Initial Spatial Distributions Research Articles

The Formation and Early Evolution of Low-Mass Stars and Brown Dwarfs

The discovery of large numbers of young low-mass stars and brown dwarfs over the past decade has made it possible to investigate star formation and early evolution in a previously unexplored mass regime. In this review, we begin by describing surveys for low-mass members of nearby associations, open clusters, star-forming regions, and the methods used to characterize their stellar properties. We then use observations of these populations to test theories of star formation and evolution at low masses. For comparison to the formation models, we consider the initial mass function, stellar multiplicity, circumstellar disks, protostellar characteristics, and kinematic and spatial distributions at birth for low-mass stars and brown dwarfs. To test the evolutionary models, we focus on measurements of dynamical masses and empirical Hertzsprung-Russell diagrams for young brown dwarfs and planetary companions.

Composite stokes singularities in coherently and incoherently superimposed vector optical fields

The composite Stokes singularities formed in the coherent and incoherent superposition of two vector monochromatic coherent optical fields are studied and compared analytically and numerically. It is shown that the dynamic behavior of composite Stokes singularities depends on the superposition scheme (coherent or incoherent superposition), initial spatial distributions of the two superimposed fields, and propagation distance, and for the coherent superposition additionally depends on the relative phase of the two fields. There exist S12 (C-points), S31, S23 composite singularities, and L-lines, in particular, V-points and C-lines at the transverse plane. Generally, both V-points and C-lines are unstable. For the coherent superposition the degree of polarization P=1, whereas for the incoherent superposition P may be less than 1 and is variable upon propagation. The results are illustrated analytically and numerically.

Effect of the initial density and angular-velocity profiles of pre-stellar cores on the properties of young stellar objects

The physical properties of young stellar objects are studied as functions of the initial spatial distributions of the gas surface density Σ and angular velocity Ω in pre-stellar cores using numerical hydrodynamic simulations. Two limiting cases are considered: spatially homogeneous cores with Σ = const and Ω = const and centrally concentrated cores with radius-dependent densities Σ ∝ r−1 and Ω ∝ r−1. The degree of gravitational instability and protostellar disk fragmentation is mostly determined by the initial core mass and the ratio of the rotational to the gravitational energy, and depends only weakly on the initial spatial configuration of pre-stellar cores, except for the earliest stages of evolution, when models with spatially homogeneous cores can be more gravitationally unstable. The accretion of disk matter onto a protostar also depends weakly on the initial distributions of Σ and Ω, with matter from the collapsing core falling onto the disk at a rate that is slightly higher in models with spatially homogeneous cores. An appreciable dependence of the disk mass, disk radius, and the disk-to-protostar mass ratio on the initial density and angular velocity profiles of the parent core is found only for class 0 young objects; this relationship is not systematic in the later I and II stages of stellar evolution. The mass of the central protostar depends weakly on the initial core configuration in all three evolutionary stages.

Spatial anisotropy: a method to study anisotropic flow in heavy ion collisions

We introduce a method to study anisotropic flow parameter vn as a collective probe to Quark Gluon Plasma in relativistic heavy ion collisions. The emphasis is put on the use of the Fourier expansion of initial spatial azimuthal distributions of participant nucleons in the overlapped region. The coefficients ∊n of Fourier expansion are called the spatial anisotropy parameter for the n-th harmonic. We propose that collective dynamics can be studied by vn/∊n. In this paper, we will discuss in particular the second (n = 2) and the fourth (n = 4) harmonics.

Uncertainty in modelling of faecal coliform overland transport associated with manure application in Maryland

