1,224 research outputs found
Shocks in dense clouds. IV. Effects of grain-grain processing on molecular line emission
Grain-grain processing has been shown to be an indispensable ingredient of
shock modelling in high density environments. For densities higher than
\sim10^5 cm-3, shattering becomes a self-enhanced process that imposes severe
chemical and dynamical consequences on the shock characteristics. Shattering is
accompanied by the vaporization of grains, which can directly release SiO to
the gas phase. Given that SiO rotational line radiation is used as a major
tracer of shocks in dense clouds, it is crucial to understand the influence of
vaporization on SiO line emission. We have developed a recipe for implementing
the effects of shattering and vaporization into a 2-fluid shock model,
resulting in a reduction of computation time by a factor \sim100 compared to a
multi-fluid modelling approach. This implementation was combined with an
LVG-based modelling of molecular line radiation transport. Using this model we
calculated grids of shock models to explore the consequences of different
dust-processing scenarios. Grain-grain processing is shown to have a strong
influence on C-type shocks for a broad range of magnetic fields: they become
hotter and thinner. The reduction in column density of shocked gas lowers the
intensity of molecular lines, at the same time as higher peak temperatures
increase the intensity of highly excited transitions compared to shocks without
grain-grain processing. For OH the net effect is an increase in line
intensities, while for CO and H2O it is the contrary. The intensity of H2
emission is decreased in low transitions and increased for highly excited
lines. For all molecules, the highly excited lines become sensitive to the
value of the magnetic field. Although vaporization increases the intensity of
SiO rotational lines, this effect is weakened by the reduced shock width. The
release of SiO early in the hot shock changes the excitation characteristics of
SiO radiation.Comment: Published in Astronomy and Astrophysics (2013). 26 pages, 16 figures,
14 table
Penetration and cratering experiments of graphite by 0.5-mm diameter steel spheres at various impact velocities
Cratering experiments have been conducted with 0.5-mm diameter AISI 52100 steel spherical projectiles and 30-mm diameter, 15-mm long graphite targets. The latter were made of a commercial grade of polycrystalline and porous graphite named EDM3 whose behavior is known as macroscopically isotropic. A two-stage light-gas gun launched the steel projectiles at velocities between 1.1 and 4.5 km s 1. In most cases, post-mortem tomographies revealed that the projectile was trapped, fragmented or not, inside the target. It showed that the apparent crater size and depth increase with the impact velocity. This is also the case of the crater volume which appears to follow a power law significantly different from those constructed in previous works for similar impact conditions and materials. Meanwhile, the projectile depth of penetration starts to decrease at velocities beyond 2.2 km s 1. This is firstly because of its plastic deformation and then, beyond 3.2 km s 1, because of its fragmentation. In addition to these three regimes of penetration behavior already described by a few authors, we suggest a fourth regime in which the projectile melting plays a significant role at velocities above 4.1 km s 1. A discussion of these four regimes is provided and indicates that each phenomenon may account for the local evolution of the depth of penetration
Resonant excitonic emission of a single quantum dot in the Rabi regime
We report on coherent resonant emission of the fundamental exciton state in a
single semiconductor GaAs quantum dot. Resonant regime with picoseconde laser
excitation is realized by embedding the quantum dots in a waveguiding
structure. As the pulse intensity is increased, Rabi oscillation is observed up
to three periods. The Rabi regime is achieved owing to an enhanced light-matter
coupling in the waveguide. This is due to a \emph{slow light effect}
(), occuring when an intense resonant pulse propagates in a
medium. The resonant control of the quantum dot fundamental transition opens
new possibilities in quantum state manipulation and quantum optics experiments
in condensed matter physics.Comment: Submitted to Phys. Rev. Let
Polarization measurements analysis II. Best estimators of polarization fraction and angle
With the forthcoming release of high precision polarization measurements,
such as from the Planck satellite, it becomes critical to evaluate the
performance of estimators for the polarization fraction and angle. These two
physical quantities suffer from a well-known bias in the presence of
measurement noise, as has been described in part I of this series. In this
paper, part II of the series, we explore the extent to which various estimators
may correct the bias. Traditional frequentist estimators of the polarization
fraction are compared with two recent estimators: one inspired by a Bayesian
analysis and a second following an asymptotic method. We investigate the
sensitivity of these estimators to the asymmetry of the covariance matrix which
may vary over large datasets. We present for the first time a comparison among
polarization angle estimators, and evaluate the statistical bias on the angle
that appears when the covariance matrix exhibits effective ellipticity. We also
address the question of the accuracy of the polarization fraction and angle
uncertainty estimators. The methods linked to the credible intervals and to the
variance estimates are tested against the robust confidence interval method.
