13,065 research outputs found
A Brief Review on Syntheses, Structures and Applications of Nanoscrolls
Nanoscrolls are papyrus-like nanostructures which present unique properties
due to their open ended morphology. These properties can be exploited in a
plethora of technological applications, leading to the design of novel and
interesting devices. During the past decade, significant advances in the
synthesis and characterization of these structures have been made, but many
challenges still remain. In this mini review we provide an overview on their
history, experimental synthesis methods, basic properties and application
perspectives
New Measurements of Venus Winds with Ground-Based Doppler Velocimetry at CFHT
operations with observations from the ground using various techniques and spectral domains (Lellouch and Witasse, 2008). We present an analysis of Venus Doppler winds at cloud tops based on observations made at the Canada France Hawaii 3.6-m telescope (CFHT) with the ESPaDOnS visible spectrograph. These observations consisted of high-resolution spectra of Fraunhofer lines in the visible range (0.37-1.05 μm) to measure the winds at cloud tops using the Doppler shift of solar radiation scattered by cloud top particles in the observer's direction (Widemann et al., 2007, 2008). The observations were made during 19-20 February 2011 and were coordinated with Visual Monitoring Camera (VMC) observations by Venus Express. The complete optical spectrum was collected over 40 spectral orders at each point with 2-5 seconds exposures, at a resolution of about 80000. The observations included various points of the dayside hemisphere at a phase angle of 67°, between +10° and -60° latitude, in steps of 10° , and from +70° to -12° longitude relative to sub-Earth meridian in steps of 12°. The Doppler shift measured in scattered solar light on Venus dayside results from two instantaneous motions: (1) a motion between the Sun and Venus upper cloud particles; (2) a motion between the observer and Venus clouds. The measured Doppler shift, which results from these two terms combined, varies with the planetocentric longitude and latitude and is minimum at meridian ΦN = ΦSun - ΦEarth where the two components subtract to each other for a pure zonal regime. Due to the need for maintaining a stable velocity reference during the course of acquisition using high resolution spectroscopy, we measure relative Doppler shifts to ΦN. The main purpose of our work is to provide variable wind measurements with respect to the background atmosphere, complementary to simultaneous measurements made with the VMC camera onboard the Venus Express. We will present first results from this work, comparing with previous results by the CFHT/ESPaDOnS and VLT-UVES spectrographs (Machado et al., 2012), with Galileo fly-by measurements and with VEx nominal mission observations (Peralta et al., 2007, Luz et al., 2011). Acknowledgements: The authors acknowledge support from FCT through projects PTDC/CTE-AST/110702/2009 and PEst-OE/FIS/UI2751/2011. PM and TW also acknowledge support from the Observatoire de Paris. Lellouch, E., and Witasse, O., A coordinated campaign of Venus ground-based observations and Venus Express measurements, Planetary and Space Science 56 (2008) 1317-1319. Luz, D., et al., Venus's polar vortex reveals precessing circulation, Science 332 (2011) 577-580. Machado, P., Luz, D. Widemann, T., Lellouch, E., Witasse, O, Characterizing the atmospheric dynamics of Venus from ground-based Doppler velocimetry, Icarus, submitted. Peralta J., R. Hueso, A. Sánchez-Lavega, A reanalysis of Venus winds at two cloud levels from Galileo SSI images, Icarus 190 (2007) 469-477. Widemann, T., Lellouch, E., Donati, J.-F., 2008, Venus Doppler winds at Cloud Tops Observed with ESPaDOnS at CFHT, Planetary and Space Science, 56, 1320-1334
Darth Fader: Using wavelets to obtain accurate redshifts of spectra at very low signal-to-noise
We present the DARTH FADER algorithm, a new wavelet-based method for
estimating redshifts of galaxy spectra in spectral surveys that is particularly
adept in the very low SNR regime. We use a standard cross-correlation method to
estimate the redshifts of galaxies, using a template set built using a PCA
analysis on a set of simulated, noise-free spectra. Darth Fader employs wavelet
filtering to both estimate the continuum & to extract prominent line features
in each galaxy spectrum. A simple selection criterion based on the number of
features present in the spectrum is then used to clean the catalogue: galaxies
with fewer than six total features are removed as we are unlikely to obtain a
reliable redshift estimate. Applying our wavelet-based cleaning algorithm to a
simulated testing set, we successfully build a clean catalogue including
extremely low signal-to-noise data (SNR=2.0), for which we are able to obtain a
5.1% catastrophic failure rate in the redshift estimates (compared with 34.5%
prior to cleaning). We also show that for a catalogue with uniformly mixed SNRs
between 1.0 & 20.0, with realistic pixel-dependent noise, it is possible to
obtain redshifts with a catastrophic failure rate of 3.3% after cleaning (as
compared to 22.7% before cleaning). Whilst we do not test this algorithm
exhaustively on real data, we present a proof of concept of the applicability
of this method to real data, showing that the wavelet filtering techniques
perform well when applied to some typical spectra from the SDSS archive. The
Darth Fader algorithm provides a robust method for extracting spectral features
from very noisy spectra. The resulting clean catalogue gives an extremely low
rate of catastrophic failures, even when the spectra have a very low SNR. For
very large sky surveys, this technique may offer a significant boost in the
number of faint galaxies with accurately determined redshifts.Comment: 22 pages, 15 figures. Accepted for publication in Astronomy &
Astrophysic
Estimations for the Single Diffractive production of the Higgs boson at the Tevatron and the LHC
The single diffractive production of the standard model Higgs boson is
computed using the diffractive factorization formalism, taking into account a
parametrization for the Pomeron structure function provided by the H1
Collaboration. We compute the cross sections at next-to-leading order accuracy
for the gluon fusion process, which includes QCD and electroweak corrections.
