3,854 research outputs found
3D-BEVIS: Bird's-Eye-View Instance Segmentation
Recent deep learning models achieve impressive results on 3D scene analysis
tasks by operating directly on unstructured point clouds. A lot of progress was
made in the field of object classification and semantic segmentation. However,
the task of instance segmentation is less explored. In this work, we present
3D-BEVIS, a deep learning framework for 3D semantic instance segmentation on
point clouds. Following the idea of previous proposal-free instance
segmentation approaches, our model learns a feature embedding and groups the
obtained feature space into semantic instances. Current point-based methods
scale linearly with the number of points by processing local sub-parts of a
scene individually. However, to perform instance segmentation by clustering,
globally consistent features are required. Therefore, we propose to combine
local point geometry with global context information from an intermediate
bird's-eye view representation.Comment: camera-ready version for GCPR '1
Cryopreservation of cell suspensions and embryogenic calluses of Citrus using a simplified freezing process
Une méthode de congélation simplifiée a été expérimentée avec une suspension cellulaire de mandarine commune et des cals embryogènes de six variétés de #citrus$. Son efficacité est comparable à celle du protocole de congélation standard développé précédemment pour ces matériels, qui nécessitait l'emploi d'un congélateur programmable. La suspension cellulaire peut être congelée sans modifier les conditions originales (0,15 M saccharose +5 % DMSO). Les cals embryogènes de 5 variétés sur les 6 expérimentées résistent à la congélation avec le procédé simplifié. Les résultats optimaux sont obtenus en augmentant la concentration en DMSO à 10 ou 15%. (Résumé d'auteur
Magnetic field dissipation in neutron star crusts: from magnetars to isolated neutron stars
We study the non--linear evolution of magnetic fields in neutron star crusts
with special attention to the influence of the Hall drift. Our goal is to
understand the conditions for fast dissipation due to the Hall term in the
induction equation. We study the interplay of Ohmic dissipation and Hall drift
in order to find a timescale for the overall crustal field decay. We solve
numerically the Hall induction equation by means of a hybrid method (spectral
in angles but finite differences in the radial coordinate). The microphysical
input consists of the most modern available crustal equation of state,
composition and electrical conductivities. We present the first long term
simulations of the non--linear magnetic field evolution in realistic neutron
star crusts with a stratified electron number density and temperature dependent
conductivity. We show that Hall drift influenced Ohmic dissipation takes place
in neutron star crusts on a timescale of 1 Myr. When the initial magnetic field
has magnetar strength, the fast Hall drift results in an initial rapid
dissipation stage that lasts 10-50 kyr. The interplay of the Hall drift with
the temporal variation and spatial gradient of conductivity tends to favor the
displacement of toroidal fields toward the inner crust, where stable
configurations can last for 1 Myr. We show that the thermally emitting isolated
neutron stars, as the Magnificent Seven, are very likely descendants of neutron
stars born as magnetars.Comment: 14 pages, 10 figure
New results on source and diffusion spectral features of Galactic cosmic rays: I- B/C ratio
In a previous study (Maurin et al., 2001), we explored the set of parameters
describing diffusive propagation of cosmic rays (galactic convection,
reacceleration, halo thickness, spectral index and normalization of the
diffusion coefficient), and we identified those giving a good fit to the
measured B/C ratio. This study is now extended to take into account a sixth
free parameter, namely the spectral index of sources. We use an updated version
of our code where the reacceleration term comes from standard minimal
reacceleration models. The goal of this paper is to present a general view of
the evolution of the goodness of fit to B/C data with the propagation
parameters. In particular, we find that, unlike the well accepted picture, and
in accordance with our previous study, a Kolmogorov-like power spectrum for
diffusion is strongly disfavored. Rather, the analysis points towards
along with source spectra index . Two
distinct energy dependences are used for the source spectra: the usual
power-law in rigidity and a law modified at low energy, the second choice being
only slightly preferred. We also show that the results are not much affected by
a different choice for the diffusion scheme. Finally, we compare our findings
to recent works, using other propagation models. This study will be further
refined in a companion paper, focusing on the fluxes of cosmic ray nuclei.Comment: 32 pages, 13 figures, accepted in A&
