557 research outputs found
MRI channel flows in vertically-stratified models of accretion disks
Simulations of the magnetorotational instability (MRI) in 'unstratified'
shearing boxes exhibit powerful coherent flows, whereby the fluid vertically
splits into countermoving planar jets or `channels'. Channel flows correspond
to certain axisymmetric linear MRI modes, and their preponderance follows from
the remarkable fact that they are approximate nonlinear solutions of the MHD
equations in the limit of weak magnetic fields. We show in this paper,
analytically and with one-dimensional numerical simulations, that this property
is also shared by certain axisymmetric MRI modes in vertically-stratified
shearing boxes. These channel flows rapidly capture significant amounts of
magnetic and kinetic energy, and thus are vulnerable to secondary shear
instabilities. We examine these parasites in the vertically stratified context,
and estimate the maximum amplitudes that channels attain before they are
destroyed. These estimates suggest that a dominant channel flow will usually
drive the disk's magnetic field to thermal strengths. The prominence of these
flows and their destruction place enormous demands on simulations, but channels
in their initial stages also offer a useful check on numerical codes. These
benchmarks are especially valuable given the increasing interest in the
saturation of the stratified MRI. Lastly we speculate on the potential
connection between 'run-away' channel flows and outburst behaviour in
protostellar and dwarf nova disks.Comment: 17 pages, 12 figures. MNRAS, accepted
Farmacologia no século XX: a ciência dos medicamentos a partir da análise do livro de Goodman e Gilman
Exposure to childhood adversity and deficits in emotion recognition: results from a large, population-based sample
© 2018 Association for Child and Adolescent Mental Health. Background: Emotion recognition skills are essential for social communication. Deficits in these skills have been implicated in mental disorders. Prior studies of clinical and high-risk samples have consistently shown that children exposed to adversity are more likely than their unexposed peers to have emotion recognition skills deficits. However, only one population-based study has examined this association. Methods: We analyzed data from children participating in the Avon Longitudinal Study of Parents and Children, a prospective birth cohort (n=6,506). We examined the association between eight adversities, assessed repeatedly from birth to age 8 (caregiver physical or emotional abuse; sexual or physical abuse; maternal psychopathology; one adult in the household; family instability; financial stress; parent legal problems; neighborhood disadvantage) and the ability to recognize facial displays of emotion measured using the faces subtest of the Diagnostic Assessment of Non-Verbal Accuracy (DANVA) at age 8.5years. In addition to examining the role of exposure (vs. nonexposure) to each type of adversity, we also evaluated the role of the timing, duration, and recency of each adversity using a Least Angle Regression variable selection procedure. Results: Over three-quarters of the sample experienced at least one adversity. We found no evidence to support an association between emotion recognition deficits and previous exposure to adversity, either in terms of total lifetime exposure, timing, duration, or recency, or when stratifying by sex. Conclusions: Results from the largest population-based sample suggest that even extreme forms of adversity are unrelated to emotion recognition deficits as measured by the DANVA, suggesting the possible immutability of emotion recognition in the general population. These findings emphasize the importance of population-based studies to generate generalizable results
A Common Explanation for the Atmospheric, Solar-Neutrino and Double Beta Decay Anomalies
We make a number of small changes, including correcting an error in our
heavy-neutrino decay rate. None of our analysis is changed, either in substance
or detail.Comment: 25 pages, 6 Figures, McGill-93/1
A New Era in the Quest for Dark Matter
There is a growing sense of `crisis' in the dark matter community, due to the
absence of evidence for the most popular candidates such as weakly interacting
massive particles, axions, and sterile neutrinos, despite the enormous effort
that has gone into searching for these particles. Here, we discuss what we have
learned about the nature of dark matter from past experiments, and the
implications for planned dark matter searches in the next decade. We argue that
diversifying the experimental effort, incorporating astronomical surveys and
gravitational wave observations, is our best hope to make progress on the dark
matter problem.Comment: Published in Nature, online on 04 Oct 2018. 13 pages, 1 figur
Geodesic rewriting systems and pregroups
In this paper we study rewriting systems for groups and monoids, focusing on
situations where finite convergent systems may be difficult to find or do not
exist. We consider systems which have no length increasing rules and are
confluent and then systems in which the length reducing rules lead to
geodesics. Combining these properties we arrive at our main object of study
which we call geodesically perfect rewriting systems. We show that these are
well-behaved and convenient to use, and give several examples of classes of
groups for which they can be constructed from natural presentations. We
describe a Knuth-Bendix completion process to construct such systems, show how
they may be found with the help of Stallings' pregroups and conversely may be
used to construct such pregroups.Comment: 44 pages, to appear in "Combinatorial and Geometric Group Theory,
Dortmund and Carleton Conferences". Series: Trends in Mathematics.
