3,565 research outputs found
Effects of anisotropy in a nonlinear crystal for squeezed vacuum generation
Squeezed vacuum (SV) can be obtained by an optical parametric amplifier (OPA)
with the quantum vacuum state at the input. We are interested in a degenerate
type-I OPA based on parametric down-conversion (PDC) where due to phase
matching requirements, an extraordinary polarized pump must impinge onto a
birefringent crystal with a large \chi(2) nonlinearity. As a consequence of the
optical anisotropy of the medium, the direction of propagation of the pump
wavevector does not coincide with the direction of propagation of its energy,
an effect known as transverse walk-off. For certain pump sizes and crystal
lengths, the transverse walk-off has a strong influence on the spatial spectrum
of the generated radiation, which in turn affects the outcome of any experiment
in which this radiation is employed. In this work we propose a method that
reduces the distortions of the two-photon amplitude (TPA) of the states
considered, by using at least two consecutive crystals instead of one. We show
that after anisotropy compensation the TPA becomes symmetric, allowing for a
simple Schmidt expansion, a procedure that in practice requires states that
come from experimental systems free of anisotropy effects
Coexisting patterns of population oscillations: the degenerate Neimark Sacker bifurcation as a generic mechanism
We investigate a population dynamics model that exhibits a Neimark Sacker
bifurcation with a period that is naturally close to 4. Beyond the bifurcation,
the period becomes soon locked at 4 due to a strong resonance, and a second
attractor of period 2 emerges, which coexists with the first attractor over a
considerable parameter range. A linear stability analysis and a numerical
investigation of the second attractor reveal that the bifurcations producing
the second attractor occur naturally in this type of system.Comment: 8 pages, 3 figure
Characterizing the radial oxygen abundance distribution in disk galaxies
We examine the possible dependence of the radial oxygen abundance
distribution on non-axisymmetrical structures (bar/spirals) and other
macroscopic parameters such as the mass, the optical radius R25, the color g-r,
and the surface brightness of the galaxy. A sample of disk galaxies from the
CALIFA DR3 is considered. We adopted the Fourier amplitude A2 of the surface
brightness as a quantitative characteristic of the strength of non-axisymmetric
structures in a galactic disk, in addition to the commonly used morphologic
division for A, AB, and B types based on the Hubble classification. To
distinguish changes in local oxygen abundance caused by the non-axisymmetrical
structures, the multiparametric mass--metallicity relation was constructed as a
function of parameters such as the bar/spiral pattern strength, the disk size,
color index g-r in the SDSS bands, and central surface brightness of the disk.
The gas-phase oxygen abundance gradient is determined by using the R
calibration. We find that there is no significant impact of the
non-axisymmetric structures such as a bar and/or spiral patterns on the local
oxygen abundance and radial oxygen abundance gradient of disk galaxies.
Galaxies with higher mass, however, exhibit flatter oxygen abundance gradients
in units of dex/kpc, but this effect is significantly less prominent for the
oxygen abundance gradients in units of dex/R25 and almost disappears when the
inner parts are avoided. We show that the oxygen abundance in the central part
of the galaxy depends neither on the optical radius R25 nor on the color g-r or
the surface brightness of the galaxy. Instead, outside the central part of the
galaxy, the oxygen abundance increases with g-r value and central surface
brightness of the disk.Comment: 11 pages, 6 figures; accepted for publication in A&
Heating and Turbulence Driving by Galaxy Motions in Galaxy Clusters
Using three-dimensional hydrodynamic simulations, we investigate heating and
turbulence driving in an intracluster medium (ICM) by orbital motions of
galaxies in a galaxy cluster. We consider Ng member galaxies on isothermal and
isotropic orbits through an ICM typical of rich clusters. An introduction of
the galaxies immediately produces gravitational wakes, providing perturbations
that can potentially grow via resonant interaction with the background gas.
When Ng^{1/2}Mg_11 < 100, where Mg_11 is each galaxy mass in units of 10^{11}
Msun, the perturbations are in the linear regime and the resonant excitation of
gravity waves is efficient to generate kinetic energy in the ICM, resulting in
the velocity dispersion sigma_v ~ 2.2 Ng^{1/2}Mg_11 km/s. When Ng^{1/2}Mg_11 >
100, on the other hand, nonlinear fluctuations of the background ICM destroy
galaxy wakes and thus render resonant excitation weak or absent. In this case,
the kinetic energy saturates at the level corresponding to sigma_v ~ 220 km/s.
