Angular Momentum Loss Rates in Be Stars Determined by the Viscous Decretion Disc Model

Circumstellar discs around Be stars are formed by the material ejected by the central star. This process removes excess angular momentum from the star as viscosity facilitates the mass and angular momentum transfer within the disc and its growth. The angular momentum loss rates (AMLR) of Be stars is a subject of debate in the literature. Through the modelling of the disc formation and dissipation phases observed from Be stars, their average AMLR can be determined and this is the goal of this work. We use the viscous decretion disc (VDD) model to provide a range of the average AMLR for Be stars and compare these rates with predicted values from the literature. We explore the reasons for discrepancies between the predicted values of average AMLR using the VDD and Geneva stellar evolution (GSE) models that were previously reported in literature and find that the largest differences occur when Be stars are rotating below their critical speeds. We show that the time over which the mass reservoir builds up is inversely proportional to the average AMLR. Also, we determine a revised value of the average AMLR for the Galactic Be star omega CMa of 4.7x10^36 g cm^2/s^2, which is in better agreement with the values expected for a typical B2 type star. Finally, the effect of disc truncation due to the presence of a companion star is investigated and we find that this has a minimal effect on the average AMLR

Localized magnetoplasmon modes arising from broken translational symmetry in semiconductor superlattices

The electromagnetic propagator associated with the localized collective magnetoplasmon excitations in a semiconductor superlattice with broken translational symmetry, is calculated analytically within linear response theory. We discuss the properties of these collective excitations in both radiative and non-radiative regimes of the electromagnetic spectra. We find that low frequency retarded modes arise when the surface density of carriers at the symmetry breaking layer is lower than the density at the remaining layers. Otherwise a doublet of localized, high-frequency magnetoplasmon-like modes occurs.Comment: Revtex file + separate pdf figure

The impact of mass-loss on the evolution and pre-supernova properties of red supergiants

The post main-sequence evolution of massive stars is very sensitive to many parameters of the stellar models. Key parameters are the mixing processes, the metallicity, the mass-loss rate and the effect of a close companion. We study how the red supergiant lifetimes, the tracks in the Hertzsprung-Russel diagram (HRD), the positions in this diagram of the pre-supernova progenitor as well as the structure of the stars at that time change for various mass-loss rates during the red supergiant phase (RSG), and for two different initial rotation velocities. The surface abundances of RSGs are much more sensitive to rotation than to the mass-loss rates during that phase. A change of the RSG mass-loss rate has a strong impact on the RSG lifetimes and therefore on the luminosity function of RSGs. At solar metallicity, the enhanced mass-loss rate models do produce significant changes on the populations of blue, yellow and red supergiants. When extended blue loops or blue ward excursions are produced by enhanced mass-loss, the models predict that a majority of blue (yellow) supergiants are post RSG objects. These post RSG stars are predicted to show much smaller surface rotational velocities than similar blue supergiants on their first crossing of the HR gap. The position in the HRD of the end point of the evolution depends on the mass of the hydrogen envelope. More precisely, whenever, at the pre-supernova stage, the H-rich envelope contains more than about 5\% of the initial mass, the star is a red supergiant, and whenever the H-rich envelope contains less than 1\% of the total mass the star is a blue supergiant. For intermediate situations, intermediate colors/effective temperatures are obtained. Yellow progenitors for core collapse supernovae can be explained by the enhanced mass-loss rate models, while the red progenitors are better fitted by the standard mass-loss rate models.Comment: 19 pages, 11 figures, 6 tables, accepted for publication in Astronomy and Astrophysic

Quiescent and active phases in Be stars : A WISE snapshot of Young Galactic Open Clusters

Through the modeling of near-infrared photometry of star-plus disk systems with the codes bedisk/beray, we successfully describe the Wide-Field Infrared Survey Explorer (WISE) photometric characteristics of Be stars in five young open clusters, NGC 663, NGC 869, NGC 884, NGC 3766, and NGC 4755, broadly studied in the literature. WISE photometry allows previously known Be stars to be detected and to find new Be candidates which could be confirmed spectroscopically. The location of Be stars in the WISE color-magnitude diagram, separates them in two groups; active (Be stars hosting a developed circumstellar disk) and quiescent objects (Be stars in a diskless phase), and this way, we can explore how often stars are observed in these different stages. The variability observed in most active variable Be stars is compatible with a disk dissipation phase. We find that 50% of Be stars in the studied open clusters are in an active phase. We can interpret this as Be stars having a developed circumstellar disk one-half of the time. The location of Be stars with a developed disk in the color-magnitude diagram require mass loss rates in agreement with values recently reported in the literature. For these objects, we expect to have a tight relation between the equivalent width of the Hα line and the mass of the disk, if the inclination is known. Also, near-infrared photometry of Be stars in stellar clusters has the potential of being useful to test whether there is a preferential viewing angle.Instituto de Astrofísica de La Plat

Características biológicas de Rhipicephalus (Boophilus) microplus mantidos em novilhas naïve da raça Angus.

