3,583 research outputs found
Reflection and Transmission at the Apparent Horizon during Gravitational Collapse
We examine the wave-functionals describing the collapse of a self-gravitating
dust ball in an exact quantization of the gravity-dust system. We show that
ingoing (collapsing) dust shell modes outside the apparent horizon must
necessarily be accompanied by outgoing modes inside the apparent horizon, whose
amplitude is suppressed by the square root of the Boltzmann factor at the
Hawking temperature. Likewise, ingoing modes in the interior must be
accompanied by outgoing modes in the exterior, again with an amplitude
suppressed by the same factor. A suitable superposition of the two solutions is
necessary to conserve the dust probability flux across the apparent horizon,
thus each region contains both ingoing and outgoing dust modes. If one
restricts oneself to considering only the modes outside the apparent horizon
then one should think of the apparent horizon as a partial reflector, the
probability for a shell to reflect being given by the Boltzmann factor at the
Hawking temperature determined by the mass contained within it. However, if one
considers the entire wave function, the outgoing wave in the exterior is seen
to be the transmission through the horizon of the interior outgoing wave that
accompanies the collapsing shells. This transmission could allow information
from the interior to be transferred to the exterior.Comment: 19 pages, no figures. To appear in Phys. Rev.
General spherically symmetric elastic stars in Relativity
The relativistic theory of elasticity is reviewed within the spherically
symmetric context with a view towards the modeling of star interiors possessing
elastic properties such as theones expected in neutron stars. Emphasis is
placed on generality in the main sections of the paper, and the results are
then applied to specific examples. Along the way, a few general results for
spacetimes admitting isometries are deduced, and their consequences are fully
exploited in the case of spherical symmetry relating them next to the the case
in which the material content of the spacetime is some elastic material. This
paper extends and generalizes the pioneering work by Magli and Kijowski [1],
Magli [2] and [3], and complements, in a sense, that by Karlovini and
Samuelsson in their interesting series of papers [4], [5] and [6].Comment: 23 page
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
Initial data and the end state of spherically symmetric gravitational collapse
Generalizing earlier results on the initial data and the final fate of dust
collapse, we study here the relevance of the initial state of a spherically
symmetric matter cloud towards determining its end state in the course of a
continuing gravitational collapse. It is shown that given an arbitrary regular
distribution of matter at the initial epoch, there always exists an evolution
from this initial data which would result either in a black hole or a naked
singularity depending on the allowed choice of free functions available in the
solution. It follows that given any initial density and pressure profiles for
the cloud, there is a non-zero measure set of configurations leading either to
black holes or naked singularities, subject to the usual energy conditions
ensuring the positivity of energy density. We also characterize here wide new
families of black hole solutions resulting from spherically symmetric collapse
without requiring the cosmic censorship assumption.Comment: Ordinary Tex file, 31 pages no figure
Elastic thick shells in general relativity
It is shown that exact spherically symmetric solutions to Einstein's field equations exist such that, over an open region of the spacetime, they are singularity free, satisfy the dominant energy condition, represent elastic matter with a well-defined constitutive function, and are such that elastic perturbations propagate causally. Two toy models are then built up in which a thick elastic, spherically symmetric shell with the above properties, separates two Robertson-Walker regions corresponding to different values of the curvature k in the first model and to the same value of k in the second model. The junction conditions (continuity of the first and second fundamental forms) are shown to be exactly satisfied across the corresponding matching spherical surfaces.- The authors are grateful to Prof. Raul Vera, from the Universidad del Pais Vasco/Euskal Herriko Unibersitatea, for helpful discussions and suggestions. One of the authors (J. C.) acknowledges financial support from the Spanish Ministerio de Economia y Competitividad through Grants No. FPA2013-41042-P and No. FPA2016-76821-P, and also acknowledges the warm hospitality, and partial financial support, from the University do Minho, where the present version of this manuscript was prepared. The research of I. B. and E. V. was partially financed by Portuguese Funds through FCT (Fundacao para a Ciencia e Tecnologia) within the Project No. UID/MAT/00013/2013. I. B. is grateful for support from FCT, through the Project No. PEstOE/MAT/UI0013/2014 and also expresses her gratitude for the hospitality at the Universitat de les Illes Balears
Busca usando sinônimos no Ainfo-Consulta.
O foco deste trabalho é na expansão de consultas com termos obtidos por meio do Thesaurus Brasileiro de Agricultura - Thesagro (BRASIL, 1999) para melhorar os resultados do Ainfo-Consulta
Dynamical Mass Constraints on Low-Mass Pre-Main-Sequence Stellar Evolutionary Tracks: An Eclipsing Binary in Orion with a 1.0 Msun Primary and an 0.7 Msun Secondary
We report the discovery of a double-lined, spectroscopic, eclipsing binary in
the Orion star-forming region. We analyze the system spectroscopically and
photometrically to empirically determine precise, distance-independent masses,
radii, effective temperatures, and luminosities for both components. The
measured masses for the primary and secondary, accurate to ~1%, are 1.01 Msun
and 0.73 Msun, respectively; thus the primary is a definitive pre-main-sequence
solar analog, and the secondary is the lowest-mass star yet discovered among
pre-main-sequence eclipsing binary systems. We use these fundamental
measurements to test the predictions of pre-main-sequence stellar evolutionary
tracks. None of the models we examined correctly predict the masses of the two
components simultaneously, and we implicate differences between the theoretical
and empirical effective temperature scales for this failing. All of the models
predict the observed slope of the mass-radius relationship reasonably well,
though the observations tend to favor models with low convection efficiencies.
Indeed, considering our newly determined mass measurements together with other
dynamical mass measurements of pre-main-sequence stars in the literature, as
well as measurements of Li abundances in these stars, we show that the data
strongly favor evolutionary models with inefficient convection in the stellar
interior, even though such models cannot reproduce the properties of the
present-day Sun.Comment: Accepted by Ap
Herpetofauna, Espora Hydroelectric Power Plant, state of Goiás, Brazil.
We provide a checklist of the herpetofaunal assemblage from Espora Hydroelectric Power Plant region(UHE Espora), southwestern of the state of Goiás, Brazil. Representatives of 32 amphibian and 71 reptile species wereobtained during faunal monitoring and faunal rescue programs carried out in the study area. The obtained species listand distribution records are here discussed in an attempt to improve the still limited knowledge on Cerradoherpetofaunal assemblages
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