AbstractConcerns for microbial safety of surface water facilitate development of predictive models that estimate concentrations and total numbers of pathogen and indicator organisms leaving manure‐fertilized fields in overland flow during runoff events. Spatial variability of bacterial concentrations in applied manure introduces high uncertainty in the model predictions. The objective of this work was to evaluate the uncertainty in model predictions of the manure‐borne bacteria overland transport caused by limited information on the spatial distribution of bacteria in surface‐applied manure. Experiments were carried out at the ARS Beltsville experimental watershed site (OPE3) in Maryland. Dairy bovine manure was applied at a 59·3 t/ha rate on the 3·55 hectare experimental field. Faecal coliform (FC) concentrations in manure measured in 2004, 2005, 2007, and 2009 varied by 4 orders of magnitude each year. Both runoff volume and FC concentrations in runoff water were monitored using a runoff flume equipped with a refrigerated pump sampler. Two runoff events occurred before the manure was incorporated into the soil. A bacteria transport add‐on module simulator of transport with infiltration and runoff (STWIR) was linked with the event‐based kinematic runoff and erosion model (KINEROS2) to simulate convective‐dispersive overland transport, bacteria release from manure, reversible attachment–detachment to soil, and surface straining of infiltrating bacteria. The model was successfully calibrated with the field experiment data. Monte Carlo simulations were carried out to account for the spatial variation in FC in applied manure and uncertainty in the FC distribution in manure caused by the small number of samples. A tenfold and twofold variation in FC concentrations in the runoff were obtained within the 90% probability interval when initial FC spatial distributions in the manure were represented by 5 and 29 samples, respectively. Copyright © 2011 John Wiley & Sons, Ltd.

Early stage of the electron kinetics in swift heavy ion tracks in dielectrics

A Monte Carlo approach was applied for simulations of the early stage (first tens of femtosecond) of kinetics of the electronic subsystem of silica $({\text{SiO}}_{2})$ in tracks of swift heavy ions (SHIs) decelerated in the electronic stopping regime. At the first step multiple ionizations of target atoms by a projectile (${\text{Ca}}^{+19}$, $E=11.4\text{ }\text{MeV}/\text{amu}$) were described that gave the initial spatial distributions of free electrons having different momenta as well as distributions of holes in different atomic shells. Spatial propagation of fast electrons results in secondary ionizations of target atoms as well as in energy transfer to the lattice at times much shorter than the times of atomic oscillations (phonons). The well detected front of excitation in the electronic and ionic subsystems is formed due to this propagation which cannot be described by models based on diffusion mechanisms (e.g., parabolic equations of heat diffusion). At times $\ensuremath{\sim}10\text{ }\text{fs}$ after the projectile passage, about $\ensuremath{\sim}0.1\mathrm{%}$ of the energy is already transferred to the lattice. About 63% of the energy deposited by the ion is accumulated in holes at these times. Calculated distributions of these holes through the atomic shells are in excellent agreement with the spectroscopy experiments. Comparison with these experiments demonstrated also that relaxation of the electronic subsystem in SHI tracks in solids cannot be described adequately without taking into account intra-atomic and interatomic Auger (Knotek-Feibelman) processes.

Limitations of the strong field approximation in ionization of the hydrogen atom by ultrashort pulses

We present a theoretical study of the ionization of hydrogen atoms as a result of the interaction with an ultrashort external electric field. Doubly-differential momentum distributions and angular momentum distributions of ejected electrons calculated in the framework of the Coulomb-Volkov and strong field approximations, as well as classical calculations are compared with the exact solution of the time dependent Schr odinger equation. We show that in the impulsive limit, the Coulomb-Volkov distorted wave theory reproduces the exact solution. The validity of the strong field approximation is probed both classically and quantum mechanically. We found that classical mechanics describes the proper quantum momentum distributions of the ejected electrons right after a sudden momentum transfer, however pronounced the differences at latter stages that arise during the subsequent electron-nucleus interaction. Although the classical calculations reproduce the quantum momentum distributions, it fails to describe properly the angular momentum distributions, even in the limit of strong fields. The origin of this failure can be attributed to the difference between quantum and classical initial spatial distributions.

On the theory of magneto-sound double simple waves

AbstractA two-dimensional double simple wave solution is given for both weakly and highly magnetized non-relativistic plasmas moving across the magnetic field. The dependence of the density and the magnetic field on the two independent phases, namely, components of the fluid velocity, is derived. It is shown that initial spatial distributions must satisfy a definite equation whose solution determines a special category for initial conditions. The time of blow up for any fixed value of the pair phase is found. A large general class of solutions for initial distributions is obtained. For any chosen initial distribution, the physical plane of flow at any instant of time splits into two regions, one forbidden and the other permitted. These regions are obtained numerically at a typical time for a special initial distribution. For this double wave solution, differential equations for streamlines and fluid trajectories are derived. Only for the simplest cases can the corresponding curves be completely integrated and these are given in this paper. The results are qualitatively similar to the one-dimensional case derived by Stenflo and Shukla.