Bogopolski, O.; Bumagin, I.; Kharlampovich, O.; Ventura, E. (Eds.) 2009,
Approx. 350 p., Hardcover. ISBN: 978-3-7643-9910-8 Birkhause
A fully coupled hydro-mechanical model for the modeling of coalbed methane recovery
Most coal seams hold important quantities of methane which is recognized as a valuable energy resource. Coal reservoir is considered not conventional because methane is held adsorbed on the coal surface. Coal is naturally fractured, it is a dual-porosity system made of matrix blocks and cleats (i.e fractures). In general, cleats are initially water saturated with the hydrostatic pressure maintaining the gas adsorbed in the coal matrix. Production of coalbed methane (CBM) first requires the mobilization of water in the cleats to reduce the reservoir pressure. Changes of coal properties during methane production are a critical issue in coalbed methane recovery. Indeed, any change of the cleat network will likely translate into modifications of the reservoir permeability. This work consists in the formulation of a consistent hydro-mechanical model for the CBM production modeling. Due to the particular structure of coal, the model is based on a dual-continuum approach to enrich the macroscale with microscale considerations. Shape factors are employed to take into account the geometry of the matrix blocks in the mass exchange between matrix and fractures. The hydro-mechanical model is fully coupled. For example, it captures the sorption-induced volumetric strain or the dependence of permeability on fracture aperture, which evolves with the stress state. The model is implemented in the finite element code Lagamine and is used for the modeling of one production well. A synthetic reservoir and then a real production case are considered. To date, attention has focused on a series of parametric analyses that can highlight the influence of the production scenario or key parameters related to the reservoir
Participation of the endogenous opioid system in the acquisition of a prenatal ethanol-related memory: Effects on neonatal and preweanling responsiveness to ethanol
The present study tested the involvement of the opioid system in the acquisition and expression of prenatal ethanol-related memories. We evaluated how this prenatal experience modulates ethanol self-administration in newborn rats, and preweanling's ingestion of the drug.During Gestational Days (GDs) 17-20, four groups of dams were treated with ethanol (2. g/kg) or water, followed immediately by naloxone (10. mg/kg) or saline administration. A fifth group received a similar dose of naloxone 20. min before ethanol administration. On PD 1, pups were tested on an operant learning procedure to obtain milk or 3% ethanol. One hour later, an extinction session was performed. At Postnatal Days (PDs) 14 and 15, preweanlings representing each prenatal treatment were evaluated in an intake test with infusions of 5% ethanol or water. Prior to the intake test on PD14, preweanlings were administered naloxone (1. mg/kg), saline or remained untreated. In both tests, animals representative of both genders were utilized.One-day-old pups rapidly learned the operant behavior to gain access to milk. In contrast, only pups prenatally treated with ethanol (administered immediately before naloxone or saline injection) increased operant responding to gain access to ethanol. On an intake test at PDs 14 and 15, those animals prenatally exposed to naloxone 20. min before ethanol administration consumed significantly lower ethanol levels than the remaining prenatal ethanol groups. Postnatal treatment with naloxone diminished intake of all solutions at PD14.These results suggest that prenatal ethanol exposure facilitates neonatal operant learning reinforced by intraoral administration of ethanol and increases ethanol consumption during PDs 14-15. The endogenous opioid system apparently is involved in the acquisition of prenatal ethanol memories, which can modulate the reinforcing attributes of the drug in neonatal and preweanling rats.Fil: Miranda Morales, Roberto Sebastián. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigación Médica Mercedes y Martín Ferreyra. Universidad Nacional de Córdoba. Instituto de Investigación Médica Mercedes y Martín Ferreyra; ArgentinaFil: Molina, Juan Carlos. Universidad Nacional de Córdoba. Facultad de Psicología; Argentina. University of Binghamton; Estados Unidos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigación Médica Mercedes y Martín Ferreyra. Universidad Nacional de Córdoba. Instituto de Investigación Médica Mercedes y Martín Ferreyra; ArgentinaFil: Spear, Norman E.. University of Binghamton; Estados UnidosFil: Abate, Paula. Universidad Nacional de Córdoba. Facultad de Psicología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigación Médica Mercedes y Martín Ferreyra. Universidad Nacional de Córdoba. Instituto de Investigación Médica Mercedes y Martín Ferreyra; Argentin
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