Interactions between brown-dwarf binaries and Sun-like stars     

M. Kaplan  D. Stamatellos  A. P. Whitworth 《Astrophysics and Space Science》2012,341(2):395-403

Several mechanisms have been proposed for the formation of brown dwarfs, but there is as yet no consensus as to which—if any—are operative in nature. Any theory of brown dwarf formation must explain the observed statistics of brown dwarfs. These statistics are limited by selection effects, but they are becoming increasingly discriminating. In particular, it appears (a) that brown dwarfs that are secondaries to Sun-like stars tend to be on wide orbits, a≳100 AU (the Brown Dwarf Desert), and (b) that these brown dwarfs have a significantly higher chance of being in a close (a≲10 AU) binary system with another brown dwarf than do brown dwarfs in the field. This then raises the issue of whether these brown dwarfs have formed in situ, i.e. by fragmentation of a circumstellar disc; or have formed elsewhere and subsequently been captured. We present numerical simulations of the purely gravitational interaction between a close brown-dwarf binary and a Sun-like star. These simulations demonstrate that such interactions have a negligible chance (<0.001) of leading to the close brown-dwarf binary being captured by the Sun-like star. Making the interactions dissipative by invoking the hydrodynamic effects of attendant discs might alter this conclusion. However, in order to explain the above statistics, this dissipation would have to favour the capture of brown-dwarf binaries over single brown-dwarfs, and we present arguments why this is unlikely. The simplest inference is that most brown-dwarf binaries—and therefore possibly also most single brown dwarfs—form by fragmentation of circumstellar discs around Sun-like protostars, with some of them subsequently being ejected into the field.  相似文献   

Epistemic uncertainty assessment using Incremental Dynamic Analysis and Neural Networks     

Dimitris G. Giovanis  Michalis Fragiadakis  Vissarion Papadopoulos 《Bulletin of Earthquake Engineering》2016,14(2):529-547

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The properties of brown dwarfs and low-mass hydrogen-burning stars formed by disc fragmentation     

Dimitris Stamatellos  Anthony P. Whitworth 《Monthly notices of the Royal Astronomical Society》2009,392(1):413-427

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The effect of Poisson noise on SPH calculations     

Annabel Cartwright  Dimitris Stamatellos  Anthony P. Whitworth 《Monthly notices of the Royal Astronomical Society》2009,395(4):2373-2380

A simple algorithm is presented which generates a lattice-like, regular spacing of smoothed particle hydrodynamics (SPH) particles in discs, with any prescribed density gradient. Using this for comparison we demonstrate the effect of Poisson noise on SPH estimates of density, pressure and viscous forces when calculated using randomly distributed particles. The standard deviation of density and pressure is typically increased by greater than an order of magnitude. In a disc with a Keplerian velocity profile, the effectiveness of the Balsara switch in reducing the shear component of SPH artificial viscosity is greatly enhanced when the particles are properly spaced, reducing the magnitude of viscosity by two orders of magnitude. Noise problems are exacerbated, not removed, by increasing the numbers of SPH particles, if the number of neighbours used is kept constant. However, comparison of the evolution of a disc created using perfectly spaced particles and a disc with identical parameters but randomly placed particles, reveals very similar results. Although there are subtle differences in the evolution, and the smooth disc takes longer to begin developing structure, in both cases the identical number of objects is created by gravitational collapse. SPH disc simulations do not depend on initial density disturbances to evolve objects by gravitational collapse, which gives added credence to the validity of the results. It also appears that complicated disc settling procedures are unnecessary.  相似文献   

Non-singular time domain BEM with applications to 3D inertial soil–structure interaction     

Dimitris L. Karabalis   《Soil Dynamics and Earthquake Engineering》2004,24(3):281-293

A direct time domain boundary element method is presented based on the Stokes fundamental solutions, discretized in both time and space, and an efficient time step-by-step solution that minimizes the accumulation of errors. A non-singular numerical integration procedure, in the Cauchy sense, is proposed for the generation of the associated influence matrices. This methodology is shown to be efficient for the solution of a number of computationally intensive problems in the area of soil–structure interaction. In addition, an algorithm for the direct calculation of the response of massive foundations to externally applied forces and/or obliquely incident seismic waves is introduced. The accuracy and computational efficiency of the proposed methodologies is established through a number of comparison studies.  相似文献   

Dynamic 3-D soil–railway track interaction by BEM–FEM     

Mohsen Mohammadi  Dimitris L. Karabalis 《地震工程与结构动力学》1995,24(9):1177-1193

A study on the dynamic response of a railway track is presented via a 3-D formulation based on the frequency domain Boundary Element Method (BEM) and the Finite Element Method (FEM). The railway track consists of a group of surface, massive, rigid footings resting on a viscoelastic half-space and connected by an overlaying rail structure. The BEM, employing the full-space fundamental solutions and quadrilateral elements, is used for the simulation of the elastic half-space while the FEM is used to model the rigid footings and the rail superstructure. The loading function consists of a set of externally applied, harmonic or transient loads. Frequency as well as transient, by way of FFT, results are presented for various modes of vibration. Various numerical studies assess the through-the-soil interaction of the adjacent footings, the influence of soil damping, the effect of the overlaying structure on the frequency content of the system, and the effective simulation of an infinitely long railway track by a truncated one.  相似文献   