The gap survival probability () is also introduced to account for
the rescattering corrections due to spectator particles present in the
interaction, and to this end we compare two different models for the survival
factor. The diffractive ratios are predicted for proton-proton collisions at
the Tevatron and the LHC for the Higgs boson mass of = 120 GeV.
Therefore, our results provide updated estimations for the diffractive ratios
of the single diffractive production of the Higgs boson in the Tevatron and LHC
kinematical regimes.Comment: 20 pages, 6 figures, 3 table
Modelling microbial exchanges between forms of soil nitrogen in contrasting ecosystems
Although nitrogen (N) is often combined with carbon (C) in organic
molecules, C passes from the air to the soil through plant photosynthesis,
whereas N passes from the soil to plants through a chain of microbial
conversions. However, dynamic models do not fully consider the
microorganisms at the centre of exchange processes between organic and
mineral forms of N. This study monitored the transfer of <sup>14</sup>C and
<sup>15</sup>N between plant materials, microorganisms, humified compartments, and
inorganic forms in six very different ecosystems along an altitudinal
transect. The microbial conversions of the <sup>15</sup>N forms appear to be
strongly linked to the previously modelled C cycle, and the same equations
and parameters can be used to model both C and N cycles. The only
difference is in the modelling of the flows between microbial and inorganic
forms. The processes of mineralization and immobilization of N appear to be
regulated by a two-way microbial exchange depending on the C : N ratios of
microorganisms and available substrates. The MOMOS (Modelling of Organic Matter of Soils) model has already been
validated for the C cycle and also appears to be valid for the prediction of
microbial transformations of N forms. This study shows that the hypothesis
of microbial homeostasis can give robust predictions at global scale.
However, the microbial populations did not appear to always be independent
of the external constraints. At some altitudes their C : N ratio could be
better modelled as decreasing during incubation and increasing with
increasing C storage in cold conditions. The ratio of potentially
mineralizable-<sup>15</sup>N/inorganic-<sup>15</sup>N and the <sup>15</sup>N stock in the
plant debris and the microorganisms was modelled as increasing with altitude,
whereas the <sup>15</sup>N storage in stable humus was modelled as decreasing with
altitude. This predicts that there is a risk that mineralization of organic
reserves in cold areas may increase global warming
Tratamento Endovascular da Disseção Crónica Isolada da Aorta Abdominal
Isolated acute abdominal aortic dissection is a relatively rare event. Its natural history is not fully understood and its optimal treatment is not established. Open surgery represents the most described treatment but endovascular intervention has had increasing application. Isolated chronic abdominal aortic dissection is even less described in the literature. We describe three patients with isolated chronic abdominal aortic dissection who underwent endovascular treatment in our institution. Mean age at presentation was 82 years. Indication for surgical intervention was aneurismal degeneration. Mean aortic diameter at presentation was 46.7 mm. There was no perioperative mortality or reinterventions. Mean follow-up was 5.3 years (2-12 years). Late reintervention was needed in one patient, eight years after initial surgery, due to type 1 endoleak. According to our experience, endovascular intervention represents an effective and durable treatment option in isolated chronic abdominal aortic dissection. However, long-term follow-up is mandatory. Furthermore, larger studies are still needed to understand this disease and its adequate treatment.info:eu-repo/semantics/publishedVersio
Influence of Disorder Strength on Phase Field Models of Interfacial Growth
We study the influence of disorder strength on the interface roughening
process in a phase-field model with locally conserved dynamics. We consider two
cases where the mobility coefficient multiplying the locally conserved current
is either constant throughout the system (the two-sided model) or becomes zero
in the phase into which the interface advances (one-sided model). In the limit
of weak disorder, both models are completely equivalent and can reproduce the
physical process of a fluid diffusively invading a porous media, where
super-rough scaling of the interface fluctuations occurs. On the other hand,
increasing disorder causes the scaling properties to change to intrinsic
anomalous scaling. In the limit of strong disorder this behavior prevails for
the one-sided model, whereas for the two-sided case, nucleation of domains in
front of the invading front are observed.Comment: Accepted for publication in PR
- …