8,781 research outputs found
Canonical Sasakian Metrics
Let be a closed manifold of Sasaki type. A polarization of is defined
by a Reeb vector field, and for one such, we consider the set of all Sasakian
metrics compatible with it. On this space, we study the functional given by the
squared -norm of the scalar curvature. We prove that its critical points,
or canonical representatives of the polarization, are Sasakian metrics that are
transversally extremal. We define a Sasaki-Futaki invariant of the
polarization, and show that it obstructs the existence of constant scalar
curvature representatives. For a fixed CR structure of Sasaki type, we define
the Sasaki cone of structures compatible with this underlying CR structure, and
prove that the set of polarizations in it that admit a canonical representative
is open.Comment: 36 pages, minor corrections made, example adde
Comment on "Two Phase Transitions in the Fully frustrated XY Model"
The conclusions of a recent paper by Olsson (Phys. Rev. Lett. 75, 2758
(1995), cond-mat/9506082) about the fully frustrated XY model in two dimensions
are questioned. In particular, the evidence presented for having two separate
chiral and U(1) phase transitions are critically considered.Comment: One page one table, to Appear in Physical Review Letter
Mononchid nematodes from Spain : one known and another new species of the genus Miconchus Andrassy, 1958
Evidence of non-thermal X-ray emission from HH 80
Protostellar jets appear at all stages of star formation when the accretion
process is still at work. Jets travel at velocities of hundreds of km/s,
creating strong shocks when interacting with interstellar medium. Several cases
of jets have been detected in X-rays, typically showing soft emission. For the
first time, we report evidence of hard X-ray emission possibly related to
non-thermal processes not explained by previous models of the post-shock
emission predicted in the jet/ambient interaction scenario. HH 80 is located at
the south head of the jet associated to the massive protostar IRAS 18162-2048.
It shows soft and hard X-ray emission in regions that are spatially separated,
with the soft X-ray emission region situated behind the region of hard X-ray
emission. We propose a scenario for HH 80 where soft X-ray emission is
associated to thermal processes from the interaction of the jet with denser
ambient matter and the hard X-ray emission is produced by synchrotron radiation
at the front shock.Comment: Accepted for publication in ApJ
The one-loop elastic coefficients for the Helfrich membrane in higher dimensions
Using a covariant geometric approach we obtain the effective bending
couplings for a 2-dimensional rigid membrane embedded into a
-dimensional Euclidean space. The Hamiltonian for the membrane has three
terms: The first one is quadratic in its mean extrinsic curvature. The second
one is proportional to its Gaussian curvature, and the last one is proportional
to its area. The results we obtain are in agreement with those finding that
thermal fluctuations soften the 2-dimensional membrane embedded into a
3-dimensional Euclidean space.Comment: 9 page
The role of temperature in the magnetic irreversibility of type-I Pb superconductors
Evidence of how temperature takes part in the magnetic irreversibility in the
intermediate state of a cylinder and various disks of pure type-I
superconducting lead is presented. Isothermal measurements of first
magnetization curves and magnetic hysteresis cycles are analyzed in a reduced
representation that defines an equilibrium state for flux penetration in all
the samples and reveals that flux expulsion depends on temperature in the disks
but not in the cylinder. The magnetic field at which irreversibility sets in
along the descending branch of the hysteresis cycle and the remnant
magnetization at zero field are found to decrease with temperature in the
disks. The contributions to irreversibility of the geometrical barrier and the
energy minima associated to stress defects that act as pinning centers on
normal-superconductor interfaces are discussed. The differences observed among
the disks are ascribed to the diverse nature of the stress defects in each
sample. The pinning barriers are suggested to decrease with the magnetic field
to account for these results
Treatment of slaughterhouse wastewater in a UASB reactor and an anaerobic filter
A study was performed to assess the feasibility of anaerobic treatment of slaughterhouse wastewaters in a UASB (Upflow Anaerobic Sludge Blanket) reactor and in an AF (Anaerobic Filter). Among the different streams generated, the slaughter line showed the highest organic content with an average COD of 8000 mg/l, of which 70% was proteins. The suspended solids content represented between 15 and 30% of the COD. Both reactors had a working volume of 21. They were operated at 37°C. The UASB reactor was run at OLR (Organic Loading Rates) of 1–6.5 kg COD/m3/day. The COD removal was 90% for OLR up to 5 kg COD/m3/day and 60% for an OLR of 6.5 kg COD/m3/day. For similar organic loading rates, the AF showed lower removal efficiencies and lower percentages of methanization. At higher OLR sludge, flotation occurred and consequently the active biomass was washed out from the filter. The results indicated that anaerobic treatment systems are applicable to slaughterhouse wastewaters and that the UASB reactor shows a better performance, giving higher COD removal efficiencies than the AF
Effect of the Reference Line on Main Roughness Parameters
In the present paper the influence of the reference system with regard to the characterization of the surface finishing is analyzed. The effect of the reference system’s choice on the most representative surface finishing parameters (e.g. roughness average Ra and root mean
square values Rq) is studied. The study can also be applied to their equivalent parameters in waviness and primary profiles. Based on ISO and ASME standards, three different types of regression lines (center, mean and orthogonal) are theoretically and experimentally analyzed, identifying the validity and applicability fields of each one depending on profile’s geometry
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