マルチエージェントモデルによる潮干狩り行動の数値シミュレーションに関する研究

When we use ocean space for human activities, we should consider sustainability and allowable human impacts.After the 1960's, we have lost 90 percent or more of the tideland of Tokyo-bay by the reclamation. "Uminokouen" is one of the artificial seashore parks developed in 1988 at the southeast of Yokohama and is well known as a relaxation place for the citizens. Over 20,000 people visit there for seashell-digging (SHIO-HIGARI) in a holiday in the spring.It is easy to understand that the human impact is quite large to an ecosystem of living bivalves in the park. Actually,the most part of living bivalves which are grew up after the last summer or autumn disappeared during the season. In this paper, we propose a computational model which predicts the human action of seashell-digging at this park using a multi-agent-algorithm. Comparing the simulation result with the actual data, the staying time of the visitors agrees well. Then, we investigate the relation among the initial spatial distributions of the bivalves, the spatial distributions of the amount of the caught short-neck clam and the trace of the visitors. Then we predict the human impact in the best weekend for the seashell-digging and examine the sustainability of the relaxation park.

Three-dimensional numerical simulation of fully coupled groundwater flow and land deformation due to groundwater pumping in an unsaturated fluvial aquifer system

A series of three-dimensional numerical simulations using a hydrogeomechanical numerical model is performed to analyze and predict fully coupled groundwater flow and land deformation due to groundwater pumping in an unsaturated fluvial aquifer system, which has irregular lateral boundaries. The aquifer system is composed of a loam layer underlain by a sand layer on bedrock. A steady-state numerical simulation is performed first to obtain initial spatial distributions of pressure head, hydraulic head, and groundwater flow velocity before groundwater pumping, and its results are illustrated and validated reasonably with respect to field measurements of hydraulic head. A transient-state numerical simulation is then performed to obtain spatial and temporal distributions of pressure head, hydraulic head, groundwater flow velocity, and land displacement vector during groundwater pumping, and its results are also illustrated and validated reasonably with respect to field measurements of both hydraulic head and vertical displacement. These numerical simulation results show that hydrogeomechanical numerical modeling can be a useful tool for analyzing and predicting long-term groundwater level fluctuation and land deformation due to groundwater pumping in actual three-dimensional aquifer systems.

Encoding information as orbital angular momentum states of light for wireless optical communications

The issues of security and obstruction of line of sight are concerns of wireless optical communications (WOC). In this paper, an analysis of light beams carrying orbital angular momentum (OAM) used for WOC is undertaken. The basic description and conception of OAM of light are given. Encoding information both as the OAM states of a single light beam and as the spatial structure of the mixed light beams with OAM is discussed and compared. Using OAM to encode the data offers an inherent security enhancement. Mixed vortex fields with OAM possess a self-reconstruction property that is able to regenerate and to reconstruct their initial spatial distributions of the complex amplitude in the condition of obstruction of line of sight.

Young Close-By Neutron Stars: The Gould Belt Vs. The Galactic Disc

We present new population synthesis calculations of close young neutron stars. In comparison with our previous investigation we use a different neutron star mass spectrum and different initial spatial and velocity distributions. The results confirm that most of ROSAT dim radioquiet isolated neutron stars had their origin in the Gould Belt. We predict that about several tens of young neutron stars can be identified in ROSAT All Sky Survey data at low galactic latitudes. Some of these sources also can have counterparts among EGRET unidentified sources.

General solution of the dispersion model for a one-dimensional stirred flow system using Danckwerts’ boundary conditions

In this contribution, we present the analytical solution of the one-dimensional dispersion model in the form of a partial differential equation for a continuous flow mixer with Danckwerts’ boundary conditions. The suggested solution enables the description of change of concentration of species in the system as dependence of time and spatial coordinates. The results are explicit relations for different initial spatial distributions of species concentration in the flow mixer as well as for changes in species concentration in the inlet flow as a function of time.

Formation of current-vortex filaments

Time evolution of a system where electric current and vorticity coexist is demonstrated by two-dimensional magnetohydrodynamic simulations. Such a system has two characteristics. The first one is that once the electric current and the vorticity share the same position, they tend to move simultaneously, staying overlapped with each other. The second one is that the electric current and the vorticity evolve to create more overlapping regions where they coexist, even if their initial spatial distributions are not the same. During the evolution, the distributions of the electric current and the vorticity are fragmented, and the current-vortex filaments are formed. Once the current-vortex filaments are formed, they survive stably. This is due to the strong correlation between the electric current and the vorticity.

