18,538 research outputs found
Detecting stars, galaxies, and asteroids with Gaia
(Abridged) Gaia aims to make a 3-dimensional map of 1,000 million stars in
our Milky Way to unravel its kinematical, dynamical, and chemical structure and
evolution. Gaia's on-board detection software discriminates stars from spurious
objects like cosmic rays and Solar protons. For this, parametrised
point-spread-function-shape criteria are used. This study aims to provide an
optimum set of parameters for these filters. We developed an emulation of the
on-board detection software, which has 20 free, so-called rejection parameters
which govern the boundaries between stars on the one hand and sharp or extended
events on the other hand. We evaluate the detection and rejection performance
of the algorithm using catalogues of simulated single stars, double stars,
cosmic rays, Solar protons, unresolved galaxies, and asteroids. We optimised
the rejection parameters, improving - with respect to the functional baseline -
the detection performance of single and double stars, while, at the same time,
improving the rejection performance of cosmic rays and of Solar protons. We
find that the minimum separation to resolve a close, equal-brightness double
star is 0.23 arcsec in the along-scan and 0.70 arcsec in the across-scan
direction, independent of the brightness of the primary. We find that, whereas
the optimised rejection parameters have no significant impact on the
detectability of de Vaucouleurs profiles, they do significantly improve the
detection of exponential-disk profiles. We also find that the optimised
rejection parameters provide detection gains for asteroids fainter than 20 mag
and for fast-moving near-Earth objects fainter than 18 mag, albeit this gain
comes at the expense of a modest detection-probability loss for bright,
fast-moving near-Earth objects. The major side effect of the optimised
parameters is that spurious ghosts in the wings of bright stars essentially
pass unfiltered.Comment: Accepted for publication in A&
Local Density of States in Mesoscopic Samples from Scanning Gate Microscopy
We study the relationship between the local density of states (LDOS) and the
conductance variation in scanning-gate-microscopy experiments on
mesoscopic structures as a charged tip scans above the sample surface. We
present an analytical model showing that in the linear-response regime the
conductance shift is proportional to the Hilbert transform of the
LDOS and hence a generalized Kramers-Kronig relation holds between LDOS and
. We analyze the physical conditions for the validity of this
relationship both for one-dimensional and two-dimensional systems when several
channels contribute to the transport. We focus on realistic Aharonov-Bohm rings
including a random distribution of impurities and analyze the LDOS-
correspondence by means of exact numerical simulations, when localized states
or semi-classical orbits characterize the wavefunction of the system.Comment: 8 pages, 8 figure
Sustentabilidade no agronegĂłcio brasileiro.
Agricultura brasileira. O que Ă© Economia Verde? AgronegĂłcio brasileiro: cenário atual comĂ©rcio mundial e projeções. Insumos utilizados pela agropecuária brasileira. ResĂduos provenientes do setor agropecuário. Mudanças climáticas e impactos na produção agrĂcola. Tecnologias agrĂcolas: oportunidades de adaptação rumo Ă economia verde e de diminuição da pegada de carbono no setor agropecuário brasileiro. PolĂticas pĂşblicas no setor agropecuário brasileiro. PolĂticas pĂşblicas de incentivo Ă pesquisa e desenvolvimento (P&D). Instrumentos econĂ´micos rumo Ă economia verde. Instrumentos econĂ´micos rumo Ă economia verde
Eletroforese de isoenzimas de plântulas na identificação de espécies de Brachiaria.
Lotes de sementes de braquiaria comercializados no Brasil vem apresentando contaminacoes som sementes de outras especies pertencentes ao mesmo genero. Deste modo, uma das especies de braquiaria atuaria como planta infestante da outra no agroecossistema e a erradicacao da especie infestante seria dificultada pela agressividade caracteristca do genero e pela falta de seletividade dos herbecidas disponiveis no mercado
Transport inefficiency in branched-out mesoscopic networks: An analog of the Braess paradox
We present evidence for a counter-intuitive behavior of semiconductor
mesoscopic networks that is the analog of the Braess paradox encountered in
classical networks. A numerical simulation of quantum transport in a two-branch
mesoscopic network reveals that adding a third branch can paradoxically induce
transport inefficiency that manifests itself in a sizable conductance drop of
the network. A scanning-probe experiment using a biased tip to modulate the
transmission of one branch in the network reveals the occurrence of this
paradox by mapping the conductance variation as a function of the tip voltage
and position.Comment: 2nd version with minor stylistic corrections. To appear in Phys. Rev.
