380 research outputs found
Multiple Photonic Shells Around a Line Singularity
Line singularities including cosmic strings may be screened by photonic
shells until they appear as a planar wall.Comment: 6 page
Linear decomposition of approximate multi-controlled single qubit gates
We provide a method for compiling approximate multi-controlled single qubit
gates into quantum circuits without ancilla qubits. The total number of
elementary gates to decompose an n-qubit multi-controlled gate is proportional
to 32n, and the previous best approximate approach without auxiliary qubits
requires 32nk elementary operations, where k is a function that depends on the
error threshold. The proposed decomposition depends on an optimization
technique that minimizes the CNOT gate count for multi-target and
multi-controlled CNOT and SU(2) gates. Computational experiments show the
reduction in the number of CNOT gates to apply multi-controlled U(2) gates. As
multi-controlled single-qubit gates serve as fundamental components of quantum
algorithms, the proposed decomposition offers a comprehensive solution that can
significantly decrease the count of elementary operations employed in quantum
computing applications
On parameters of the Levi-Civita solution
The Levi-Civita (LC) solution is matched to a cylindrical shell of an
anisotropic fluid. The fluid satisfies the energy conditions when the mass
parameter is in the range . The mass per unit
length of the shell is given explicitly in terms of , which has a
finite maximum. The relevance of the results to the non-existence of horizons
in the LC solution and to gauge cosmic strings is pointed out.Comment: Latex, no figure
Temperature Evolution Law of Imperfect Relativistic Fluids
The first-order general relativistic theory of a generic dissipative
(heat-conducting, viscous, particle-creating) fluid is rediscussed from a
unified covariant frame-independent point of view. By generalizing some
previous works in the literature, we derive a formula for the temperature
variation rate, which is valid both in Eckart's (particle) and in the
Landau-Lifshitz (energy) frames. Particular attention is paid to the case of
gravitational particle creation and its possible cross-effect with the bulk
viscosity mechanism.Comment: 14 pages, no figure, revte
On the thickness of a mildly relativistic collisional shock wave
We consider an imperfect relativistic fluid which develops a shock wave and
discuss its structure and thickness, taking into account the effects of
viscosity and heat conduction in the form of sound absorption. The junction
conditions and the non linear equations describing the evolution of the shock
are derived with the corresponding Newtonian limit discussed in detail. As
happens in the non relativistic regime, the thickness is inversely proportional
to the discontinuity in the pressure, but new terms of purely relativistic
origin are present. Particularizing for a polytropic gas, it is found that the
pure viscous relativistic shock is thicker than its nonrelativistic
counterpart, while the opposite holds for pure heat conduction.Comment: 11 pages, no figures, title changed, improved introduction and
discussion. New author adde
Levi-Civita Solutions Coupled with Electromagnetic Fields
The local and global properties of the Levi-Civita (LC) solutions coupled
with an electromagnetic field are studied and some limits to the vacuum LC
solutions are given. By doing such limits, the physical and geometrical
interpretations of the free parameters involved in the solutions are made
clear. Sources for both the LC vacuum solutions and the LC solutions coupled
with an electromagnetic field are studied, and in particular it is found that
all the LC vacuum solutions with can be produced by
cylindrically symmetric thin shells that satisfy all the energy conditions,
weak, dominant, and strong. When the electromagnetic field is present, the
situation changes dramatically. In the case of a purely magnetic field, all the
solutions with or can be
produced by physically acceptable cylindrical thin shells, while in the case of
a purely electric field, no such shells are found for any value of .Comment: Typed in Revtex, including two figure
Resolved photometry of extragalactic young massive star clusters
We present colour-magnitude diagrams (CMDs) for a sample of seven young
massive clusters in the galaxies NGC 1313, NGC 1569, NGC 1705, NGC 5236 and NGC
7793. The clusters have ages in the range 5-50 million years and masses of 10^5
-10^6 Msun. Although crowding prevents us from obtaining photometry in the
central regions of the clusters, we are still able to measure up to 30-100
supergiant stars in each of the richest clusters, along with the brighter main
sequence stars. The resulting CMDs and luminosity functions are compared with
photometry of artificially generated clusters, designed to reproduce the
photometric errors and completeness as realistically as possible. In agreement
with previous studies, our CMDs show no clear gap between the H-burning main
sequence and the He-burning supergiant stars, contrary to predictions by common
stellar isochrones. In general, the isochrones also fail to match the observed
number ratios of red-to-blue supergiant stars, although the difficulty of
separating blue supergiants from the main sequence complicates this comparison.
