115 research outputs found
Activity time series of old stars from late F to early K VI. Exoplanet mass characterisation and detectability in radial velocity
Stellar variability impacts radial velocities at various timescales and
therefore the detectability of exoplanets and the mass determination based on
this technique. It is necessary to implement systematic studies, to delineate
the current limitations of RV techniques to detect Earth-like planets. This
paper aims are to investigate whether the targeted 10% mass uncertainty from RV
follow-up of transits detected by PLATO can be reached, and to analyse and
quantify Earth-like planet detectability for various spectral types. We
implemented blind tests based on a large data set of realistic synthetic time
series reproducing different phenomena leading to stellar variability such as
complex magnetic activity patterns as well as flows, covering F6-K4 stars and a
wide range of activity levels. The 10% mass uncertainty for a 1 MEarth in the
habitable zone of a G2 star cannot be reached, even with an improved version of
a usual correction of stellar activity and even for long-duration (ten years)
well-sampled observations. This level can be reached for masses above 3 MEarth
or for K4 stars alone. We quantify the maximum dispersion of the RV residuals
needed to reach this 10% level, assuming the correction method and models do
not affect the planetary signal. Several other methods were tested and do not
allow a significantly improvement of this limited performance. Similarly, such
low-mass planets in the habitable zone cannot be detected with a similar
correction: blind tests lead to very low detection rates for 1 MEarth and a
very high level of false positives. Very significant and new improvements with
respect to methods based on activity indicators to correct for stellar activity
must be devised at all timescales to reach the objective of 10% uncertainty on
the mass or to detect such planets in RV. Methods based on the correlation with
activity indicators are unlikely to be sufficient.Comment: Paper accepted in Astronomy and Astrophysic
Curvature-Controlled Defect Localization in Elastic Surface Crystals
We investigate the influence of curvature and topology on crystalline dimpled patterns on the surface of generic elastic bilayers. Our numerical analysis predicts that the total number of defects created by adiabatic compression exhibits universal quadratic scaling for spherical, ellipsoidal, and toroidal surfaces over a wide range of system sizes. However, both the localization of individual defects and the orientation of defect chains depend strongly on the local Gaussian curvature and its gradients across a surface. Our results imply that curvature and topology can be utilized to pattern defects in elastic materials, thus promising improved control over hierarchical bending, buckling, or folding processes. Generally, this study suggests that bilayer systems provide an inexpensive yet valuable experimental test bed for exploring the effects of geometrically induced forces on assemblies of topological charges.MIT Masdar ProgramSwiss National Science Foundation (148743)Solomon Buchsbaum AT&T Research FundAlfred P. Sloan Foundation (Sloan Research Fellowship)National Science Foundation (U.S.) (CAREER CMMI-1351449
Aeroelastic instability of cantilevered flexible plates in uniform flow
We address the flutter instability of a flexible plate immersed in an axial
flow. This instability is similar to flag flutter and results from the
competition between destabilising pressure forces and stabilising bending
stiffness. In previous experimental studies, the plates have always appeared
much more stable than the predictions of two-dimensional models. This
discrepancy is discussed and clarified in this paper by examining
experimentally and theoretically the effect of the plate aspect ratio on the
instability threshold. We show that the two-dimensional limit cannot be
achieved experimentally because hysteretical behaviour and three-dimensional
effects appear for plates of large aspect ratio. The nature of the instability
bifurcation (sub- or supercritical) is also discussed in the light of recent
numerical results
Dynamique d'un fluide dans un cylindre en précession
Pour des petits nombres de Reynolds
lâĂ©coulement d'un fluide dans un cylindre en prĂ©cession est stable et est une
superposition de modes de Kelvin. Pour Re suffisamment grand lâĂ©coulement est instable.
Un mécanisme de résonance triadique entre modes de Kelvin permet de déterminer le seuil
de stabilitĂ©. Une analyse faiblement non-linĂ©aire permet de prĂ©dire lâĂ©coulement moyen,
mĂȘme en rĂ©gime turbulent. Ces prĂ©dictions sont en trĂšs bon accord avec les
expériences
Etude de la stabilité d'un fluide dans un cylindre en précession
Dans ce papier nous prĂ©sentons une analyse de stabilitĂ© linĂ©aire basĂ©e sur un mĂ©canisme de rĂ©sonance triadique pour expliquer l'instabilitĂ© d'un fluide dans un cylindre en prĂ©cession. Nous Ă©tablissons une expression formelle du taux de croissance dans le cas oĂč un mode de Kelvin forcĂ© par la prĂ©cession rĂ©sonne avec deux modes de Kelvin libres. Cette expression est Ă©valuĂ©e dans le cas particulier de modes de nombres d'onde azimutaux m=1 et m=2 lorsqu'on considĂšre la premiĂšre rĂ©sonance des modes 1 et 3 de l'Ă©coulement de base. Nous montrons alors qu'il existe des modes pour lesquels cette thĂ©orie prĂ©voit une interaction constructive
COVID-19 symptoms at hospital admission vary with age and sex: results from the ISARIC prospective multinational observational study
Background:
The ISARIC prospective multinational observational study is the largest cohort of hospitalized patients with COVID-19. We present relationships of age, sex, and nationality to presenting symptoms.
Methods:
International, prospective observational study of 60â109 hospitalized symptomatic patients with laboratory-confirmed COVID-19 recruited from 43 countries between 30 January and 3 August 2020. Logistic regression was performed to evaluate relationships of age and sex to published COVID-19 case definitions and the most commonly reported symptoms.
Results:
âTypicalâ symptoms of fever (69%), cough (68%) and shortness of breath (66%) were the most commonly reported. 92% of patients experienced at least one of these. Prevalence of typical symptoms was greatest in 30- to 60-year-olds (respectively 80, 79, 69%; at least one 95%). They were reported less frequently in children (â€â18 years: 69, 48, 23; 85%), older adults (â„â70 years: 61, 62, 65; 90%), and women (66, 66, 64; 90%; vs. men 71, 70, 67; 93%, each Pâ<â0.001). The most common atypical presentations under 60 years of age were nausea and vomiting and abdominal pain, and over 60 years was confusion. Regression models showed significant differences in symptoms with sex, age and country.
Interpretation:
This international collaboration has allowed us to report reliable symptom data from the largest cohort of patients admitted to hospital with COVID-19. Adults over 60 and children admitted to hospital with COVID-19 are less likely to present with typical symptoms. Nausea and vomiting are common atypical presentations under 30 years. Confusion is a frequent atypical presentation of COVID-19 in adults over 60 years. Women are less likely to experience typical symptoms than men
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