2,515 research outputs found
Fiscal Policy, Business Cycles and Labor-Market Fluctuations
In this paper we study the effects and transmission of fiscal policy in a dynamic general equilibrium sticky-price model with non-Ricardian agents, distortionary taxation and a Walrasian labor market. We derive a simple analytical framework for fiscal policy similar to the workhorse 'new synthesis' model widely used in the monetary policy literature. We then explore theoretical conditions under which government spending (whether financed by lump-sum or income taxes) can increase private consumption as observed in the data. We conclude that making the model fare better in this respect necessarily makes it fare worse in what concerns real wage fluctuations. Additionally, we show that the model can generate non-Keynesian effects of fiscal policy when participation to asset markets is limited enough and the monetary policy rule is passive.Fiscal Policy; Dynamic General Equilibrium; Distortionary Taxation; Sticky Prices; Non-Ricardian Agents; Government Debt; Non-Keynesian Effects.
Product Attributes and Consumer Willingness to Pay for Environmental Management Systems in Agriculture: Using the Choice Modeling Technique
Consumer concerns in food purchasing contain a number of elements, including food safety, environment, animal welfare, and other social issues. The purpose of this study was to examine consumer perceptions of the potential benefits of products that are produced using an environmental management system (EMS) in agriculture, and to identify those factors that influence choice. The choice modeling technique uses consumer responses (preferences) to estimate Montrealers= willingness to pay (WTP) for production practices that decrease the impacts on the environment, as well as for other potential benefits of EMS production. Results indicate that consumers are willing to pay a price premium for these environmental benefits. This could provide a justification for government to provide incentives for environmental farm management practices and support to certification and labelling programs.Consumer/Household Economics,
Imaging a boson star at the Galactic center
Millimeter very long baseline interferometry will soon produce accurate
images of the closest surroundings of the supermassive compact object at the
center of the Galaxy, Sgr A*. These images may reveal the existence of a
central faint region, the so-called shadow, which is often interpreted as the
observable consequence of the event horizon of a black hole. In this paper, we
compute images of an accretion torus around Sgr A* assuming this compact object
is a boson star, i.e. an alternative to black holes within general relativity,
with no event horizon and no hard surface. We show that very relativistic
rotating boson stars produce images extremely similar to Kerr black holes,
showing in particular shadow-like and photon-ring-like structures. This result
highlights the extreme difficulty of unambiguously telling the existence of an
event horizon from strong-field images.Comment: 21 pages, 9 figures, accepted in CQG; main difference wrt previous
version is the last paragraph of the conclusio
A magnetized torus for modeling Sgr A* millimeter images and spectra
Context. The supermassive black hole, Sagittarius (Sgr) A*, in the centre of
our Galaxy has the largest angular size in the sky among all astrophysical
black holes. Its shadow, assuming no rotation, spans ~ 50 microarcsec.
Resolving such dimensions has long been out of reach for astronomical
instruments until a new generation of interferometers being operational during
this decade. Of particular interest is the Event Horizon Telescope (EHT) with
resolution ~ 20 microarcsec in the millimeter-wavelength range 0.87 mm - 1.3
mm. Aims. We investigate the ability of the fully general relativistic
Komissarov (2006) analytical magnetized torus model to account for observable
constraints at Sgr A* in the centimeter and millimeter domains. The impact of
the magnetic field geometry on the observables is also studied. Methods. We
calculate ray-traced centimeter- and millimeter-wavelength synchrotron spectra
and images of a magnetized accretion torus surrounding the central black hole
in Sgr A*. We assume stationarity, axial symmetry, constant specific angular
momentum and polytropic equation of state. A hybrid population of thermal and
non-thermal electrons is considered. Results. We show that the torus model is
capable of reproducing spectral constraints in the millimeter domain, and in
particular in the observable domain of the EHT. However, the torus model is not
yet able to fit the centimeter spectrum. 1.3 mm images at high inclinations are
in agreement with observable constraints. Conclusions. The ability of the torus
model to account for observations of Sgr A* in the millimeter domain is
interesting in the perspective of the future EHT. Such an analytical model
allows very fast computations. It will thus be a suitable test bed for
investigating large domains of physical parameters, as well as non-black-hole
compact object candidates and alternative theories of gravity.Comment: Major changes wrt the June 2014 version. Accepted by A&
Biological synthesis of fluorescent nanoparticles by cadmium and tellurite resistant Antarctic bacteria: exploring novel natural nanofactories
Indexación: Web of ScienceBackground: Fluorescent nanoparticles or quantum dots (QDs) have been intensely studied for basic and applied research due to their unique size-dependent properties. There is an increasing interest in developing ecofriendly methods to synthesize these nanoparticles since they improve biocompatibility and avoid the generation of toxic byproducts. The use of biological systems, particularly prokaryotes, has emerged as a promising alternative. Recent studies indicate that QDs biosynthesis is related to factors such as cellular redox status and antioxidant defenses. Based on this, the mixture of extreme conditions of Antarctica would allow the development of natural QDs producing bacteria.
Results: In this study we isolated and characterized cadmium and tellurite resistant Antarctic bacteria capable of synthesizing CdS and CdTe QDs when exposed to these oxidizing heavy metals. A time dependent change in fluorescence emission color, moving from green to red, was determined on bacterial cells exposed to metals. Biosynthesis was observed in cells grown at different temperatures and high metal concentrations. Electron microscopy analysis of treated cells revealed nanometric electron-dense elements and structures resembling membrane vesicles mostly associated to periplasmic space. Purified biosynthesized QDs displayed broad absorption and emission spectra characteristic of biogenic Cd nanoparticles.
