7,154 research outputs found
Equilibrium, radial stability and non-adiabatic gravitational collapse of anisotropic neutron stars
In this work we construct families of anisotropic neutron stars for an
equation of state compatible with the constraints of the gravitational-wave
event GW170817 and for four anisotropy ansatze. Such stars are subjected to a
radial perturbation in order to study their stability against radial
oscillations and we develop a dynamical model to describe the non-adiabatic
gravitational collapse of the unstable anisotropic configurations whose
ultimate fate is the formation of a black hole. We find that the standard
criterion for radial stability is not always compatible with
the calculation of the oscillation frequencies for some anisotropy ansatze, and
each anisotropy parameter is constrained taking into account the recent
restriction of maximum mass of neutron stars. We further generalize the TOV
equations within a non-adiabatic context and we investigate the dynamical
behaviour of the equation of state, heat flux, anisotropy factor and mass
function as an unstable anisotropic star collapses. After obtaining the
evolution equations we recover, as a static limit, the background equations.Comment: 17 pages, 7 figure
Thermodynamics of Ion Separation by Electrosorption
We present a simple, top-down approach for the calculation of minimum energy
consumption of electrosorptive ion separation using variational form of the
(Gibbs) free energy. We focus and expand on the case of electrostatic
capacitive deionization (CDI), and the theoretical framework is independent of
details of the double-layer charge distribution and is applicable to any
thermodynamically consistent model, such as the Gouy-Chapman-Stern (GCS) and
modified Donnan (mD) models. We demonstrate that, under certain assumptions,
the minimum required electric work energy is indeed equivalent to the free
energy of separation. Using the theory, we define the thermodynamic efficiency
of CDI. We explore the thermodynamic efficiency of current experimental CDI
systems and show that these are currently very low, less than 1% for most
existing systems. We applied this knowledge and constructed and operated a CDI
cell to show that judicious selection of the materials, geometry, and process
parameters can be used to achieve a 9% thermodynamic efficiency (4.6 kT energy
per removed ion). This relatively high value is, to our knowledge, by far the
highest thermodynamic efficiency ever demonstrated for CDI. We hypothesize that
efficiency can be further improved by further reduction of CDI cell series
resistances and optimization of operational parameters
Metabolism And The Rise Of Fungus Cultivation By Ants
Most ant colonies are comprised of workers that cooperate to harvest resources and feed developing larvae. Around 50 million years ago (MYA), ants of the attine lineage adopted an alternative strategy, harvesting resources used as compost to produce fungal gardens. While fungus cultivation is considered a major breakthrough in ant evolution, the associated ecological consequences remain poorly understood. Here, we compare the energetics of attine colony-farms and ancestral hunter-gatherer colonies using metabolic scaling principles within a phylogenetic context. We find two major energetic transitions. First, the earliest lower-attine farmers transitioned to lower mass-specific metabolic rates while shifting significant fractions of biomass from ant tissue to fungus gardens. Second, a transition 20 MYA to specialized cultivars in the higher-attine clade was associated with increased colony metabolism (without changes in garden fungal content) and with metabolic scaling nearly identical to hypometry observed in hunter-gatherer ants, although only the hunter-gatherer slope was distinguishable from isometry. Based on these evolutionary transitions, we propose that shifting living-tissue storage from ants to fungal mutualists provided energetic storage advantages contributing to attine diversification and outline critical assumptions that, when tested, will help link metabolism, farming efficiency, and colony fitness.Integrative Biolog
Multi-Fluid Simulation of the Magnetic Field Evolution in Neutron Stars
Using a numerical simulation, we study the effects of ambipolar diffusion and
ohmic diffusion on the magnetic field evolution in the interior of an isolated
neutron star. We are interested in the behavior of the magnetic field on a long
time scale, over which all Alfven and sound waves have been damped. We model
the stellar interior as an electrically neutral plasma composed of neutrons,
protons and electrons, which can interact with each other through collisions
and electromagnetic forces. Weak interactions convert neutrons and charged
particles into each other, erasing chemical imbalances. As a first step, we
assume that the magnetic field points in one fixed Cartesian direction but can
vary along an orthogonal direction. We start with a uniform-density background
threaded by a homogeneous magnetic field and study the evolution of a magnetic
perturbation as well as the density fluctuations it induces in the particles.
We show that the system evolves through different quasi-equilibrium states and
estimate the characteristic time scales on which these quasi-equilibria occur.Comment: It will be published in AIP Proceedings of the Conference '40 Years
of Pulsars: Milisecond Pulsars, Magnetars and More' held at University of
McGill, Montreal, Canada, August 2007. Contributed Talk at Conference '40
Years of Pulsars: Milisecond Pulsars, Magnetars and More
Constraint on Einstein-Gauss-Bonnet Gravity from Neutron Stars
Within the framework of Einstein-Gauss-Bonnet theory in five-dimensional
spacetime ( EGB), we derive the hydrostatic equilibrium equations and solve
them numerically to obtain the neutron stars for both isotropic and anisotropic
distribution of matter. The mass-radius relations are obtained for SLy equation
of state, which describes both the solid crust and the liquid core of neutron
stars, and for a wide range of the Gauss-Bonnet coupling parameter .
More specifically, we find that the contribution of the Gauss-Bonnet term leads
to substantial deviations from the Einstein gravity. We also discuss that after
a certain value of , the theory admits higher maximum masses compared
with general relativity, however, the causality condition is violated in the
high-mass region. Finally, our results are compared with the recent
observations data on mass-radius diagram.Comment: 9 pages, 5 figure
An Intelligent Interactive Knowledge Model for Decision Support in Real Time Traffic Management
This paper proposes the use of advanced knowledge models to support real time decision for management problems as an adequate response to the current needs and technology. The new conditions for human operation created by the telematics technology are discussed and a general architecture using knowledge modelling techniques is proposed. Then, the application of the approach to support real time management of the private traffic in the city of Turin is described
Ecología térmica de Microlophus occipitalis (Sauria: Tropiduridae) en el Bosque Seco de Llanura de Tumbes, Perú
The thermal ecology of Microlophus occipitalis Peters 1871 in the plain dry forests of Tumbes (northewestern Peru) was studied. Mean body temperature was 36.1 ± 1.8 ºC, similar to body temperatures exposed by Microlophus peruvianus in northern Peru. There were no differences between body temperature and degree of thermoregulation of males and females, due to a possible association to their social structure and microhabitat use. Air and substrate temperature affects the body temperature of Microlophus occipitalis, where air temperature accounts for a significant proportion of body temperature variation. We suggest more detailed studies on this lizard species, especially under climate change scenarios in northwestern Peru.Se estudió la ecología termal de Microlophus occipitalis Peters 1871 en el Bosque Seco de Llanura de Tumbes (noroeste del Perú). La temperatura corporal promedio fue de 36,1 ± 1,8 ºC, similar a las temperaturas exhibidas por Microlophus peruvianus en el norte del Perú. No se identificaron diferencias entre la temperatura corporal y el grado de termorregulación de hembras y machos, posiblemente asociado a su estructura social y uso de microhábitat. La temperatura del aire y del sustrato afectaron la temperatura corporal de Microlophus occipitalis, aunque la temperatura del aire afecta en mayor grado la variación de la temperatura corporal. Se sugiere realizar estudios más detallados en esta especie, especialmente bajo escenarios de cambio climático en el noroeste del Perú
- …