106 research outputs found
Numerically generated quasi-equilibrium orbits of black holes: Circular or eccentric?
We make a comparison between results from numerically generated,
quasi-equilibrium configurations of compact binary systems of black holes in
close orbits, and results from the post-Newtonian approximation. The
post-Newtonian results are accurate through third PN order (O(v/c)^6 beyond
Newtonian gravity), and include rotational and spin-orbit effects, but are
generalized to permit orbits of non-zero eccentricity. Both treatments ignore
gravitational radiation reaction. The energy E and angular momentum J of a
given configuration are compared between the two methods as a function of the
orbital angular frequency \Omega. For small \Omega, corresponding to orbital
separations a factor of two larger than that of the innermost stable orbit, we
find that, if the orbit is permitted to be slightly eccentric, with e ranging
from \approx 0.03 to \approx 0.05, and with the two objects initially located
at the orbital apocenter (maximum separation), our PN formulae give much better
fits to the numerically generated data than do any circular-orbit PN methods,
including various ``effective one-body'' resummation techniques. We speculate
that the approximations made in solving the initial value equations of general
relativity numerically may introduce a spurious eccentricity into the orbits.Comment: 6 pages, 4 figures, to be submitted to Phys. Rev.
Edaphic fauna in soil profile after three decades of different soil management and cover crops in a subtropical region
Abstract This research evaluated the effects of long-term (30 years) winter cover crops under conventional farming system and no-tillage system on edaphic fauna in a Rhodic Hapludox soil, from Paraná State, Brazil. We used three winter cover crops (black oat, hairy vetch and fallow), and as a reference a fragment of natural forest. Soil monoliths were collected at two times, one during the flowering of maize (April 2013) and the other during the flowering of soybean (January 2014). The extraction of the monoliths was carried out in three layers in the soil profile (0-10, 10-20 and 20-30 cm). Seventeen taxonomic groups were sampled. The density of the edaphic fauna is inversely related to soil depth. The winter crops associated with the no-tillage system in long-term resulted in fauna densities similar to the natural environment, with a higher density (density increase of 2.2x) at a depth of 10-20 cm in areas with black oat. At 0-10 cm depth, black oat and vetch under no-tillage systems resulted in an increase of 62% and 69% (April 2013) and 46 and 44% (January 2014), respectively, in the density of soil fauna, when compared to the same winter crops in conventional farming system
Dilatonic current-carrying cosmic strings
We investigate the nature of ordinary cosmic vortices in some scalar-tensor
extensions of gravity. We find solutions for which the dilaton field condenses
inside the vortex core. These solutions can be interpreted as raising the
degeneracy between the eigenvalues of the effective stress-energy tensor,
namely the energy per unit length U and the tension T, by picking a privileged
spacelike or timelike coordinate direction; in the latter case, a phase
frequency threshold occurs that is similar to what is found in ordinary neutral
current-carrying cosmic strings. We find that the dilaton contribution for the
equation of state, once averaged along the string worldsheet, vanishes, leading
to an effective Nambu-Goto behavior of such a string network in cosmology, i.e.
on very large scales. It is found also that on small scales, the energy per
unit length and tension depend on the string internal coordinates in such a way
as to permit the existence of centrifugally supported equilibrium
configuration, also known as vortons, whose stability, depending on the very
short distance (unknown) physics, can lead to catastrophic consequences on the
evolution of the Universe.Comment: 10 pages, ReVTeX, 2 figures, minor typos corrected. This version to
appear in Phys. Rev.
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