Nonthermalized Electron Transport in Dye-Sensitized Nanocrystalline TiO2 Films: Transient Photocurrent and Random-Walk Modeling Studies

A random-walk approach is developed to model the electron-transport dynamics in dye-sensitized TiO2 solar cells within a multiple-trapping framework, and the predicted results are compared with those measured by transient photocurrent. The illumination geometry and the wavelength of the probe light are used to create certain initial spatial distributions of photoinjected electrons in the TiO2 films. Both have a dramatic effect on the shape of the measured photocurrent transient. Cells are probed with light incident from either the collecting (substrate) electrode side or the counter-electrode side. Excellent correspondence between simulated and measured current transients is observed. When electrons are injected far from the collecting electrode, their diffusion is found to be classical, corresponding to thermalized (nondispersive) transport. Nonthermalized (dispersive) electron transport is shown to be important when electrons are injected near the collecting electrode, which corresponds to the illuminat...

Modelling the impact of physical and chemical heterogeneity on solute leaching in pyritic overburden mine spoils

Spatial variability of physical and chemical properties may affect acidification and acid mine drainage in sulfide-bearing overburden mine spoils. This study compares heterogeneous with vertically layered and horizontally averaged spatial distributions of chemical components in a generic 2D-vertical spoil cross-section with heterogeneous distributions of water and air contents and of water flux densities. The initial spatial distributions of the fraction of sulfur and the pre-oxidized zones follow autocorrelated random functions. The initial background chemistry distribution is correlated to the degree of pre-oxidation using five classes of differing initial chemical systems. Processes considered include variably saturated water flow, oxygen diffusion, shrinking-core kinetics of pyrite oxidation, multicomponent reactive solute transport, and geochemical equilibrium reactions between aqueous and mineral components. Spatial distributions were generated using the SGSIM-GSLIB geostatistical simulation algorithm and estimated parameters. Results show differences in vertical concentration profiles of chemical components and in integrated breakthrough curves at 30 m depth between the heterogeneous and the layered case for both—high and low—sulfur content scenarios. Differences are relatively large in the beginning and with respect to the distribution of solid phase components. A heterogeneous sulfide mineral distribution results in vertical spreading of the oxidation front while precipitation and dissolution of the secondary minerals affects the acid mine drainage, by locally retarding and releasing the solutes.

The cellular environment in computer simulations of radiation-induced damage to DNA.

Radiation-induced DNA single- and double-strand breaks were modeled for 660 keV photon radiation and scavenger capacity mimicking the cellular environment. Atomistic representation of DNA in B form with a first hydration shell was utilized to model direct and indirect damage. Monte Carlo generated electron tracks were used to model energy deposition in matter and to derive initial spatial distributions of species which appear in the medium following radiolysis. Diffusion of species was followed with time, and their reactions with DNA and each other were modeled in an encounter-controlled manner. Three methods to account for hydroxyl radical diffusion in a cellular environment were tested: assumed exponential survival, time-limited modeling and modeling of reactions between hydroxyl radicals and scavengers in an encounter-controlled manner. Although the method based on modeling scavenging in an encounter-controlled manner is more precise, it requires substantially more computer resources than either the exponential or time-limiting method. Scavenger concentrations of 0.5 and 0.15 M were considered using exponential and encounter-controlled methods with reaction rate set at 3x10(9) dm3 mol(-1) s(-1). Diffusion length and strand break yields, predicted by these two methods for the same scavenger molarity, were different by 20%-30%. The method based on limiting time of chemistry follow-up to 10(-9) s leads to DNA damage and radical diffusion estimates similar to 0.5 M scavenger concentration in the other two methods. The difference observed in predictions made by the methods considered could be tolerated in computer simulations of DNA damage.