In several cases we observe a large spread (1-2 mag) in the luminosities of the
supergiant stars that cannot be accounted for by observational errors. This
spread can be reproduced by including an age spread of 10-30 million years in
the models. However, age spreads cannot fully account for the observed
morphology of the CMDs and other processes, such as the evolution of
interacting binary stars, may also play a role.Comment: 15 pages, 12 figures, accepted for publication in A&
On the jet structure and magnetic field configuration of GRB 020813
The polarization curve of GRB 020813 is discussed and compared to different
models for the structure, evolution and magnetisation properties of the jet and
the interstellar medium onto which the fireball impacts. GRB 020813 is best
suited for this kind of analysis for the smoothness of its afterglow light
curve, ensuring the applicability of current models. The polarization dataset
allows us to rule out the standard GRB jet, in which the energy and Lorentz
factor have a well defined value inside the jet opening angle and the magnetic
field is generated at the shock front. We explore alternative models finding
that a structured jet or a jet with a toroidal component of the magnetic field
can fit equally well the polarization curve. Stronger conclusions cannot be
drawn due to the incomplete sampling of the polarization curve. A more dense
sampling, especially at early times, is required to pin down the structure of
the jet and the geometry of its magnetic field.Comment: 7 pages, 5 postscript figures, minor revisions according to the
referee comments. A&A in pres
The dust SED of dwarf galaxies
Context. High-resolution data from Spitzer, Herschel, and Planck allow us to probe the entire spectral energy distribution (SED) of morphologically separated components of the dust emission from nearby galaxies and allow a more detailed comparison between data and models. Aims. We wish to establish the physical origin of dust heating and emission based on radiation transfer models, that self-consistently connect the emission components from diffuse dust and the dust in massive star forming regions. Methods. NGC 4214 is a nearby dwarf galaxy with a large set of ancillary data, ranging from the ultraviolet (UV) to radio, including maps from Spitzer and Herschel and detections from Planck. We mapped this galaxy with MAMBO at 1.2mm at the IRAM 30m telescope. We extracted separate dust emission components for the HII regions (plus their associated PDRs on pc scales) and for the diffuse dust (on kpc scales). We analysed the full UV to FIR/submm SED of the galaxy using a radiation transfer model that self-consistently treats the dust emission from diffuse and star forming (SF) complexes components, considering the illumination of diffuse dust both by the distributed stellar populations and by escaping light from the HII regions. While maintaining consistency within the framework of this model, we additionally used a model that provides a detailed description of the dust emission from the HII regions and their surrounding PDRs on pc scales. Thanks to the large amount of available data and many previous studies for NGC 4214, very few free parameters remained in the model fitting process. Results. We achieve a satisfactory fit for the emission from HII + PDR regions on pc scales, with the exception of the emission at 8 μm, which is underpredicted by the model. For the diffuse emission we achieve a good fit if we assume that about 40-65% of the emission escaping the HII + PDR regions is able to leave the galaxy without passing through a diffuse ISM, which is not an unlikely scenario for a dwarf galaxy that has recently undergone a nuclear starburst. We determine a dust-to-gas mass ratio of 350-470, which is close to the expected value based on the metallicity. © 2012 ESO
The Deformable Universe
The concept of smooth deformations of a Riemannian manifolds, recently
evidenced by the solution of the Poincar\'e conjecture, is applied to
Einstein's gravitational theory and in particular to the standard FLRW
cosmology. We present a brief review of the deformation of Riemannian geometry,
showing how such deformations can be derived from the Einstein-Hilbert
dynamical principle. We show that such deformations of space-times of general
relativity produce observable effects that can be measured by four-dimensional
observers. In the case of the FLRW cosmology, one such observable effect is
shown to be consistent with the accelerated expansion of the universe.Comment: 20 pages, LaTeX, 3 figure
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