2,352 research outputs found
Large quantum gravity effects: Cylindrical waves in four dimensions
Linearly polarized cylindrical waves in four-dimensional vacuum gravity are
mathematically equivalent to rotationally symmetric gravity coupled to a
Maxwell (or Klein-Gordon) field in three dimensions. The quantization of this
latter system was performed by Ashtekar and Pierri in a recent work. Employing
that quantization, we obtain here a complete quantum theory which describes the
four-dimensional geometry of the Einstein-Rosen waves. In particular, we
construct regularized operators to represent the metric. It is shown that the
results achieved by Ashtekar about the existence of important quantum gravity
effects in the Einstein-Maxwell system at large distances from the symmetry
axis continue to be valid from a four-dimensional point of view. The only
significant difference is that, in order to admit an approximate classical
description in the asymptotic region, states that are coherent in the Maxwell
field need not contain a large number of photons anymore. We also analyze the
metric fluctuations on the symmetry axis and argue that they are generally
relevant for all of the coherent states.Comment: Version accepted for publication in Int. J. Mod. Phys.
Direct measurement of the 14N(p,g)15O S-factor
We have measured the 14N(p,g)15O excitation function for energies in the
range E_p = 155--524 keV. Fits of these data using R-matrix theory yield a
value for the S-factor at zero energy of 1.64(17) keV b, which is significantly
smaller than the result of a previous direct measurement. The corresponding
reduction in the stellar reaction rate for 14N(p,g)15O has a number of
interesting consequences, including an impact on estimates for the age of the
Galaxy derived from globular clusters.Comment: 5 pages, 3 figures, submitted to Phys. Rev. Let
Chemical and stable isotope composition (18O/16O, 2H/1H) of formation waters from the Carabobo Oilfield, Venezuela
In this short note, we present the first data on stable isotope composition of the oilfield waters from Carabobo area of the Faja PetrolĂfera del Orinoco âHugo ChĂĄvezâ (Orinoco Oil Belt). From a chemical point of view, the formation waters show a main Na-Cl level (TDS up to 30g/l) with a dilution trend toward Na-HCO3 composition (down to 1g/l). Until now, such a clear net chemical compositional trend was ascribed to a meteoric dilution (fresh/ brackish bicarbonate) of the seawater endmember (the saltiest chloride). The isotope results of this study reveal that the seawater mother water was modified during a high-temperature thrusting event (120â125°C), forming 18O-enriched diagenetic water (up to +4â°), which was diluted in recent times by glacial meltwater and presentday meteoric water. The hypothetical presence of flood by a meteoric paleo-water also offers new hints to explain the low API gravity (<10°API biodegraded, extra heavy oil) and composition of the local crude
Conductance Distributions in Random Resistor Networks: Self Averaging and Disorder Lengths
The self averaging properties of conductance are explored in random
resistor networks with a broad distribution of bond strengths
P(g)\simg^{\mu-1}. Distributions of equivalent conductances are estimated
numerically on hierarchical lattices as a function of size and distribution
tail parameter . For networks above the percolation threshold, convergence
to a Gaussian basin is always the case, except in the limit --> 0. A {\it
disorder length} is identified beyond which the system is effectively
homogeneous. This length diverges as ( is the
regular percolation correlation length exponent) as -->0. This suggest
that exactly the same critical behavior can be induced by geometrical disorder
and bu strong bond disorder with the bond occupation probability .
Only lattices at the percolation threshold have renormalized probability
distribution in a {\it Levy-like} basin. At the threshold the disorder length
diverges at a vritical tail strength as , with
, a new exponent. Critical path analysis is used in a generalized
form to give form to give the macroscopic conductance for lattice above .Comment: 16 pages plain TeX file, 6 figures available upon
request.IBC-1603-01
Polytopic bis(oxazoline)-based ligands for recoverable catalytic systems applied to the enantioselective Henry reaction
Several kinds of polytopic chiral ligands (including ditopic, tritopic and tetratopic), based on the bis(oxazoline) and azabis(oxazoline) motifs, have been tested in the preparation of recoverable catalytic systems for the Henry reaction. The results obtained with the different ligands are, in general, good, but they point to the existence of a delicate balance between the coordinating ability of the ligand, the catalytic activity and the recovery of the catalyst by formation of the coordination polymer, related to the easiness to form oligomeric species in solution
Modeling the dynamic rupture propagation on heterogeneous faults with rate- and state-dependent friction
We investigate the effects of non-uniform distribution of constitutive parameters on the dynamic propagation of
an earthquake rupture. We use a 2D finite difference numerical method and we assume that the dynamic rupture
propagation is governed by a rate- and state-dependent constitutive law. We first discuss the results of several
numerical experiments performed with different values of the constitutive parameters a (to account for the direct
effect of friction), b (controlling the friction evolution) and L (the characteristic length-scale parameter) to
simulate the dynamic rupture propagation on homogeneous faults. Spontaneous dynamic ruptures can be simulated
on velocity weakening (a < b) fault patches: our results point out the dependence of the traction and slip velocity
evolution on the adopted constitutive parameters. We therefore model the dynamic rupture propagation on
heterogeneous faults. We use in this study the characterization of different frictional regimes proposed by
Boatwright and Cocco (1996) based on different values of the constitutive parameters a, b and L. Our numerical
simulations show that the heterogeneities of the L parameter affect the dynamic rupture propagation, control
the peak slip velocity and weakly modify the dynamic stress drop and the rupture velocity. Moreover, a barrier
can be simulated through a large contrast of L parameter. The heterogeneity of a and b parameters affects the
dynamic rupture propagation in a more complex way. A velocity strengthening area (a > b) can arrest a dynamic
rupture, but can be driven to an instability if suddenly loaded by the dynamic rupture front. Our simulations
provide a picture of the complex interactions between fault patches having different frictional properties and illustrate
how the traction and slip velocity evolutions are modified during the propagation on heterogeneous
faults. These results involve interesting implications for slip duration and fracture energy
Asymptotic normalization coefficients (nuclear vertex constants) for and the direct astrophysical S-factors at solar energies
A new analysis of the precise experimental astrophysical S-factors for the
direct capture reaction [A.J.Junghans et al.Phys.Rev. C
68 (2003) 065803 and L.T. Baby et al. Phys.Rev. C 67 (2003) 065805] is carried
out based on the modified two - body potential approach in which the direct
astrophysical S-factor, , is expressed in terms of the
asymptotic normalization constants for and two additional
conditions are involved to verify the peripheral character of the reaction
under consideration. The Woods-Saxon potential form is used for the bound
()- state wave function and for the - scattering wave function.
New estimates are obtained for the ^{\glqq}indirectly measured\grqq values of
the asymptotic normalization constants (the nuclear vertex constants) for the
and at E 115 keV, including =0. These
values of and asymptotic normalization constants have been used for
getting information about the ^{\glqq}indirectly measured\grqq values of the
wave average scattering length and the wave effective range parameters
for - scattering.Comment: 27 pages, 6 figure
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