425 research outputs found
Charged massive particle at rest in the field of a Reissner-Nordstr\"om black hole
The interaction of a Reissner-Nordstr\"om black hole and a charged massive
particle is studied in the framework of perturbation theory. The particle
backreaction is taken into account, studying the effect of general static
perturbations of the hole following the approach of Zerilli. The solutions of
the combined Einstein-Maxwell equations for both perturbed gravitational and
electromagnetic fields at first order of the perturbation are exactly
reconstructed by summing all multipoles, and are given explicit closed form
expressions. The existence of a singularity-free solution of the
Einstein-Maxwell system requires that the charge to mass ratios of the black
hole and of the particle satisfy an equilibrium condition which is in general
dependent on the separation between the two bodies. If the black hole is
undercritically charged (i.e. its charge to mass ratio is less than one), the
particle must be overcritically charged, in the sense that the particle must
have a charge to mass ratio greater than one. If the charge to mass ratios of
the black hole and of the particle are both equal to one (so that they are both
critically charged, or "extreme"), the equilibrium can exist for any separation
distance, and the solution we find coincides with the linearization in the
present context of the well known Majumdar-Papapetrou solution for two extreme
Reissner-Nordstr\"om black holes. In addition to these singularity-free
solutions, we also analyze the corresponding solution for the problem of a
massive particle at rest near a Schwarzschild black hole, exhibiting a strut
singularity on the axis between the two bodies. The relations between our
perturbative solutions and the corresponding exact two-body solutions belonging
to the Weyl class are also discussed.Comment: 42 pages, 3 eps figures, revtex macro
The Stagger-grid: A grid of 3D stellar atmosphere models - IV. Limb darkening coefficients
We compute the emergent stellar spectra from the UV to far infrared for
different viewing angles using realistic 3D model atmospheres for a large range
in stellar parameters to predict the stellar limb darkening. We have computed
full 3D LTE synthetic spectra based on 3D radiative hydrodynamic atmosphere
models from the Stagger-grid. From the resulting intensities at different
wavelength, we derived coefficients for the standard limb darkening laws
considering a number of often-used photometric filters. Furthermore, we
calculated theoretical transit light curves, in order to quantify the
differences between predictions by the widely used 1D model atmosphere and our
3D models. The 3D models are often found to predict steeper limb darkening
compared to the 1D models, mainly due to the temperature stratifications and
temperature gradients being different in the 3D models compared to those
predicted with 1D models based on the mixing length theory description of
convective energy transport. The resulting differences in the transit light
curves are rather small; however, these can be significant for high-precision
observations of extrasolar transits, and are able to lower the residuals from
the fits with 1D limb darkening profiles. We advocate the use of the new limb
darkening coefficients provided for the standard four-parameter non-linear
power law, which can fit the limb darkening more accurately than other choices.Comment: Accepted for publication in A&A, 10 pages, 9 figures, 1 tabl
Planet transit and stellar granulation detection with interferometry
Aims. We used realistic three-dimensional (3D) radiative hydrodynamical (RHD)
simulations from the Stagger-grid and synthetic images computed with the
radiative transfer code Optim3D to provide interferometric observables to
extract the signature of stellar granulation and transiting planets. Methods.
We computed intensity maps from RHD simulations for twelve interferometric
instruments covering wavelengths ranging from optical to infrared. The stellar
surface asymmetries in the brightness distribution mostly affect closure
phases. We compared the closure phases of the system star with a transiting
planet and the star alone and considered the impact of magnetic spots
constructing a hypothetical starspots image. Results. All the simulations show
departure from the axisymmetric case at all wavelengths. We presented two
possible targets (Beta Com and Procyon) and found that departures up to 16 deg
can be detected on the 3rd lobe and higher. In particular, MIRC is the most
appropriate instrument because it combines good UV coverage and long baselines.
