7,537 research outputs found
Einstein-Cartan-Dirac gravity with symmetry breaking
Einstein-Cartan theory is an extension of the standard formulation of General
Relativity where torsion (the antisymmetric part of the affine connection) is
non-vanishing. Just as the space-time metric is sourced by the stress-energy
tensor of the matter fields, torsion is sourced via the spin density tensor,
whose physical effects become relevant at very high spin densities. In this
work we introduce an extension of the Einstein-Cartan-Dirac theory with an
electromagnetic (Maxwell) contribution minimally coupled to torsion. This
contribution breaks the gauge symmetry, which is suggested by the
possibility of a torsion-induced phase transition in the early Universe,
yielding new physics in extreme (spin) density regimes. We obtain the
generalized gravitational, electromagnetic and fermionic field equations for
this theory, estimate the strength of the corrections, and discuss the
corresponding phenomenology. In particular, we briefly address some
astrophysical considerations regarding the relevance of the effects which might
take place inside ultra-dense neutron stars with strong magnetic fields
(magnetars).Comment: 15 double column pages; v2: removed one section and added content to
other sections. Version accepted for publication on EPJ
Vacuum Polarization by a Magnetic Flux Tube at Finite Temperature in the Cosmic String Spacetime
In this paper we analyse the effect produced by the temperature in the vacuum
polarization associated with charged massless scalar field in the presence of
magnetic flux tube in the cosmic string spacetime. Three different
configurations of magnetic fields are taken into account: a homogeneous
field inside the tube, a field proportional to and a
cylindrical shell with -function. In these three cases, the axis of the
infinitely long tube of radius coincides with the cosmic string. Because
the complexity of this analysis in the region inside the tube, we consider the
thermal effect in the region outside. In order to develop this analysis, we
construct the thermal Green function associated with this system for the three
above mentioned situations considering points in the region outside the tube.
We explicitly calculate in the high-temperature limit, the thermal average of
the field square and the energy-momentum tensor.Comment: 16 pages, 1 figur
First detection of CF+ towards a high-mass protostar
We report the first detection of the J = 1 - 0 (102.6 GHz) rotational lines
of CF+ (fluoromethylidynium ion) towards CygX-N63, a young and massive
protostar of the Cygnus X region. This detection occurred as part of an
unbiased spectral survey of this object in the 0.8-3 mm range, performed with
the IRAM 30m telescope. The data were analyzed using a local thermodynamical
equilibrium model (LTE model) and a population diagram in order to derive the
column density. The line velocity (-4 km s-1) and line width (1.6 km s-1)
indicate an origin from the collapsing envelope of the protostar.
We obtain a CF+ column density of 4.10e11 cm-2. The CF+ ion is thought to be
a good tracer for C+ and assuming a ratio of 10e-6 for CF+/C+, we derive a
total number of C+ of 1.2x10e53 within the beam. There is no evidence of carbon
ionization caused by an exterior source of UV photons suggesting that the
protostar itself is the source of ionization. Ionization from the protostellar
photosphere is not efficient enough. In contrast, X-ray ionization from the
accretion shock(s) and UV ionization from outflow shocks could provide a large
enough ionizing power to explain our CF+ detection.
Surprisingly, CF+ has been detected towards a cold, massive protostar with no
sign of an external photon dissociation region (PDR), which means that the only
possibility is the existence of a significant inner source of C+. This is an
important result that opens interesting perspectives to study the early
development of ionized regions and to approach the issue of the evolution of
the inner regions of collapsing envelopes of massive protostars. The existence
of high energy radiations early in the evolution of massive protostars also has
important implications for chemical evolution of dense collapsing gas and could
trigger peculiar chemistry and early formation of a hot core.Comment: 6 page
Filamentary fragmentation in a turbulent medium
We present the results of smoothed particle hydrodynamic simulations
investigating the evolution and fragmentation of filaments that are accreting
from a turbulent medium. We show that the presence of turbulence, and the
resulting inhomogeneities in the accretion flow, play a significant role in the
fragmentation process. Filaments which experience a weakly turbulent accretion
flow fragment in a two-tier hierarchical fashion, similar to the fragmentation
pattern seen in the Orion Integral Shaped Filament. Increasing the energy in
the turbulent velocity field results in more sub-structure within the
filaments, and one sees a shift from gravity-dominated fragmentation to
turbulence-dominated fragmentation. The sub-structure formed in the filaments
is elongated and roughly parallel to the longitudinal axis of the filament,
similar to the fibres seen in observations of Taurus, and suggests that the
fray and fragment scenario is a possible mechanism for the production of
fibres. We show that the formation of these fibre-like structures is linked to
the vorticity of the velocity field inside the filament and the filament's
accretion from an inhomogeneous medium. Moreover, we find that accretion is
able to drive and sustain roughly sonic levels of turbulence inside the
filaments, but is not able to prevent radial collapse once the filaments become
supercritical. However, the supercritical filaments which contain fibre-like
structures do not collapse radially, suggesting that fibrous filaments may not
necessarily become radially unstable once they reach the critical line-density.Comment: (Accepted for publication in MNRAS
Cultivares de abacaxi.
O abacaxizeiro é uma fruteira amplamente cultivada no Brasil e representa excelente fonte de renda para diversas regiões do País. Estima-se que cerca de 70% da produção mundial de abacaxi provém da cultivar Smooth Cayenne. O predomínio do plantio desta cultivar, nos principais países produtores do mundo, vem proporcionando o desaparecimento de cultivares de interesse local ou regional. No Brasil e em outros países da América Latina, ocorrem diversas cultivares de abacaxi que estão sendo substituídas por Smooth Cayenne. Essas cultivares são importantes para o futuro da cultura e deveriam ser preservadas, caracterizadas e avaliadas. As cultivares de abacaxi mais conhecidas no mundo são classificadas em cinco grupos distintos (Cayenne, Spanish, Queen, Pernambuco ou Pérola e Mordilonus-Perolera), de acordo com um conjunto de caracteres comuns.bitstream/item/81508/1/cultivares-de-abacaxi-Rnato-Cabral-Circular-tecnica-33-1999.pdfMemória
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