434 research outputs found
Polarization of stars with debris disks: comparing observations with models
The Space telescope carried out an unprecedented survey of nearby
stars for debris disks. The dust present in these debris disks scatters and
polarizes stellar light in the visible part of the spectrum. We explore what
can be learned with aperture polarimetry and detailed radiative transfer
modelling about stellar systems with debris disks. We present a polarimetric
survey, with measurements from the literature, of candidate stars observed by
DEBRIS and DUNES surveys. We perform a statistical analysis of the
polarimetric data with the detection of far-infrared excess by and
with a sample of 223 stars. Monte Carlo simulations were performed to
determine the effects of various model parameters on the polarization level and
find the mass required for detection with current instruments. Eighteen stars
were detected with a polarization per cent and
, but only two of them have a debris disk. No statistically
significant difference is found between the different groups of stars, with,
without, and unknown status for far-infrared excess, and presence of
polarization. The simulations show that the integrated polarization is rather
small, usually per cent for typical masses detected by their
far-infrared excess for hot and most warm disks. Masses observed in cold disks
can produce polarization levels above per cent since there is usually
more dust in them than in closer disks. We list five factors which can explain
the observed low-polarization detection rate. Observations with high-precision
polarimeters should lead to additional constraints on models of unresolved
debris disks.Comment: Corrected some quotations and typos and deleted superfluous
references. 20 pages, 5 figure
Relevé polarimétrique d'étoiles candidates pour des disques de débris
Le relevĂ© DEBRIS est effectuĂ© par le tĂ©lescope spatial Herschel. Il permet dâĂ©chantillonner les disques de dĂ©bris autour dâĂ©toiles de lâenvironnement solaire. Dans la premiĂšre partie de ce mĂ©moire, un relevĂ© polarimĂ©trique de 108 Ă©toiles des candidates de DEBRIS est prĂ©sentĂ©. Utilisant le polarimĂštre de lâObservatoire du Mont-MĂ©gantic, des observations ont Ă©tĂ© effectuĂ©es afin de dĂ©tecter la polarisation due Ă la prĂ©sence de disques de dĂ©bris. En raison dâun faible taux de dĂ©tection dâĂ©toiles polarisĂ©es, une analyse statistique a Ă©tĂ© rĂ©alisĂ©e dans le but de comparer la polarisation dâĂ©toiles possĂ©dant un excĂšs dans lâinfrarouge et la polarisation de celles nâen possĂ©dant pas. Utilisant la thĂ©orie de diffusion de Mie, un modĂšle a Ă©tĂ© construit afin de prĂ©dire la polarisation due Ă un disque de dĂ©bris. Les rĂ©sultats du modĂšle sont cohĂ©rents avec les observations.
La deuxiĂšme partie de ce mĂ©moire prĂ©sente des tests optiques du polarimĂštre POL-2, construit Ă lâUniversitĂ© de MontrĂ©al. Lâimageur du tĂ©lescope James-Clerk-Maxwell passe de lâinstrument SCUBA Ă lâinstrument SCUBA-2, qui sera au moins cent fois plus rapide que son prĂ©dĂ©cesseur. De mĂȘme, le polarimĂštre suit lâamĂ©lioration et un nouveau polarimĂštre, POL-2, a Ă©tĂ© installĂ© sur SCUBA-2 en juillet 2010. Afin de vĂ©rifier les performances optiques de POL-2, des tests ont Ă©tĂ© exĂ©cutĂ©s dans les laboratoires sub-millimĂ©triques de lâUniversitĂ© de Western Ontario en juin 2009 et de lâUniversitĂ© de Lethbridge en septembre 2009. Ces tests et leurs implications pour les observations futures sont discutĂ©s.The DEBRIS survey, being performed by the space telescope Herschel, is an unbiased sampling of the debris disks candidates in the solar neighbourhood. In the first part of this thesis, a ground-based polarimetric survey of 108 DEBRIS candidate stars is presented. Using the polarimeter, âLa Belle et la Bete,â at the Mont-Megantic Observatory, observations were carried out in order to detect polarization induced by the presence of a debris disk. Due to a low rate of detection, a statistical analysis was performed to compare the polarization between stars owning a debris disk with stars without one. Using Mie scattering theory, a basic model was constructed to estimate the level of polarization produced by dust grains in a debris disk. The results of this model are consistent with our observations.