Ionospheric specification and forecasting based on observations from European ionosondes participating in DIAS project     

Anna Belehaki  Ljiljana Cander  Bruno Zolesi  Juergen Bremer  Christer Juren  Iwona Stanisławska  Dimitris Dialetis  Michael Hatzopoulos 《Acta Geophysica》2007,55(3):398-409

There are two main objectives of the DIAS (European Digital Upper Atmosphere Server) project. First, it establishes a pan-European repository of raw and derived digital data describing the state of ionospheric part of the upper atmosphere, which is capable of ingesting real-time information and maintaining historical data collections provided by most operating ionospheric stations in Europe. Second, the DIAS system produces and distributes, based on the raw data collection, several operational products required by various user groups for ionospheric nowcasting, prediction and forecasting purposes. The project completed on May 2006 and the DIAS server operates since then continuously. The basic products that are delivered are real-time and historical ionograms from all DIAS-affiliated ionospheric stations, frequency plots and maps of the ionosphere over Europe based on the foF2, M(3000)F2, MUF and electron density parameters, as well as long term prediction and short term forecasting up to 24 hour ahead. The paper describes use of the ionospheric measurements in the DIAS modelling techniques for specification, predict-tion and forecasting of the ionosphere over the European region, and details the final products available to the DIAS user community.  相似文献   

The diffuse-to-global and diffuse-to-direct-beam spectral irradiance ratios as turbidity indexes in an urban environment     

Dimitris G. Kaskaoutis  Harry D. Kambezidis 《Journal of Atmospheric and Solar》2009,71(2):246-256

Continuous measurements of solar spectral radiation using the Multi-filter Rotating Shadow Band Radiometer (MFRSR) are performed at the Actinometric Station of the National Observatory of Athens (ASNOA). The present study utilizes three clear-sky days of continuous observations, from local sunrise to local sunset, in order to investigate the daily variation of the radiation components (diffuse, global and direct-beam) as well as their ratios (diffuse-to-global, DGR, and diffuse-to-direct-beam, DDR) under different atmospheric conditions. Both ratios have received great scientific interest, especially for investigating solar irradiance modifications under various atmospheric conditions, aerosol load and optical properties. Apart from this, the present study shows that the DDR can constitute a measure of atmospheric turbidity when it is determined at longer wavelengths, while the DGR cannot. The effect of the solar zenith angle (SZA) on both ratios is significant at the shorter wavelengths with varying sensitivities depending on the aerosol field and sun elevation. The present study confirms the results obtained by previous solar irradiance measurements in Athens and also those computed via radiative transfer codes and sheds light on the scientific knowledge of the use of spectral DDR as an atmospheric turbidity index.  相似文献   

Numerical simulation of dynamic soil–structure interaction in shaking table testing     

Dimitris Pitilakis  Matt Dietz  David Muir Wood  Didier Clouteau  Arezou Modaressi 《Soil Dynamics and Earthquake Engineering》2008,28(6):453-467

This paper provides an insight into the numerical simulation of soil–structure interaction (SSI) phenomena studied in a shaking table facility. The shaking table test is purposely designed to confirm the ability of the numerical substructure technique to simulate the SSI phenomenon. A model foundation–structure system with strong SSI potential is embedded in a dry bed of sand deposited within a purpose designed shaking-table soil container. The experimental system is subjected to a strong ground motion. The numerical simulation of the complete soil–foundation–structure system is conducted in the linear viscoelastic domain using the substructure approach. The matching of the experimental and numerical responses in both frequency and in time domain is satisfying. Many important aspects of SSI that are apparent in the experiment are captured by the numerical simulation. Furthermore, the numerical modelling is shown to be adequate for practical engineering design purposes.  相似文献   

Meta-analysis of electron cyclotron resonance absorption features detected in high-mass X-ray binaries     

Dimitris M.Christodoulou  Silas G.T.Laycock  Demosthenes Kazanas 《天文和天体物理学研究(英文版)》2019,(10)

Using recent compilations of detailed X-ray observations and spectral models of exceptional quality, we record the electron cyclotron resonance absorption(ECRA) features that have been detected in 45 pulsating high-mass X-ray binaries(HMXBs) and ultraluminous X-ray(ULX) sources harboring neutron stars, although seven of these detections are still questionable and another 21 are single and/or not independently confirmed. From the comprehensive catalogs of Jaisawal Naik and Staubert et al.and from several additional recent observations, we produce two lists of HMXB ECRA sources: a list of 17 sources in which multiple ECRA lines or single very low-energy lines are seen, in which we can reasonably assume that the lowest energy reveals the fundamental cyclotron level for each source; and a"contaminated" list of 38 sources including the 21 detections of single ECRA lines that may(not) be higherlevel harmonics. Both lists confirm a previous result that we have obtained independently by modeling the propeller lines of Magellanic HMXB pulsars: the surface dipolar magnetic fields B*of HMXB neutron stars are segregated around five distinct values with B*= 0.28 ± 0.08, 0.55 ± 0.11, 1.3 ± 0.37, 3.0 ± 0.68 and 7.9 ± 3.1, in units of TG. However, an explanation of this phenomenon is currently lacking. We have found no correlation between these B*values and the corresponding observed spin periods, spin period derivatives, orbital periods, maximum X-ray luminosities, neutron star masses or companion star masses.  相似文献