A stochastic simulation model for evaluating the concept of patch spraying

The long‐term economic benefits of `patch' spraying are likely to be related to the initial spatial distribution of the target weeds, the demographic characteristics of the species and the weed control and crop husbandry practices to which they are subjected. This paper describes a stochastic simulation model developed to investigate the interaction between weed seed dispersal and patch spraying. Simulated weed plant and seed populations are generated and compared with data from field observations. Lloyd's Patchiness index is used to quantify the patchiness of the weed density distribution, and the parameter k of the negative binomial distribution is used as a measure of distribution shape. A method of assessing the spatial scale of weed aggregation is proposed, in which spatial weed density information is transformed into the frequency domain, using a discrete two‐dimensional Fourier transform. In this paper, we simulate `on/off' patch spraying (full or zero herbicide application rate). A quantitative analysis of the effects of sprayer resolution and weed seed dispersal range on the herbicide reduction and yield benefits from patch spraying is performed for three initial spatial seedbank distributions. The model is parameterized for the grass weed Alopecurus myosuroides Huds. Herbicide is applied in square areas (whose size is defined by the spatial resolution of the sprayer) in which mean weed density is greater than or equal to one plant m–2. For a system conforming to this specification we show that for the control of A. myosuroides, it is unlikely that patch spraying would be profitable in the long term if the control area is larger than 6 m × 6 m. In some circumstances higher resolution may be required.

Binaries in Globular Clusters

Binary stars in a globular cluster (hereafter, GC) may be primordial (i.e. formed along with the cluster), or the result of cluster dynamics. “Dynamical” binaries can result from conservative three-body encounters (e.g. Spitzer, 1987) if a third star can carry away enough kinetic energy to leave two others bound, or from dissipative two-body encounters, if two stars happen to pass within a few stellar radii of one other (Fabian, Pringle, & Rees, 1975). Such non-primordial systems are likely to be found primarily in evolved GC cores, both because conditions are more favorable for making them there, and because of mass segregation. Knowledge of the formation process allows reasonable estimates to be made of their mass and energy distributions. The initial spatial, mass, and energy distributions of primordial binaries, on the other hand, are largely unknown.

Propagation of linear and nonlinear spin wave excitations in YIG and Lu2.04Bi0.96Fe5O12 films studied by space resolved Brillouin light scattering (abstract)

We have investigated the propagation of surface and backward volume spin waves (BVSW) with the in-plane wavevector k∥=10–900 cm−1 excited by microwaves in YIG and Lu2.04Bi0.96Fe5O12 (LBIG) films with the film thicknesses 5 and 1.5 μm, respectively. In all cases the magnetization and k∥ were in the film plane. The frequencies of the spin wave excitations as well as the spatial distribution of their intensities in the linear and in the nonlinear excitation regime were measured by means of a fully automated, high-stabilization, small-angle Brillouin light scattering (BLS) setup with a spatial resolution of 30 μm. It is well known1 that in the case of BVSW modes, the Lighthill criterion2 for modulational instability is fulfilled for both the longitudinal and the transverse perturbations of the initially constant-amplitude wave beam. Therefore, this mode is very much suitable for the investigation of the evolution of a plane-front, constant-amplitude initial beam, and for a direct experimental study of two-dimensional nonlinear diffraction effects of the beam leading to self-focusing. The garnet films with in-plane dimensions of 2×10 mm2 were grown by liquid phase epitaxy onto a single crystalline (111)-oriented gallium gadolinium garnet substrate. Two strip antennas, 35 μm wide and situated at the ends of the films, were used for the excitation of the spin waves and for monitoring. The working frequency was 8.10 GHz. Special efforts were taken to minimize the beam divergence due to the finite length of the input antenna. For all nonlinear studies the initial angular beam divergence was not larger than Θmax=0.05 rad. In the linear regime the attenuation factor was measured, and the reflection of the spin waves from the sample boundaries was studied. We also observed the interference between the two lowest order lateral modes having different initial spatial distributions of the magnetization. The BLS technique was shown to provide a signal to noise ratio exceeding 60 dB. In the nonlinear regime the propagation of the BVSW mode was thoroughly investigated. We have found clear evidence for the self-focusing effect, which exists in a narrow interval of input powers. In these studies overheating of the sample by the microwave power was carefully avoided by using an intermittent source of maximum average power of 100 mW working at a frequency of 4 kHz. That allowed us to claim that the observed focusing of the spin waves was not an artifact caused by sample heating. The distance between the focusing maximum and the antenna was 1.5 mm for LBIG sample and 6 mm for YIG sample. These values are in a good agreement with our theoretical calculations, if one takes into account the thicknesses of the films. For a comparison of the observed values of the width and the length of the focusing spot and their dependence on the microwave power with the theory, more numerical calculations are needed. Our results demonstrate the wide applicability of an advanced combined microwave-BLS technique for a two-dimensional mapping of the spin wave amplitudes in the media.