Conclusions: Our work presents a novel and simple biological approach to produce QDs at room temperature by using heavy metal resistant Antarctic bacteria, highlighting the unique properties of these microorganisms as potent natural producers of nano-scale materials and promising candidates for bioremediation purposes.http://microbialcellfactories.biomedcentral.com/articles/10.1186/s12934-016-0477-
Circular geodesics and thick tori around rotating boson stars
Accretion disks play an important role in the evolution of their relativistic
inner compact objects. The emergence of a new generation of interferometers
will allow to resolve these accretion disks and provide more information about
the properties of the central gravitating object. Due to this instrumental leap
forward it is crucial to investigate the accretion disk physics near various
types of inner compact objects now to deduce later constraints on the central
objects from observations. A possible candidate for the inner object is the
boson star. Here, we will try to analyze the differences between accretion
structures surrounding boson stars and black holes. We aim at analysing the
physics of circular geodesics around boson stars and study simple thick
accretion tori (so-called Polish doughnuts) in the vicinity of these stars. We
realize a detailed study of the properties of circular geodesics around boson
stars. We then perform a parameter study of thick tori with constant angular
momentum surrounding boson stars. This is done using the boson star models
computed by a code constructed with the spectral solver library KADATH. We
demonstrate that all the circular stable orbits are bound. In the case of a
constant angular momentum torus, a cusp in the torus surface exists only for
boson stars with a strong gravitational scalar field. Moreover, for each inner
radius of the disk, the allowed specific angular momentum values lie within a
constrained range which depends on the boson star considered. We show that the
accretion tori around boson stars have different characteristics than in the
vicinity of a black hole. With future instruments it could be possible to use
these differences to constrain the nature of compact objects.Comment: Accepted for publication in CQ
Vertical dissipation profiles and the photosphere location in thin and slim accretion disks
We calculate optically thick but geometrically thin (and slim) accretion disk
models and perform a ray-tracing of photons (in the Kerr geometry) to calculate
the observed disk spectra. Previously, it was a common practice to ray-trace
photons assuming that they are emitted from the Kerr geometry equatorial plane,
z = 0. We show that the spectra calculated with this assumption differ from
these calculated under the assumption that photons are emitted from the actual
surface of the disc, z = H(r). This implies that a knowledge of the location of
the thin disks effective photosphere is relevant for calculating the spectra.
In this paper we investigate, in terms of a simple toy model, a possible
influence of the (unknown, and therefore ad hoc assumed) vertical dissipation
profiles on the vertical structure of the disk and thus on the location of the
effective photosphere, and on the observed spectra. For disks with moderate and
high mass accretion rates (\dot m>0.01\dot m_C) we find that the photosphere
location in the inner, radiation pressure dominated, disk region (where most of
the radiation comes from) does not depend on the dissipation profile and
therefore emerging disk spectra are insensitive to the choice of the
dissipation function. For lower accretion rates the photosphere location
depends on the assumed vertical dissipation profile down to the disk inner
edge, but the dependence is very weak and thus of minor importance. We conclude
that the spectra of optically thick accretion disks around black holes should
be calculated with the ray-tracing from the effective photosphere and that,
fortunately, the choice of a particular vertical dissipation profile does not
substantially influence the calculated spectrum.Comment: 7 pages, 7 figure
Pd/Cu Site Interchange and Non-Fermi-Liquid Behavior in UCu_4Pd
X-ray-absorption fine-structure measurements of the local structure in
UCu_4Pd are described which indicate a probable lattice-disorder origin for
non-Fermi-liquid behavior in this material. Short Pd-Cu distances are observed,
consistent with 24 +/- 3% of the Pd atoms occupying nominally Cu sites. A
"Kondo disorder" model, based on the effect on the local Kondo temperature T_K
of this interchange and some additional bond-length disorder, agrees
quantitatively with previous experimental susceptibility data, and therefore
also with specific heat and magnetic resonance experiments.Comment: 4 pages, 3 PostScript figures, to be published in PR
Epicyclic oscillations of non-slender fluid tori around Kerr black holes
Considering epicyclic oscillations of pressure-supported perfect fluid tori
orbiting Kerr black holes we examine non-geodesic (pressure) effects on the
epicyclic modes properties. Using a perturbation method we derive fully general
relativistic formulas for eigenfunctions and eigenfrequencies of the radial and
vertical epicyclic modes of a slightly non-slender, constant specific angular
momentum torus up to second-order accuracy with respect to the torus thickness.
The behaviour of the axisymmetric and lowest-order () non-axisymmetric
epicyclic modes is investigated. For an arbitrary black hole spin we find that,
in comparison with the (axisymmetric) epicyclic frequencies of free test
particles, non-slender tori receive negative pressure corrections and exhibit
thus lower frequencies. Our findings are in qualitative agreement with the
results of a recent pseudo-Newtonian study of analogous problem defined within
the Paczy{\'n}ski-Wiita potential. Implications of our results on the
high-frequency QPO models dealing with epicyclic oscillations are addressed.Comment: 24 pages, 8 figure
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