6,001 research outputs found
No-horizon theorem for spacetimes with spacelike G1 isometry groups
We consider four-dimensional spacetimes which obey the
Einstein equations , and admit a global spacelike
isometry group. By means of dimensional reduction and local
analyis on the reduced (2+1) spacetime, we obtain a sufficient condition on
which guarantees that cannot contain apparent
horizons. Given any (3+1) spacetime with spacelike translational isometry, the
no-horizon condition can be readily tested without the need for dimensional
reduction. This provides thus a useful and encompassing apparent horizon test
for -symmetric spacetimes. We argue that this adds further evidence
towards the validity of the hoop conjecture, and signals possible violations of
strong cosmic censorship.Comment: 8 pages, LaTeX, uses IOP package; published in Class. Quantum Gra
Modeling the spectrum of gravitational waves in the primordial Universe
Recent observations from type Ia Supernovae and from cosmic microwave
background (CMB) anisotropies have revealed that most of the matter of the
Universe interacts in a repulsive manner, composing the so-called dark energy
constituent of the Universe. The analysis of cosmic gravitational waves (GW)
represents, besides the CMB temperature and polarization anisotropies, an
additional approach in the determination of parameters that may constrain the
dark energy models and their consistence. In recent work, a generalized
Chaplygin gas model was considered in a flat universe and the corresponding
spectrum of gravitational waves was obtained. The present work adds a massless
gas component to that model and the new spectrum is compared to the previous
one. The Chaplygin gas is also used to simulate a -CDM model by means
of a particular combination of parameters so that the Chaplygin gas and the
-CDM models can be easily distinguished in the theoretical scenarios
here established. The lack of direct observational data is partialy solved when
the signature of the GW on the CMB spectra is determined.Comment: Proc. of the Conference on Magnetic Fields in the Universe: from
laboratories and stars to primordial structures, AIP(NY), eds. E. M. de
Gouveia Dal Pino, G. Lugones & A. Lazarian (2005), in press. (8 pages, 11
figures
Phases of massive scalar field collapse
We study critical behavior in the collapse of massive spherically symmetric
scalar fields. We observe two distinct types of phase transition at the
threshold of black hole formation. Type II phase transitions occur when the
radial extent of the initial pulse is less than the Compton
wavelength () of the scalar field. The critical solution is that
found by Choptuik in the collapse of massless scalar fields. Type I phase
transitions, where the black hole formation turns on at finite mass, occur when
. The critical solutions are unstable soliton stars with
masses \alt 0.6 \mu^{-1}. Our results in combination with those obtained for
the collapse of a Yang-Mills field~{[M.~W. Choptuik, T. Chmaj, and P. Bizon,
Phys. Rev. Lett. 77, 424 (1996)]} suggest that unstable, confined solutions to
the Einstein-matter equations may be relevant to the critical point of other
matter models.Comment: 5 pages, RevTex, 4 postscript figures included using psfi
Competição de gramĂneas forrageiras em relação a ervas invasoras de pastagens em Porto Velho-RO.
O presente trabalho teve por finalidade testar mĂ©todos de controle e gramĂneas mais competitivas com as ervas invasoras, visando minimizar os custos de seu controle em sistemas integrados.bitstream/item/56193/1/COT1602-0001.pd
Comportamento de variedades de mirtileiro sob cultivo agroecolĂłgico.
bitstream/item/36304/1/comunicado-248.pd
Adaptability and stability in rubber tree progenies under different environmental conditions.
The aim of this work was to select progenies with high adaptability and stability from the dry rubber yield (PBS), of genotypes from a three-year-old rubber tree population, installed in three different locations (SelvĂria, MS, Votuporanga, SP and Colina, SP), by the MHPRVG (Harmonic mean of the relative performance of the genetic value) method predicted by BLUP. The progenies were installed in a randomized block design with 30 treatments (progenies), three replications and 10 plants per plot, with spacing of 3.00 x 3.00 m (SelvĂria, MS) and 1.50 x 1.50 m (Colina, SP and Votuporanga, SP). The PBS presented considerable genetic coefficient variability, ranging from 23.03 to 27.82% and average heritability ranging from 0.47 to 0.99, indicating the high value of the progeny tests in rubber tree breeding programs. The MHPRVG method provided a genetic gain ranging from 11 to 38% in 10 progenies to the PBS and allowed the selection of progenies with high predicted yield potential
Star-forming dwarf galaxies in the Virgo cluster: the link between molecular gas, atomic gas, and dust
We present CO(1-0) and CO(2-1) observations of a sample of 20
star-forming dwarfs selected from the Herschel Virgo Cluster Survey, with
oxygen abundances ranging from 12 + log(O/H) ~ 8.1 to 8.8. CO emission is
observed in ten galaxies and marginally detected in another one. CO fluxes
correlate with the FIR 250 m emission, and the dwarfs follow the same
linear relation that holds for more massive spiral galaxies extended to a wider
dynamical range. We compare different methods to estimate H2 molecular masses,
namely a metallicity-dependent CO-to-H2 conversion factor and one dependent on
H-band luminosity. The molecular-to-stellar mass ratio remains nearly constant
at stellar masses <~ 10 M, contrary to the atomic hydrogen
fraction, M/M, which increases inversely with M. The flattening
of the M/M ratio at low stellar masses does not seem to be related
to the effects of the cluster environment because it occurs for both
HI-deficient and HI-normal dwarfs. The molecular-to-atomic ratio is more
tightly correlated with stellar surface density than metallicity, confirming
that the interstellar gas pressure plays a key role in determining the balance
between the two gaseous components of the interstellar medium. Virgo dwarfs
follow the same linear trend between molecular gas mass and star formation rate
as more massive spirals, but gas depletion timescales, , are not
constant and range between 100 Myr and 6 Gyr. The interaction with the Virgo
cluster environment is removing the atomic gas and dust components of the
dwarfs, but the molecular gas appears to be less affected at the current stage
of evolution within the cluster. However, the correlation between HI deficiency
and the molecular gas depletion time suggests that the lack of gas
replenishment from the outer regions of the disc is lowering the star formation
activity.Comment: 19 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
EficiĂȘncia de mĂ©todos para a superação da dormĂȘncia de sementes de videira.
O presente trabalho teve como objetivo avaliar dois mĂ©todos para a quebra de dormĂȘncia das sementes de videira. Sementes do cruzamento âA1118â xâSuperior Seedlessâ realizada pela Embrapa Semi-Ărido em 2004, foram submetidas a do is tratamentos: trĂȘs concentraçÔes de ĂĄcido giberĂ©lico (0, 2000 e 4000 mg.L-1) e 4 perĂodos de tempo em geladeira comum sob temperatura de 5ÂșC (0, 30, 60 e 90 dias)
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