The second part of the thesis presents the optical tests of the polarimeter POL-2, built at Université de Montréal. The James Clerk-Maxwell-Telescope imager SCUBA has been upgraded to SCUBA-2, which is more than one hundred times faster than its predecessor with 500 times more pixels. Likewise, the polarimeter follows the improvement as a new polarimeter, POL-2, was installed on SCUBA-2 in July 2010. In order to verify the optical performance of POL-2, tests were completed at the submillimeter laboratories at the University of Western Ontario in June 2009, and the University of Lethbridge in September 2009. These tests and their implications for future observations are discussed
An observational estimate of the direct response of the cold-season atmospheric circulation to the Arctic Sea ice loss
Author Posting. © American Meteorological Society, 2020. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 33(9), (2020): 3863-3882, doi:10.1175/JCLI-D-19-0687.1.The direct response of the cold-season atmospheric circulation to the Arctic sea ice loss is estimated from observed sea ice concentration (SIC) and an atmospheric reanalysis, assuming that the atmospheric response to the long-term sea ice loss is the same as that to interannual pan-Arctic SIC fluctuations with identical spatial patterns. No large-scale relationship with previous interannual SIC fluctuations is found in October and November, but a negative North Atlantic Oscillation (NAO)/Arctic Oscillation follows the pan-Arctic SIC fluctuations from December to March. The signal is field significant in the stratosphere in December, and in the troposphere and tropopause thereafter. However, multiple regressions indicate that the stratospheric December signal is largely due to concomitant Siberian snow-cover anomalies. On the other hand, the tropospheric JanuaryâMarch NAO signals can be unambiguously attributed to SIC variability, with an Iceland high approaching 45 m at 500 hPa, a 2°C surface air warming in northeastern Canada, and a modulation of blocking activity in the North Atlantic sector. In March, a 1°C northern Europe cooling is also attributed to SIC. An SIC impact on the warm Arcticâcold Eurasia pattern is only found in February in relation to January SIC. Extrapolating the most robust results suggests that, in the absence of other forcings, the SIC loss between 1979 and 2016 would have induced a 2°â3°C decadeâ1 winter warming in northeastern North America and a 40â60 m decadeâ1 increase in the height of the Iceland high, if linearity and perpetual winter conditions could be assumed.This research was supported by the Blue-Action project (European Unionâs Horizon 2020 research and innovation program, Grant 727852) and by the National Science Foundation (OPP 1736738).2020-10-0
Moving beyond the costâloss ratio : economic assessment of streamflow forecasts for a risk-averse decision maker
A large effort has been made over the past 10
years to promote the operational use of probabilistic or ensemble
streamflow forecasts. Numerous studies have shown
that ensemble forecasts are of higher quality than deterministic
ones. Many studies also conclude that decisions based
on ensemble rather than deterministic forecasts lead to better
decisions in the context of flood mitigation. Hence, it is
believed that ensemble forecasts possess a greater economic
and social value for both decision makers and the general
population. However, the vast majority of, if not all, existing
hydro-economic studies rely on a costâloss ratio framework
that assumes a risk-neutral decision maker. To overcome
this important flaw, this study borrows from economics
and evaluates the economic value of early warning flood systems
using the well-known Constant Absolute Risk Aversion
(CARA) utility function, which explicitly accounts for the
level of risk aversion of the decision maker. This new framework
allows for the full exploitation of the information related
to a forecastsâ uncertainty, making it especially suited
for the economic assessment of ensemble or probabilistic
forecasts. Rather than comparing deterministic and ensemble
forecasts, this study focuses on comparing different types of
ensemble forecasts. There are multiple ways of assessing and
representing forecast uncertainty. Consequently, there exist
many different means of building an ensemble forecasting
system for future streamflow. One such possibility is to dress
deterministic forecasts using the statistics of past error forecasts.