Quantitative assessment of pinning forces and the superconducting gap in NbN thin films from complementary magnetic force microscopy and transport measurements
Epitaxial niobium-nitride thin films with a critical temperature of Tc=16K
and a thickness of 100nm were fabricated on MgO(100) substrates by pulsed laser
deposition. Low-temperature magnetic force microscopy (MFM) images of the
supercurrent vortices were measured after field cooling in a magnetic field of
3mT at various temperatures. Temperature dependence of the penetration depth
has been evaluated by a two-dimensional fitting of the vortex profiles in the
monopole-monopole model. Its subsequent fit to a single s-wave gap function
results in the superconducting gap amplitude Delta(0) = 2.9 meV = 2.1*kB*Tc, in
perfect agreement with previous reports. The pinning force has been
independently estimated from local depinning of individual vortices by lateral
forces exerted by the MFM tip and from transport measurements. A good
quantitative agreement between the two techniques shows that for low fields, B
<< Hc2, MFM is a powerful and reliable technique to probe the local variations
of the pinning landscape. We also demonstrate that the monopole model can be
successfully applied even for thin films with a thickness comparable to the
penetration depth.Comment: 6 pages, 6 figures, 2 table
Continuous monitoring of the boundary-layer top with lidar
International audienceContinuous lidar observations of the top height of the boundary layer (BL top) have been performed at Leipzig (51.3° N, 12.4° E), Germany, since August 2005. The results of measurements taken with a compact, automated Raman lidar over a one-year period (February 2006 to January 2007) are presented. Four different methods for the determination of the BL top are discussed. The most promising technique, the wavelet covariance algorithm, is improved by implementing some modifications so that an automated, robust retrieval of BL depths from lidar data is possible. Three case studies of simultaneous observations with the Raman lidar, a vertical-wind Doppler lidar, and accompanying radiosonde profiling of temperature and humidity are discussed to demonstrate the potential and the limits of the four lidar techniques at different aerosol and meteorological conditions. The lidar-derived BL top heights are compared with respective values derived from predictions of the regional weather forecast model COSMO of the German Meteorological Service. The comparison shows a general underestimation of the BL top by about 20% by the model. The statistical analysis of the one-year data set reveals that the seasonal mean of the daytime maximum BL top is 1400 m in spring, 1800 m in summer, 1200 m in autumn, and 800 m in winter at the continental, central European site. BL top typically increases by 100?300 m per hour in the morning of convective days
Gravitational Instability in Radiation Pressure Dominated Backgrounds
I consider the physics of gravitational instabilities in the presence of
dynamically important radiation pressure and gray radiative diffusion, governed
by a constant opacity, kappa. For any non-zero radiation diffusion rate on an
optically-thick scale, the medium is unstable unless the classical gas-only
isothermal Jeans criterion is satisfied. When diffusion is "slow," although the
dynamical Jeans instability is stabilized by radiation pressure on scales
smaller than the adiabatic Jeans length, on these same spatial scales the
medium is unstable to a diffusive mode. In this regime, neglecting gas
pressure, the characteristic timescale for growth is independent of spatial
scale and given by (3 kappa c_s^2)/(4 pi G c), where c_s is the adiabatic sound
speed. This timescale is that required for a fluid parcel to radiate away its
thermal energy content at the Eddington limit, the Kelvin-Helmholz timescale
for a radiation pressure supported self-gravitating object. In the limit of
"rapid" diffusion, radiation does nothing to suppress the Jeans instability and
the medium is dynamically unstable unless the gas-only Jeans criterion is
satisfied. I connect with treatments of Silk damping in the early universe. I
discuss several applications, including photons diffusing in regions of extreme
star formation (starburst galaxies & pc-scale AGN disks), and the diffusion of
cosmic rays in normal galaxies and galaxy clusters. The former (particularly,
starbursts) are "rapidly" diffusing and thus cannot be supported against
dynamical instability in the linear regime by radiation pressure alone. The
latter are more nearly "slowly" diffusing. I speculate that the turbulence in
starbursts may be driven by the dynamical coupling between the radiation field
and the self-gravitating gas, perhaps mediated by magnetic fields. (Abridged)Comment: 15 pages; accepted to Ap
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