14,575 research outputs found
Green's function approach to Chern-Simons extended electrodynamics: an effective theory describing topological insulators
Boundary effects produced by a Chern-Simons (CS) extension to electrodynamics
are analyzed exploiting the Green's function (GF) method. We consider the
electromagnetic field coupled to a -term in a way that has been
proposed to provide the correct low energy effective action for topological
insulators (TI). We take the -term to be piecewise constant in
different regions of space separated by a common interface , to be
called the -boundary. Features arising due to the presence of the
boundary, such as magnetoelectric effects, are already known in CS extended
electrodynamics and solutions for some experimental setups have been found with
specific configuration of sources. In this work we illustrate a method to
construct the GF that allows to solve the CS modified field equations for a
given -boundary with otherwise arbitrary configuration of sources. The
method is illustrated by solving the case of a planar -boundary but can
also be applied for cylindrical and spherical geometries for which the
-boundary can be characterized by a surface where a given coordinate
remains constant. The static fields of a point-like charge interacting with a
planar TI, as described by a planar discontinuity in , are calculated
and successfully compared with previously reported results. We also compute the
force between the charge and the -boundary by two different methods,
using the energy momentum tensor approach and the interaction energy calculated
via the GF. The infinitely straight current-carrying wire is also analyzed
Photoionization models for extreme Ly 1216 and HeII 1640 ratios in quasar halos, and PopIII vs AGN diagnostics
We explore mechanisms to produce extremely high Ly-alpha/HeII flux ratios, or
to enhance the observed number of Ly-alpha photons per incident ionizing
photon, in extended AGN-photoionized nebulae at high-redshift. Using
photoionization models, we explore the impact of ionization parameter, gas
metallicity, ionizing spectrum, electron energy distribution, and cloud viewing
angle on the relative fluxes of Ly-alpha, HeII and other lines, and on the
observed number of Ly-alpha photons per incident ionizing photon. We find that
low ionization parameter, a relatively soft or filtered ionizing spectrum, low
gas metallicity, kappa-distributed electron energies, or reflection of Ly-alpha
photons by HI can all result in significantly enhanced Ly-alpha relative to
other lines (>10%), with log Ly-alpha/HeII reaching values up to 4.6. In the
cases of low gas metallicity, reflection by HI, or a hard or filtered ionizing
spectrum, the observed number of Ly-alpha photons per incident ionizing photon
is itself significantly enhanced above the nominal Case B value of 0.66 due to
collisional excitation, reaching values up to 5.3 in our 'extreme case' model.
At low gas metallicity (e.g. 0.1 x Solar), the production of Ly-alpha is
predominantly via collisional excitation rather than recombination. In
addition, we find that collisional excitation of Ly-alpha becomes more
efficient if the ionizing continuum is pre-filtered through an optically thin
screen of gas closer to the AGN. We also show that Ly-alpha / HeII ratios of
the z~3.5 quasars studied by Borisova et al. (2016) are consistent with
AGN-photoionization of gas with moderate to low metallicity and/or low
ionization parameter, without requiring exotic ionization/excitation mechanisms
such as strong line-transfer effects. We also present UV-optical diagnostic
diagrams to distinguish between photoionization by Pop III stars and AGN
photoionization.Comment: Accepted for publication in A&A. 14 pages, 9 figures. Abstract
slightly shortened to meet arxiv character limi
A complete gauge-invariant formalism for arbitrary second-order perturbations of a Schwarzschild black hole
Using recently developed efficient symbolic manipulations tools, we present a
general gauge-invariant formalism to study arbitrary radiative
second-order perturbations of a Schwarzschild black hole. In particular, we
construct the second order Zerilli and Regge-Wheeler equations under the
presence of any two first-order modes, reconstruct the perturbed metric in
terms of the master scalars, and compute the radiated energy at null infinity.
The results of this paper enable systematic studies of generic second order
perturbations of the Schwarzschild spacetime. In particular, studies of
mode-mode coupling and non-linear effects in gravitational radiation, the
second-order stability of the Schwarzschild spacetime, or the geometry of the
black hole horizon.Comment: 14 page
Influence of general convective motions on the exterior of isolated rotating bodies in equilibrium
The problem of describing isolated rotating bodies in equilibrium in General
Relativity has so far been treated under the assumption of the circularity
condition in the interior of the body. For a fluid without energy flux, this
condition implies that the fluid flow moves only along the angular direction,
i.e. there is no convection. Using this simplification, some recent studies
have provided us with uniqueness and existence results for asymptotically flat
vacuum exterior fields given the interior sources. Here, the generalisation of
the problem to include general sources is studied. It is proven that the
convective motions have no direct influence on the exterior field, and hence,
that the aforementioned results on uniqueness and existence of exterior fields
apply equally in the general case.Comment: 8 pages, LaTex, uses iopart style files. To appear in Class. Quatum
Gra
Hokupa'a-Gemini Discovery of Two Ultracool Companions to the Young Star HD 130948
We report the discovery of two faint ultracool companions to the nearby
(d~17.9 pc) young G2V star HD 130948 (HR 5534, HIP 72567) using the Hokupa'a
adaptive optics instrument mounted on the Gemini North 8-meter telescope. Both
objects have the same common proper motion as the primary star as seen over a 7
month baseline and have near-IR photometric colors that are consistent with an
early-L classification. Near-IR spectra taken with the NIRSPEC AO instrument on
the Keck II telescope reveal K I lines, FeH, and water bandheads. Based on
these spectra, we determine that both objects have spectral type dL2 with an
uncertainty of 2 spectral subclasses. The position of the new companions on the
H-R diagram in comparison with theoretical models is consistent with the young
age of the primary star (<0.8 Gyr) estimated on the basis of X-ray activity,
lithium abundance and fast rotation. HD 130948 B and C likely constitute a pair
of young contracting brown dwarfs with an orbital period of about 10 years, and
will yield dynamical masses for L dwarfs in the near future.Comment: 10 pages, 3 figures, (13 total pages
Scanning Electron Microscopy to Establish the Marble Weathering Mechanism in the Alhambra of Granada (Spain)
The weathering processes of the white marble from Macael quarries (Almeria, Spain) used in the Alhambra of Granada have been studied with the aim of establishing the weathering factors and mechanism. Unaltered quarry samples have been submitted to diverse accelerated weathering experiments and their morphology compared to the altered materials from the monument. The weathering process stages have been established and the degree of weathering in some monumental dependencies evaluated
Yukawa terms in noncommutative SO(10) and E6 GUTs
We propose a method for constructing Yukawa terms for noncommutative SO(10)
and E6 GUTs, when these GUTs are formulated within the enveloping-algebra
formalism. The most general noncommutative Yukawa term that we propose
contains, at first order in thetamunu, the most general BRS invariant Yukawa
contribution whose only dimensionful parameter is the noncommutativity
parameter. This noncommutative Yukawa interaction is thus renormalisable at
first order in thetamunu.Comment: 14 pages, no figure
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