Such dressing methods are popular among operational agencies because of their simplicity and intuitiveness. Another
approach is the use of ensemble meteorological forecasts
for precipitation and temperature, which are then provided
as inputs to one or many hydrological model(s). In
this study, three concurrent ensemble streamflow forecasting
systems are compared: simple statistically dressed deterministic
forecasts, forecasts based on meteorological ensembles,
and a variant of the latter that also includes an estimation
of state variable uncertainty. This comparison takes
place for the Montmorency River, a small flood-prone watershed
in southern central Quebec, Canada. The assessment
of forecasts is performed for lead times of 1 to 5 days, both
in terms of forecastsâ quality (relative to the corresponding
record of observations) and in terms of economic value, using
the new proposed framework based on the CARA utility
function. It is found that the economic value of a forecast
for a risk-averse decision maker is closely linked to the forecast
reliability in predicting the upper tail of the streamflow
distribution. Hence, post-processing forecasts to avoid overforecasting
could help improve both the quality and the value
of forecasts
The neural correlates of referential communication : taking advantage of sparse-sampling fMRI to study verbal communication with a real interaction partner
This paper introduces an innovative functional magnetic resonance imaging (fMRI) protocol to study real verbal interactions while limiting the impact of speech-related movement artefacts. This protocol is based on a sparse sampling acquisition technique and allowed participants to complete a referential communication task with a real interaction partner. During verbal interactions, speakers adjust their verbal productions depending on their interlocutors' knowledge of the referents being mentioned. These adjustments have been linked to theory of mind (ToM), the ability to infer other's mental states. We thus sought to determine if the brain regions supporting ToM would also be activated during a referential communication task in which participants have to present movie characters that vary in their likelihood of being known by their interlocutor. This pilot study establishes that the sparse sampling strategy is a viable option to study the neural correlates of referential communication while minimizing movement artefacts. In addition, the brain regions supporting ToM were recruited during the task, though specifically for the conditions where participants could adjust their verbal productions to the interlocutor's likely knowledge of the referent. This study therefore demonstrates the feasibility and relevance of a sparse-sampling approach to study verbal interactions with fMRI, including referential communication
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Improving music genre classification using automatically induced harmony rules
We present a new genre classification framework using both low-level signal-based features and high-level harmony features. A state-of-the-art statistical genre classifier based on timbral features is extended using a first-order random forest containing for each genre rules derived from harmony or chord sequences. This random forest has been automatically induced, using the first-order logic induction algorithm TILDE, from a dataset, in which for each chord the degree and chord category are identified, and covering classical, jazz and pop genre classes. The audio descriptor-based genre classifier contains 206 features, covering spectral, temporal, energy, and pitch characteristics of the audio signal. The fusion of the harmony-based classifier with the extracted feature vectors is tested on three-genre subsets of the GTZAN and ISMIR04 datasets, which contain 300 and 448 recordings, respectively. Machine learning classifiers were tested using 5 Ă 5-fold cross-validation and feature selection. Results indicate that the proposed harmony-based rules combined with the timbral descriptor-based genre classification system lead to improved genre classification rates
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Note complÚte ; Résumé ; Capsule: Ingénieure ou infirmier: les choix traditionnels et non traditionnels des filles et des garçons
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Leupaxin Expression Is Dispensable for B Cell Immune Responses.
The generation of a potent humoral immune response by B cells relies on the integration of signals induced by the B cell receptor, toll-like receptors and both negative and positive co-receptors. Several reports also suggest that integrin signaling plays an important role in this process. How integrin signaling is regulated in B cells is however still partially understood. Integrin activity and function are controlled by several mechanisms including regulation by molecular adaptors of the paxillin family. In B cells, Leupaxin (Lpxn) is the most expressed member of the family and in vitro studies suggest that it could dampen BCR signaling. Here, we report that Lpxn expression is increased in germinal center B cells compared to naĂŻve B cells. Moreover, Lpxn deficiency leads to decreased B cell differentiation into plasma cells in vitro. However, Lpxn seems dispensable for the generation of a potent B cell immune response in vivo. Altogether our results suggest that Lpxn is dispensable for T-dependent and T-independent B cell immune responses
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