From this pool of estimators, we build recipes adapted to different use-cases:
build a mask, compute large maps, and deal with low S/N data. More generally,
we show that the traditional estimators suffer from discontinuous distributions
at low S/N, while the asymptotic and Bayesian methods do not. Attention is
given to the shape of the output distribution of the estimators, and is
compared with a Gaussian. In this regard, the new asymptotic method presents
the best performance, while the Bayesian output distribution is shown to be
strongly asymmetric with a sharp cut at low S/N.Finally, we present an
optimization of the estimator derived from the Bayesian analysis using adapted
priors
Local disorder and optical properties in V-shaped quantum wires : towards one-dimensional exciton systems
The exciton localization is studied in GaAs/GaAlAs V-shaped quantum wires
(QWRs) by high spatial resolution spectroscopy. Scanning optical imaging of
different generations of samples shows that the localization length has been
enhanced as the growth techniques were improved. In the best samples, excitons
are delocalized in islands of length of the order of 1 micron, and form a
continuum of 1D states in each of them, as evidenced by the sqrt(T) dependence
of the radiative lifetime. On the opposite, in the previous generation of QWRs,
the localization length is typically 50 nm and the QWR behaves as a collection
of quantum boxes. These localization properties are compared to structural
properties and related to the progresses of the growth techniques. The presence
of residual disorder is evidenced in the best samples and explained by the
separation of electrons and holes due to the large in-built piezo-electric
field present in the structure.Comment: 8 figure
Engineering the spatial confinement of exciton-polaritons in semiconductors
We demonstrate the spatial confinement of electronic excitations in a solid
state system, within novel artificial structures that can be designed having
arbitrary dimensionality and shape. The excitations under study are
exciton-polaritons in a planar semiconductor microcavity. They are confined
within a micron-sized region through lateral trapping of their photon
component. Striking signatures of confined states of lower and upper polaritons
are found in angle-resolved light emission spectra, where a discrete energy
spectrum and broad angular patterns are present. A theoretical model supports
unambiguously our observations
Magnetic field morphology in nearby molecular clouds as revealed by starlight and submillimetre polarization
Within four nearby (d < 160 pc) molecular clouds, we statistically evaluate
the structure of the interstellar magnetic field, projected on the plane of the
sky and integrated along the line of sight, as inferred from the polarized
thermal emission of Galactic dust observed by Planck at 353 GHz and from the
optical and NIR polarization of background starlight. We compare the dispersion
of the field orientation directly in vicinities with an area equivalent to that
subtended by the Planck effective beam at 353 GHz (10') and using the
second-order structure functions of the field orientation angles. We find that
the average dispersion of the starlight-inferred field orientations within
10'-diameter vicinities is less than 20 deg, and that at these scales the mean
field orientation is on average within 5 deg of that inferred from the
submillimetre polarization observations in the considered regions. We also find
that the dispersion of starlight polarization orientations and the polarization
fractions within these vicinities are well reproduced by a Gaussian model of
the turbulent structure of the magnetic field, in agreement with the findings
reported by the Planck collaboration at scales greater than 10' and for
comparable column densities. At scales greater than 10', we find differences of
up to 14.7 deg between the second-order structure functions obtained from
starlight and submillimetre polarization observations in the same positions in
the plane of the sky, but comparison with a Gaussian model of the turbulent
structure of the magnetic field indicates that these differences are small and
are consistent with the difference in angular resolution between both
techniques.Comment: 15 pages, 10 figures, submitted to A&
A regionally resolved inventory of High Mountain Asia surge-type glaciers, derived from a multi-factor remote sensing approach
Knowledge about the occurrence and characteristics of surge-type glaciers is crucial due to the impact of surging on glacier melt and glacier-related hazards. One of the super-clusters of surge-type glaciers is High Mountain Asia (HMA). However, no consistent region-wide inventory of surge-type glaciers in HMA exists. We present a regionally resolved inventory of surge-type glaciers based on their behaviour across High Mountain Asia between 2000 and 2018. We identify surge-type behaviour from surface velocity, elevation and feature change patterns using a multi-factor remote sensing approach that combines yearly ITS_LIVE velocity data, DEM differences and very-high-resolution imagery (Bing Maps, Google Earth). Out of the â95 000 glaciers in HMA, we identified 666 that show diagnostic surge-type glacier behaviour between 2000 and 2018, which are mainly found in the Karakoram (223) and the Pamir regions (223). The total area covered by the 666 surge-type glaciers represents 19.5 % of the glacierized area in Randolph Glacier Inventory (RGI) V6.0 polygons in HMA. Only 68 glaciers were already identified as âsurge typeâ in the RGI V6.0. We further validate 107 glaciers previously labelled as âprobably surge typeâ and newly identify 491 glaciers, not previously reported in other inventories covering HMA. We finally discuss the possibility of self-organized criticality in glacier surges. Across all regions of HMA, the surge-affected area within glacier complexes displays a significant power law dependency with glacier length
LO-phonon assisted polariton lasing in a ZnO based microcavity
Polariton relaxation mechanisms are analysed experimentally and theoretically
in a ZnO-based polariton laser. A minimum lasing threshold is obtained when the
energy difference between the exciton reservoir and the bottom of the lower
polariton branch is resonant with the LO phonon energy. Tuning off this
resonance increases the threshold, and exciton-exciton scattering processes
become involved in the polariton relaxation. These observations are
qualitatively reproduced by simulations based on the numerical solution of the
semi-classical Boltzmann equations
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