SIEPE

Efecto de Confinamiento en Muestras Mesoscópicas Superconductoras.

En superconductores mesoscópicos el tamaño y la forma de la muestra tienen un efecto considerable sobre las propiedades físicas del estado superconductor. Este efecto es estudiado convencionalmente mediante la ecuación de Ginzburg-Landau (GL) utilizando la condición de frontera que implica que la componente normal de la corriente superconductora es igual a cero. Sin embargo esta aproximación no incluye la contribución a la energía de los pares de Cooper debida al confinamiento. En el presente trabajo utilizamos la generalización realizada por Shanenko e Ivanov a la teoría GL, la cual tiene en cuenta la interacción debida al confinamiento, para estudiar el campo crítico termodinámico de una muestra mesoscópica superconductora con simetría cilíndrica en ausencia de campo magnético

Modelos y análisis para datos de degradación

La degradación es una debilidad que eventualmente puede causar la falla (e. g., el desgaste que sufren los neumáticos de un automóvil). Cuando es posible medirla, esta puede proporcionar mayor información que los datos de tiempo de falla, para propósitos de determinación y mejoramiento de la confiabilidad de un producto. Este artículo es de carácter divulgativo y desarrolla técnicas que son propuestas por Meeker y Escobar (1998). Se cree importante hacer conocer este tópico, hoy en la frontera de la Teoría de Confiabilidad (Lawless 2000)). En este trabajo se compara el análisis clásico aproximado de degradación con el modelo de degradación explícito. Estos últimos modelos implican la utilización de modelos físicos de degradación a los cuales se les introduce efectos aleatorios. Se implementan las técnicas para la estimación de modelos mixtos en S-PLUS siguiendo a Pinheiro y Bates (2000) y se utiliza el bootstrap para intervalos de confianza.Degradation is a weakness that eventually can cause failure (e.g. car tire wear). When it is possible to measure degradation, such measures often provide more information than failure-time data for purposes of assessing and improving product reliability. This is a paper which mainly pretends to divulge techniques that had been developed by Meeker and amp; Escobar (1998). We think it is worth to make this topics known, because they are in the research frontier of the Reliability Theory (Lawless 2000). We compare in this work the explicit degradation models with the approximate degradation analysis. The explicit degradation model requires specific models developed by engineers and physical scientists, which are treated as mixed models with random effects. To obtain ML estimates we use S-PLUS following Pinheiro and amp; Bates (2000), and also use bootstrap confidence intervals

Anticoagulation in Patients With COVID-19: JACC Review Topic of the Week.

Clinical, laboratory, and autopsy findings support an association between coronavirus disease-2019 (COVID-19) and thromboembolic disease. Acute COVID-19 infection is characterized by mononuclear cell reactivity and pan-endothelialitis, contributing to a high incidence of thrombosis in large and small blood vessels, both arterial and venous. Observational studies and randomized trials have investigated whether full-dose anticoagulation may improve outcomes compared with prophylactic dose heparin. Although no benefit for therapeutic heparin has been found in patients who are critically ill hospitalized with COVID-19, some studies support a possible role for therapeutic anticoagulation in patients not yet requiring intensive care unit support. We summarize the pathology, rationale, and current evidence for use of anticoagulation in patients with COVID-19 and describe the main design elements of the ongoing FREEDOM COVID-19 Anticoagulation trial, in which 3,600 hospitalized patients with COVID-19 not requiring intensive care unit level of care are being randomized to prophylactic-dose enoxaparin vs therapeutic-dose enoxaparin vs therapeutic-dose apixaban. (FREEDOM COVID-19 Anticoagulation Strategy [FREEDOM COVID]; NCT04512079).Dr Farkouh has received research grants from Amgen, Novo Nordisk, and Novartis. Dr Stone has received speaker honoraria from Infraredx; has served as a consultant to Valfix, TherOx, Robocath, HeartFlow, Ablative Solutions, Miracor, Neovasc, Abiomed, Ancora, Vectorious, Elucid Bio, Occlutech, CorFlow, Apollo Therapeutics, Impulse Dynamics, Cardiomech, Gore, and Amgen; and has equity/ options from Ancora, Cagent, Applied Therapeutics, Biostar family of funds, SpectraWave, Orchestra Biomed, Aria, Cardiac Success, Valfix, and Xenter. Dr Godoy is supported by the Frederick Banting and Charles Best Canada Graduate Scholarship (Doctoral Research Award) from the Canadian Institutes of Health Research. All other authors have reported that they have no relationships relevant to the contents of this paper to disclose.S