46,082 research outputs found
Gravastars and Black Holes of Anisotropic Dark Energy
Dynamical models of prototype gravastars made of anisotropic dark energy are
constructed, in which an infinitely thin spherical shell of a perfect fluid
with the equation of state divides the whole spacetime
into two regions, the internal region filled with a dark energy fluid, and the
external Schwarzschild region. The models represent "bounded excursion" stable
gravastars, where the thin shell is oscillating between two finite radii, while
in other cases they collapse until the formation of black holes. Here we show,
for the first time in the literature, a model of gravastar and formation of
black hole with both interior and thin shell constituted exclusively of dark
energy. Besides, the sign of the parameter of anisotropy () seems to
be relevant to the gravastar formation. The formation is favored when the
tangential pressure is greater than the radial pressure, at least in the
neighborhood of the isotropic case ().Comment: 16 pages, 8 figures. Accepted for publication in Gen. Rel. Gra
How the Charge Can Affect the Formation of Gravastars
In recent work we physically interpreted a special gravastar solution
characterized by a zero Schwarzschild mass. In fact, in that case, none
gravastar was formed and the shell expanded, leaving behind a de Sitter or a
Minkowski spacetime, or collapsed without forming an event horizon, originating
what we called a massive non-gravitational object. This object has two
components of non zero mass but the exterior spacetime is Minkowski or de
Sitter. One of the component is a massive thin shell and the other one is de
Sitter spacetime inside. The total mass of this object is zero Schwarzschild
mass, which characterizes an exterior vacuum spacetime. Here, we extend this
study to the case where we have a charged shell. Now, the exterior is a
Reissner-Nordstr\"om spacetime and, depending on the parameter
of the equation of state of the shell, and the charge, a
gravastar structure can be formed. We have found that the presence of the
charge contributes to the stability of the gravastar, if the charge is greater
than a critical value. Otherwise, a massive non-gravitational object is formed
for small charges.Comment: 17 pages and 7 figures, several typos corrected, accepted for
publication in JCA
Lattice dynamics and electron-phonon interaction in (3,3) carbon nanotubes
We present a detailed study of the lattice dynamics and electron-phonon
coupling for a (3,3) carbon nanotube which belongs to the class of small
diameter based nanotubes which have recently been claimed to be
superconducting. We treat the electronic and phononic degrees of freedom
completely by modern ab-initio methods without involving approximations beyond
the local density approximation. Using density functional perturbation theory
we find a mean-field Peierls transition temperature of approx 40K which is an
order of magnitude larger than the calculated superconducting transition
temperature. Thus in (3,3) tubes the Peierls transition might compete with
superconductivity. The Peierls instability is related to the special 2k_F
nesting feature of the Fermi surface. Due to the special topology of the (n,n)
tubes also a q=0 coupling between the two bands crossing the Fermi energy at
k_F is possible which leads to a phonon softening at the Gamma point.Comment: 4 pages, 3 figures; to be published in Phys. Rev. Let
In-vivo magnetic resonance imaging of hyperpolarized silicon particles
Silicon-based micro and nanoparticles have gained popularity in a wide range
of biomedical applications due to their biocompatibility and biodegradability
in-vivo, as well as a flexible surface chemistry, which allows drug loading,
functionalization and targeting. Here we report direct in-vivo imaging of
hyperpolarized 29Si nuclei in silicon microparticles by MRI. Natural physical
properties of silicon provide surface electronic states for dynamic nuclear
polarization (DNP), extremely long depolarization times, insensitivity to the
in-vivo environment or particle tumbling, and surfaces favorable for
functionalization. Potential applications to gastrointestinal, intravascular,
and tumor perfusion imaging at sub-picomolar concentrations are presented.
