating the prepolymer to a temperature sufficient to complete polymerization and foaming of the reactants. In another embodiment of the process, the reactants are heated to form a prepolymer. The prepolymer is then cured at higher temperatures to complete foaming and polymerization
Metal drilling with portable hand drills
Study of metal drilling solves problems of excessive burring, oversized holes, and out-of-round holes. Recommendations deal with using the proper chemical coolants, applying the coolants effectively, employing cutting oils, and dissipating the heat caused by drilling
Generation of Cosmological Seed Magnetic Fields from Inflation with Cutoff
Inflation has the potential to seed the galactic magnetic fields observed
today. However, there is an obstacle to the amplification of the quantum
fluctuations of the electromagnetic field during inflation: namely the
conformal invariance of electromagnetic theory on a conformally flat underlying
geometry. As the existence of a preferred minimal length breaks the conformal
invariance of the background geometry, it is plausible that this effect could
generate some electromagnetic field amplification. We show that this scenario
is equivalent to endowing the photon with a large negative mass during
inflation. This effective mass is negligibly small in a radiation and matter
dominated universe. Depending on the value of the free parameter of the theory,
we show that the seed required by the dynamo mechanism can be generated. We
also show that this mechanism can produce the requisite galactic magnetic field
without resorting to a dynamo mechanism.Comment: Latex, 16 pages, 2 figures, 4 references added, minor corrections;
v4: more references added, boundary term written in a covariant form,
discussion regarding other gauge fields added, submitted to PRD; v5: matched
with the published versio
Sub-Milliarcsecond Precision of Pulsar Motions: Using In-Beam Calibrators with the VLBA
We present Very Long Baseline Array phase-referenced measurements of the
parallax and proper motion of two pulsars, B0919+06 and B1857-26.
Sub-milliarcsecond positional accuracy was obtained by simultaneously observing
a weak calibrator source within the 40' field of view of the VLBA at 1.5 GHz.
We discuss the merits of using weak close calibrator sources for VLBI
observations at low frequencies, and outline a method of observation and data
reduction for these type of measurements. For the pulsar B1919+06 we measure a
parallax of 0.31 +/- 0.14 mas. The accuracy of the proper motions is
approximately 0.5 mas, an order of magnitude improvement over most previous
determinations.Comment: 11 pages plus 4 figures. In press, Astronomical Journa
Heisenberg exchange in magnetic monoxides
The superexchange intertacion in transition-metal oxides, proposed initially
by Anderson in 1950, is treated using contemporary tight-binding theory and
existing parameters. We find also a direct exchange for nearest-neighbor metal
ions, larger by a factor of order five than the superexchange. This direct
exchange arises from Vddm coupling, rather than overlap of atomic charge
densities, a small overlap exchange contribution which we also estimate. For
FeO and CoO there is also an important negative contribution, related to Stoner
ferromagnetism, from the partially filled minority-spin band which broadens
when ionic spins are aligned. The corresponding J1 and J2 parameters are
calculated for MnO, FeO, CoO, and NiO. They give good accounts of the Neel and
the Curie-Weiss temperatures, show appropriate trends, and give a reasonable
account of their volume dependences. For MnO the predicted value for the
magnetic susceptibility at the Neel temperature and the crystal distortion
arising from the antiferromagnetic transition were reasonably well given.
Application to CuO2 planes in the cuprates gives J=1220oK, compared to an
experimental 1500oK, and for LiCrO2 gives J1=4 50oK compared to an experimental
230oK.Comment: 21 pages, 1 figure, submitted to Phys. Rev. B 1/19/07. Realized
J=4V^2/U applies generally, as opposed to J=2V^2/U from one-electron theory
(1/28 revision
Production Systems Involving Stocker Cattle and Soft Red Winter Wheat
A three year study at the Livestock and Forestry Research Station near Batesville, Arkansas evaluated production systems involving stocker cattle and soft red winter wheat. Grazing of soft red winter wheat forage from October through February followed by harvesting wheat grain or grazing through April with stocker cattle offers an alternative to conventional farming. Soft red winter wheat, when planted by September 15, produces an ample supply of high-quality forage that supports rapid growth of stocker cattle during October through April. Net income from stocker cattle averaged over 75,000,000 per year if 750,000 acres of wheat are grazed
Optical matrix elements in tight-binding models with overlap
We investigate the effect of orbital overlap on optical matrix elements in
empirical tight-binding models. Empirical tight-binding models assume an
orthogonal basis of (atomiclike) states and a diagonal coordinate operator
which neglects the intra-atomic part. It is shown that, starting with an atomic
basis which is not orthogonal, the orthogonalization process induces
intra-atomic matrix elements of the coordinate operator and extends the range
of the effective Hamiltonian. We analyze simple tight-binding models and show
that non-orthogonality plays an important role in optical matrix elements. In
addition, the procedure gives formal justification to the nearest-neighbor
spin-orbit interaction introduced by Boykin [Phys. Rev \textbf{B} 57, 1620
(1998)] in order to describe the Dresselahaus term which is neglected in
empirical tight-binding models.Comment: 16 pages 6 figures, to appear in Phys. Rev.
The extensive nature of group quality
We consider groups of interacting nodes engaged in an activity as many-body,
complex systems and analyse their cooperative behaviour from a mean-field point
of view. We show that inter-nodal interactions rather than accumulated
individual node strengths dominate the quality of group activity, and give rise
to phenomena akin to phase transitions, where the extensive relationship
between group quality and quantity reduces. The theory is tested using
empirical data on quantity and quality of scientific research groups, for which
critical masses are determined.Comment: 6 pages, 6 figures containing 13 plots. Very minor changes to
coincide with published versio
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