Moreover, we explored the impact of convection on interferometric planet
signature for three prototypes of planets. It is possible to disentangle the
signature of the planet at particular wavelengths (either in the infrared or in
the optical) by comparing the closure phases of the star at difference phases
of the planetary transit. Conclusions. The detection and characterisation of
planets must be based on a comprehensive knowledge of the host star; this
includes the detailed study of the stellar surface convection with
interferometric techniques. In this context, RHD simulations are crucial to
reach this aim. We emphasize that interferometric observations should be pushed
at high spatial frequencies by accumulating observations on closure phases at
short and long baselines.Comment: accepted in Astronomy and Astrophysics, 13 pages. Some figures have
reduced resolution to decrease the size of the output file. Please contact
[email protected] to have the high resolution version of the pape
A metrological characterization of the Kinect V2 time-of-flight camera
A metrological characterization process for time-of-flight (TOF) cameras is proposed in this paper and applied to the Microsoft Kinect V2. Based on the Guide to the Expression of Uncertainty in Measurement (GUM), the uncertainty of a three-dimensional (3D) scene reconstruction is analysed. In particular, the random and the systematic components of the uncertainty are evaluated for the single sensor pixel and for the complete depth camera. The manufacturer declares an uncertainty in the measurement of the central pixel of the sensor of about few millimetres (Kinect for Windows Features, 2015), which is considerably better than the first version of the Microsoft Kinect (Chow et al., 2012 [1]). This work points out that performances are highly influenced by measuring conditions and environmental parameters of the scene; actually the 3D point reconstruction uncertainty can vary from 1.5 to tens of millimetres
Magnetic hot-spot generation at optical frequencies: from plasmonic metamolecules to all-dielectric nanoclusters
AbstractThe weakness of magnetic effects at optical frequencies is directly related to the lack of symmetry between electric and magnetic charges. Natural materials cease to exhibit appreciable magnetic phenomena at rather low frequencies and become unemployable for practical applications in optics. For this reason, historically important efforts were spent in the development of artificial materials. The first evidence in this direction was provided by split-ring resonators in the microwave range. However, the efficient scaling of these devices towards the optical frequencies has been prevented by the strong ohmic losses suffered by circulating currents. With all of these considerations, artificial optical magnetism has become an active topic of research, and particular attention has been devoted to tailor plasmonic metamolecules generating magnetic hot spots. Several routes have been proposed in these directions, leading, for example, to plasmon hybridization in 3D complex structures or Fano-like magnetic resonances. Concurrently, with the aim of electromagnetic manipulation at the nanoscale and in order to overcome the critical issue of heat dissipation, alternative strategies have been introduced and investigated. All-dielectric nanoparticles made of high-index semiconducting materials have been proposed, as they can support both magnetic and electric Mie resonances. Aside from their important role in fundamental physics, magnetic resonances also provide a new degree of freedom for nanostructured systems, which can trigger unconventional nanophotonic processes, such as nonlinear effects or electromagnetic field localization for enhanced spectroscopy and optical trapping
Photoinduced Temperature Gradients in Sub-wavelength Plasmonic Structures: The Thermoplasmonics of Nanocones
Plasmonic structures are renowned for their capability to efficiently convert
light into heat at the nanoscale. However, despite the possibility to generate
deep sub-wavelength electromagnetic hot spots, the formation of extremely
localized thermal hot spots is an open challenge of research, simply because of
the diffusive spread of heat along the whole metallic nanostructure. Here we
tackle this challenge by exploiting single gold nanocones. We theoretically
show how these structures can indeed realize extremely high temperature
gradients within the metal, leading to deep sub-wavelength thermal hot spots,
owing to their capability of concentrating light at the apex under resonant
conditions even under continuous wave illumination. A three-dimensional Finite
Element Method model is employed to study the electromagnetic field in the
structure and subsequent thermoplasmonic behaviour, in terms of the
three-dimensional temperature distribution. We show how the latter is affected
by nanocone size, shape, and composition of the surrounding environment.
Finally, we anticipate the use of photoinduced temperature gradients in
nanocones for applications in optofluidics and thermoelectrics or for thermally
induced nanofabrication
Clinical, epidemiological and virological features of acute hepatitis B in Italy
Purpose To evaluate the association of hepatitis B virus
(HBV) genotypes, basal core promoter (BCP)/precore (PC)
and S gene mutations with the clinical-epidemiological
characteristics of acute hepatitis B (AHB) in Italy.
Methods During July 2005–January 2007, 103 symptomatic AHB patients were enrolled and prospectively followed up at 15 national hospitals. HBV genotypes, BCP/
PC and S gene variants were determined by nested-PCR
and direct sequence analysis.
Results Genotype D, A and F were detected in 49, 45
and 6 % of patients, respectively. BCP, PC, and BCP
plus PC variants were found in 3.1, 11.3 and 7.2 % of
patients, respectively. At enrollment, 68.3 % of patients
were hepatitis B e antigen (HBeAg)-positive and 31.7 %
HBeAg-negative. BCP/PC mutations were more common in HBeAg-negative than in HBeAg-positive patients
(p < 0.0001). Compared to genotype D patients, those harboring non-D genotypes were more frequently males
(p = 0.023), HBeAg-positive (p < 0.001), had higher
bilirubin (p = 0.014) and viremia (p = 0.034) levels and
less frequently carried BCP/PC mutations (p < 0.001).
Non-D genotype patients more often were from Central Italy (p = 0.001) and reported risky sexual exposure
(p = 0.021). Two patients had received vaccination before
AHB: one harbored genotype F; the other showed a S gene
mutation. Four patients developed fulminant AHB; mutations were found in 2 of 3 patients who underwent BCP/
PC sequencing. After a 6-month follow-up, only 2 (2.8 %)
patients developed persistent infection.
Conclusion AHB by non-D genotypes is increasing in
Italy and is associated with risky sexual exposure. The ability of some genotypes to cause persistent and/or severe
infection in Italy warrants larger studies for clarificatio
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