Lett.: Editorially approved for publication 6 January 201
Frictional sliding without geometrical reflection symmetry
The dynamics of frictional interfaces play an important role in many physical
systems spanning a broad range of scales. It is well-known that frictional
interfaces separating two dissimilar materials couple interfacial slip and
normal stress variations, a coupling that has major implications on their
stability, failure mechanism and rupture directionality. In contrast,
interfaces separating identical materials are traditionally assumed not to
feature such a coupling due to symmetry considerations. We show, combining
theory and experiments, that interfaces which separate bodies made of
macroscopically identical materials, but lack geometrical reflection symmetry,
generically feature such a coupling. We discuss two applications of this novel
feature. First, we show that it accounts for a distinct, and previously
unexplained, experimentally observed weakening effect in frictional cracks.
Second, we demonstrate that it can destabilize frictional sliding which is
otherwise stable. The emerging framework is expected to find applications in a
broad range of systems.Comment: 14 pages, 5 figures + Supplementary Material. Minor change in the
title, extended analysis in the second par
Hidden Sp(2s+1)- or SO(2s+1)-symmetry and new exactly solvable models in ultracold atomic systems
The high spin ultracold atom models with a special form of contact
interactions, i.e., the scattering lengthes in the total spin-
channels are equal but may be different from that in the spin-0 channel, is
studied. It is found that those models have either -symmetry for the
fermions or -symmetry for the bosons in the spin sector. Based on the
symmetry analysis, a new class of exactly solvable models is proposed and
solved via the Bethe ansatz. The ground states for repulsive fermions are also
discussed.Comment: 6 pages, 2 figure
Cosmic String Network Evolution in arbitrary Friedmann-Lemaitre models
We use the velocity-dependent one-scale model by Martins & Shellard to
investigate the evolution of a GUT long cosmic string network in arbitrary
Friedmann-Lemaitre models. Four representative models are used to show that in
general there is no scaling solution. The implications for structure formation
are briefly discussed.Comment: 8 pages, 4 postscript figures included, submitted to Phys. Rev.
Evidence for a spectroscopic direct detection of reflected light from 51 Peg b
The detection of reflected light from an exoplanet is a difficult technical
challenge at optical wavelengths. Even though this signal is expected to
replicate the stellar signal, not only is it several orders of magnitude
fainter, but it is also hidden among the stellar noise. We apply a variant of
the cross-correlation technique to HARPS observations of 51 Peg to detect the
reflected signal from planet 51 Peg b. Our method makes use of the
cross-correlation function of a binary mask with high-resolution spectra to
amplify the minute planetary signal that is present in the spectra by a factor
proportional to the number of spectral lines when performing the cross
correlation. The resulting cross-correlation functions are then normalized by a
stellar template to remove the stellar signal. Carefully selected sections of
the resulting normalized CCFs are stacked to increase the planetary signal
further. The recovered signal allows probing several of the planetary
properties, including its real mass and albedo. We detect evidence for the
reflected signal from planet 51 Peg b at a significance of 3\sigma_noise. The
detection of the signal permits us to infer a real mass of 0.46^+0.06_-0.01
M_Jup (assuming a stellar mass of 1.04\;M_Sun) for the planet and an orbital
inclination of 80^+10_-19 degrees. The analysis of the data also allows us to
infer a tentative value for the (radius-dependent) geometric albedo of the
planet. The results suggest that 51Peg b may be an inflated hot Jupiter with a
high albedo (e.g., an albedo of 0.5 yields a radius of 1.9 \pm 0.3 R_Jup for a
signal amplitude of 6.0\pm0.4 x 10^-5). We confirm that the method we perfected
can be used to retrieve an exoplanet's reflected signal, even with current
observing facilities. The advent of next generation of observing facilities
will yield new opportunities for this type of technique to probe deeper into
exoplanets.Comment: 9 pages, 6 figure
Wigner and Kondo physics in quantum point contacts revealed by scanning gate microscopy
Quantum point contacts exhibit mysterious conductance anomalies in addition
to well known conductance plateaus at multiples of 2e^2/h. These 0.7 and
zero-bias anomalies have been intensively studied, but their microscopic origin
in terms of many-body effects is still highly debated. Here we use the charged
tip of a scanning gate microscope to tune in situ the electrostatic potential
of the point contact. While sweeping the tip distance, we observe repetitive
splittings of the zero-bias anomaly, correlated with simultaneous appearances
of the 0.7 anomaly. We interpret this behaviour in terms of alternating
equilibrium and non-equilibrium Kondo screenings of different spin states
localized in the channel. These alternating Kondo effects point towards the
presence of a Wigner crystal containing several charges with different
parities. Indeed, simulations show that the electron density in the channel is
low enough to reach one-dimensional Wigner crystallization over a size
controlled by the tip position
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