These results demonstrate a new background-free imaging modality applicable to
a range of inexpensive, readily available, and biocompatible Si particles.Comment: Supplemental Material include
Stable Gravastars of Anisotropic Dark Energy
Dynamical models of prototype gravastars made of phantom energy are
constructed, in which an infinitely thin spherical shell of a perfect fluid
with the equation of state divides the whole spacetime
into two regions, the internal region filled with a dark energy (or phantom)
fluid, and the external Schwarzschild region. It is found that in some cases
the models represent the "bounded excursion" stable gravastars, where the thin
shell is oscillating between two finite radii, while in other cases they
collapse until the formation of black holes or normal stars. In the phase
space, the region for the "bounded excursion" gravastars is very small in
comparison to that of black holes, but not empty, as found in our previous
papers. Therefore, although the existence of gravastars can not be completely
excluded from current analysis, the opposite is not possible either, that is,
even if gravastars exist, they do not exclude the existence of black holes.Comment: 35 pages, 43 figures, added some clarifying texts and corrected some
typos, accepted for publication in JCA
Facile O-atom insertion into C-C and C-H bonds by a trinuclear copper complex designed to harness a singlet oxene
Two trinuclear copper [CuICuICuI(L)]1+ complexes have been prepared with the multidentate ligands (L) 3,3'-(1,4-diazepane-1,4-diyl)bis(1-((2-(dimethylamino)ethyl)(methyl)amino)propan-2-ol) (7-Me) and (3,3'-(1,4-diazepane-1,4-diyl)bis(1-((2-(diethylamino) ethyl)(ethyl) amino)propan-2-ol) (7-Et) as models for the active site of the particulate methane monooxygenase (pMMO). The ligands were designed to form the proper spatial and electronic geometry to harness a "singlet oxene," according to the mechanism previously suggested by our laboratory. Consistent with the design strategy, both [CuICuICuI(L)]1+ reacted with dioxygen to form a putative bis(µ3-oxo)CuIICuIICuIII species, capable of facile O-atom insertion across the central C-C bond of benzil and 2,3-butanedione at ambient temperature and pressure. These complexes also catalyze facile O-atom transfer to the C-H bond of CH3CN to form glycolonitrile. These results, together with our recent biochemical studies on pMMO, provide support for our hypothesis that the hydroxylation site of pMMO contains a trinuclear copper cluster that mediates C-H bond activation by a singlet oxene mechanism
Topological dilaton black holes
In four-dimensional spacetime, when the two-sphere of black hole event
horizons is replaced by a two-dimensional hypersurface with zero or negative
constant curvature, the black hole is referred to as a topological black hole.
In this paper we present some exact topological black hole solutions in the
Einstein-Maxwell-dilaton theory with a Liouville-type dilaton potential.Comment: 8 pages, Revtex, no figure
Exact Black Hole and Cosmological Solutions in a Two-Dimensional Dilaton-Spectator Theory of Gravity
Exact black hole and cosmological solutions are obtained for a special
two-dimensional dilaton-spectator () theory of gravity. We show how
in this context any desired spacetime behaviour can be determined by an
appropriate choice of a dilaton potential function and a ``coupling
function'' in the action. We illustrate several black hole solutions
as examples. In particular, asymptotically flat double- and multiple- horizon
black hole solutions are obtained. One solution bears an interesting
resemblance to the string-theoretic black hole and contains the same
thermodynamic properties; another resembles the Reissner-Nordstrom
solution. We find two characteristic features of all the black hole solutions.
First the coupling constants in must be set equal to constants of
integration (typically the mass). Second, the spectator field and its
derivative both diverge at any event horizon. A test particle with
``spectator charge" ({\it i.e.} one coupled either to or ),
will therefore encounter an infinite tidal force at the horizon or an
``infinite potential barrier'' located outside the horizon respectively. We
also compute the Hawking temperature and entropy for our solutions. In
cosmology, two non-singular solutions which resemble two exact solutions
in string-motivated cosmology are obtained. In addition, we construct a
singular model which describes the standard non-inflationary big bang
cosmology (). Motivated by the
similaritiesbetween and gravitational field equations in
cosmology, we briefly discuss a special dilaton-spectator action
constructed from the bosonic part of the low energy heterotic string action andComment: 34 pgs. Plain Tex, revised version contains some clarifying comments
concerning the relationship between the constants of integration and the
coupling constants
Bolometric light curves of supernovae and post-explosion magnetic fields
The various effects leading to diversity in the bolometric light curves of
supernovae are examined: nucleosynthesis, kinematic differences, ejected mass,
degree of mixing, and configuration and intensity of the magnetic field are
discussed. In Type Ia supernovae, a departure in the bolometric light curve
from the full-trapping decline of Co can occur within the two and a half
years after the explosion, depending on the evolutionary path followed by the
WD during the accretion phase. If convection has developed in the WD core
during the presupernova evolution, starting several thousand years before the
explosion, a tangled magnetic field close to the equipartition value should
have grown in the WD. Such an intense magnetic field would confine positrons
where they originate from the Co decays, and preclude a strong departure
from the full-trapping decline, as the supernova expands. This situation is
expected to occur in C+O Chandrasekhar WDs as opposed to edge-lit detonated
sub-Chandrasekhar WDs. If the pre-explosion magnetic field of the WD is less
intense than 10G, a lack of confinement of the positrons emitted in the
Co decay and a departure from full-trapping decline would occur. The
time at which it takes place can provide estimates of the original magnetic
field of the WD, its configuration, and also of the mass of the supernova
ejecta. In SN 1991bg, the bolometric light curve suggests absence of a
significant tangled magnetic field (intensity lower than G).
Chandrasekhar-mass models do not reproduce the bolometric light curve of this
supernova. For SN 1972E, on the contrary, there is evidence for a tangled
configuration of the magnetic field and its light curve is well reproduced by a
Chandrasekhar WD explosion.Comment: 54 pages, including 8 figures. To appear in Ap
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