2,212 research outputs found
On Effective Spacetime Dimension in the Ho\v{r}ava-Lifshitz Gravity
In this manuscript we explicitly compute the effective dimension of spacetime
in some backgrounds of Ho\v{r}ava-Lifshitz (H-L) gravity. For all the cases
considered, the results are compatible with a dimensional reduction of the
spacetime to , at high energies (ultraviolet limit), which is confirmed
by other quantum gravity approaches, as well as to , at low energies
(infrared limit). This is obtained by computing the free energy of massless
scalar and gauge fields. We find that the only effect of the background is to
change the proportionality constant between the internal energy and
temperature. Firstly, we consider both the non-perturbative and perturbative
models involving the matter action, without gravitational sources but with
manifest time and space symmetry breaking, in order to calculate modifications
in the Stephan-Boltzmann law. When gravity is taken into account, we assume a
scenario in which there is a spherical source with mass and radius in
thermal equilibrium with radiation, and consider the static and spherically
symmetric solution of the H-L theory found by Kehagias-Sfetsos (K-S), in the
weak and strong field approximations. As byproducts, for the weak field regime,
we used the current uncertainty of the solar radiance measurements to establish
a constraint on the free parameter of the K-S solution. We also
calculate the corrections, due to gravity, to the recently predicted attractive
force that black bodies exert on nearby neutral atoms and molecules.Comment: references adde
Dependence of the Black-body Force on Spacetime Geometry and Topology
In this paper we compute the corrections to the black-body force (BBF)
potential due to spacetime geometry and topology. This recently discovered
attractive force on neutral atoms is caused by the thermal radiation emitted
from black bodies and here we investigate it in relativistic gravitational
systems with spherical and cylindrical symmetries. For some astrophysical
objects we find that the corrected black-body potential is greater than the
flat case, showing that this kind of correction can be quite relevant when
curved spaces are considered. Then we consider four cases: The Schwarzschild
spacetime, the global monopole, the non-relativistic infinity cylinder and the
static cosmic string. For the spherically symmetric case of a massive body, we
find that two corrections appear: One due to the gravitational modification of
the temperature and the other due to the modification of the solid angle
subtended by the atom. We apply the found results to a typical neutron star and
to the Sun. For the global monopole, the modification in the black-body
potential is of topological nature and it is due to the central solid angle
deficit that occurs in the spacetime generated by that object. In the
cylindrical case, which is locally flat, no gravitational correction to the
temperature exists, as in the global monopole case. However, we find the
curious fact that the BBF depends on the topology of the spacetime through the
modification of the azimuthal angle and therefore of the solid angle. For the
static cosmic string we find that the force is null for the zero thickness
case.Comment: 8 pages, 5 figures. Revised versio
Near-Infrared Time-Series Photometry in the Field of Cygnus OB2 Association I - Rotational Scenario For Candidate Members
In the last decades, the early pre main sequence stellar rotational evolution
picture has been constrained by studies targeting different young regions at a
variety of ages. Observational studies suggest a mass-rotation dependence, and
for some mass ranges a connection between rotation and the presence of a
circumstellar disk. Not still fully explored, though, is the role of
environmental conditions on the rotational regulation.
We investigate the rotational properties of candidate members of the young
massive association Cygnus OB2. The Stetson variability index, Lomb-Scargle
periodogram, Saunders statistics, string/rope length method, and visual
verification of folded light curves were applied to select 1224 periodic
variable stars. Completeness and contamination of the periodic sample was
derived from Monte Carlo simulations, out of which 894 periods were considered
reliable. Our study was considered reasonably complete for periods from 2 to 30
days.
The general rotational scenario seen in other young regions is confirmed by
Cygnus OB2 period distributions, with disked stars rotating on average slower
than non-disked stars. A mass-rotation dependence was also verified, but as in
NGC 6530, lower mass stars are rotating on average slower than higher mass
stars, with an excess of slow rotators among the lower mass population. The
effect of the environment on the rotational properties of the association was
investigated by re-analysing the results while taking into account the incident
UV radiation arising from O stars in the association. Results compatible with
the disk-locking scenario were verified for stars with low UV incidence, but no
statistical significant relation between rotation and disk presence was
verified for stars with high UV incidence suggesting that massive stars can
have an important role on regulating the rotation of nearby low mass stars.Comment: Submitted on December 23, 201
Efeito da suplemantacao alimentar sobre o comportamento reprodutivo de tourinhos Canchim. I. caracteristicas puberais.
Avaliacao do efeito da suplentacao alimentar durante o periodo seco sobre a puberdade de tourinhos Canchim
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