The angle-averaged velocity power spectra of turbulence driven in our models
have slopes in the range of -3.7 to -4.3. With the nonlinear saturation of
resonant excitation, none of the cooling models considered are able to halt
cooling catastrophe, suggesting that the galaxy motions alone are unlikely to
solve the cooling flow problem.Comment: 12 pages including 3 figures, To appear in ApJ
Calibration of radii and masses of open clusters with a simulation
Context: A recent new approach to apply a simple dynamical mass estimate of
tidally limited star clusters is based on the identification of the tidal
radius in a King profile with the dynamical Jacobi radius. The application to
an unbiased open cluster catalogue yields significantly higher cluster masses
compared to the classical methods. Aims: We quantify the bias in the mass
determination as function of projection direction and cluster age by analysing
a simulated star cluster. Methods: We use direct -body simulations of a star
cluster including stellar evolution in an analytic Milky Way potential and
apply a best fit to the projected number density of cluster stars. Results: We
obtain significantly overestimated star cluster masses which depend strongly on
the viewing direction. The overestimation is typically in the range of 10-50
percent and reaches a factor of 3.5 for young clusters. Mass segregation
reduces the derived limiting radii systematically.Comment: 9 pages, 10+1 figures, accepted by Astronomy and Astrophysic
Islam, Religiosity, and Immigrant Political Action in Western Europe
Cataloged from PDF version of article.The issues of migration and immigrant political integration in western democracies have
become increasingly intertwined with debates on religion, particularly Islam. To date, however,
we have surprisingly little systematic research on how religious beliefs are related to
immigrantsâ political engagement. In this study, we argue that religion has a capacity to
mobilize immigrants politically but the strength of this relationship depends on immigrant
generation, religiosity, and the type of religion. Using survey data collected as part of the
European Social Survey (ESS) 2002â2010 in 18 West European democracies, our analyses
reveal that religion is indeed linked to political engagement of immigrants in a complex
way: while belonging to a religion is generally associated with less political participation,
exposure to religious institutions appears to have the opposite effect. Moreover, we find
that, compared to foreign-born Muslims, second-generation Muslim immigrants are not
only more religious and more politically dissatisfied with their host countries, but also that
religiosity is more strongly linked to their political engagement. This relationship, however,
is limited to uninstitutionalized political action
High resolution simulations of unstable modes in a collisionless disc
We present N-body simulations of unstable spiral modes in a dynamically cool
collisionless disc. We show that spiral modes grow in a thin collisionless disk
in accordance with the analytical perturbation theory. We use the particle-mesh
code SUPERBOX with nested grids to follow the evolution of unstable spirals
that emerge from an unstable equilibrium state. We use a large number of
particles (up to 40 million particles) and high-resolution spatial grids in our
simulations (128^3 cells). These allow us to trace the dynamics of the unstable
spiral modes until their wave amplitudes are saturated due to nonlinear
effects. In general, the results of our simulations are in agreement with the
analytical predictions. The growth rate and the pattern speed of the most
unstable bar-mode measured in N-body simulations agree with the linear
analysis. However the parameters of secondary unstable modes are in lesser
agreement because of the still limited resolution of our simulations.Comment: 11 pages, 8 figures in 22 files, A&A in print: Oct. 1st 200
Towards predicting post-editing productivity
Machine translation (MT) quality is generally measured via automatic metrics, producing scores that have no meaning for translators who are required to post-edit MT output or for project managers who have to plan and budget for transla- tion projects. This paper investigates correlations between two such automatic metrics (general text matcher and translation edit rate) and post-editing productivity. For the purposes of this paper, productivity is measured via processing speed and cognitive measures of effort using eye tracking as a tool. Processing speed, average fixation time and count are found to correlate well with the scores for groups of segments. Segments with high GTM and TER scores require substantially less time and cognitive effort than medium or low-scoring segments. Future research involving score thresholds and confidence estimation is suggested
Dynamical Friction in a Gaseous Medium
Using time-dependent linear perturbation theory, we evaluate the dynamical
friction force on a massive perturber M_p traveling at velocity V through a
uniform gaseous medium of density rho_0 and sound speed c_s. This drag force
acts in the direction -\hat V, and arises from the gravitational attraction
between the perturber and its wake in the ambient medium. For supersonic motion
(M=V/c_s>1), the enhanced-density wake is confined to the Mach cone trailing
the perturber; for subsonic motion (M<1), the wake is confined to a sphere of
radius c_s t centered a distance V t behind the perturber. Inside the wake,
surfaces of constant density are hyperboloids or oblate spheroids for
supersonic or subsonic perturbers, respectively, with the density maximal
nearest the perturber. The dynamical drag force has the form F_df= - I 4\pi (G
M_p)^2\rho_0/V^2. We evaluate I analytically; its limits are I\to M^3/3 for
M>1. We compare our results to the
Chandrasekhar formula for dynamical friction in a collisionless medium, noting
that the gaseous drag is generally more efficient when M>1 but less efficient
when M<1. To allow simple estimates of orbit evolution in a gaseous protogalaxy
or proto-star cluster, we use our formulae to evaluate the decay times of a
(supersonic) perturber on a near-circular orbit in an isothermal \rho\propto
r^{-2} halo, and of a (subsonic) perturber on a near-circular orbit in a
constant-density core. We also mention the relevance of our calculations to
protoplanet migration in a circumstellar nebula.Comment: 17 pages, 5 postscript figures, to appear in ApJ 3/1/9
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