63,396 research outputs found
Stability of coalescing binary stars against gravitational collapse: hydrodynamical simulations
We perform simulations of relativistic binary stars in post-Newtonian gravity
to investigate their dynamical stability prior to merger against gravitational
collapse in a tidal field. In general, our equations are only strictly accurate
to first post-Newtonian order, but they recover full general relativity for
spherical, static stars. We study both corotational and irrotational binary
configurations of identical stars in circular orbits. We adopt a soft,
adiabatic equation of state with , for which the onset of
instability occurs at a sufficiently small value of the compaction that a
post-Newtonian approximation is quite accurate. For such a soft equation of
state there is no innermost stable circular orbit, so that we can study
arbitrarily close binaries. This choice still allows us to study all the
qualitative features exhibited by any adiabatic equation of state regarding
stability against gravitational collapse. We demonstrate that, independent of
the internal stellar velocity profile, the tidal field from a binary companion
stabilizes a star against gravitational collapse.Comment: 13 pages, 10 figures, RevTex, to appear in Phys. Rev.
Impact of surface imperfections on the Casimir force for lenses of centimeter-size curvature radii
The impact of imperfections, which are always present on surfaces of lenses
with centimeter-size curvature radii, on the Casimir force in the lens-plate
geometry is investigated. It is shown that the commonly used formulation of the
proximity force approximation is inapplicable for spherical lenses with surface
imperfections, such as bubbles and pits. More general expressions for the
Casimir force are derived that take surface imperfections into account. Using
these expressions we show that surface imperfections can both increase and
decrease the magnitude of the Casimir force up to a few tens of percent when
compared with the case of a perfectly spherical lens. We demonstrate that the
Casimir force between a perfectly spherical lens and a plate described by the
Drude model can be made approximately equal to the force between a sphere with
some surface imperfection and a plate described by the plasma model, and vice
versa. In the case of a metallic sphere and semiconductor plate, approximately
the same Casimir forces are obtained for four different descriptions of charge
carriers in the semiconductor if appropriate surface imperfections on the lens
surface are present. The conclusion is made that there is a fundamental problem
in the interpretation of measurement data for the Casimir force, obtained by
using spherical lenses of centimeter-size radii, and their comparison with
theory.Comment: 28 pages, 7 figures, 1 table. To appear in Phys. Rev.
A spherical model with directional interactions: I. Static properties
We introduce a simple spherical model whose structural properties are similar
to the ones generated by models with directional interactions, by employing a
binary mixture of large and small hard spheres, with a square-well attraction
acting only between particles of different size. The small particles provide
the bonds between the large ones. With a proper choice of the interaction
parameters, as well as of the relative concentration of the two species, it is
possible to control the effective valence. Here we focus on a specific choice
of the parameters which favors tetrahedral ordering and study the equilibrium
static properties of the system in a large window of densities and
temperatures. Upon lowering the temperature we observe a progressive increase
in local order, accompanied by the formation of a four-coordinated network of
bonds. Three different density regions are observed: at low density the system
phase separates into a gas and a liquid phase; at intermediate densities a
network of fully bonded particles develops; at high densities -- due to the
competition between excluded volume and attractive interactions -- the system
forms a defective network. The very same behavior has been previously observed
in numerical studies of non-spherical models for molecular liquids, such as
water, and in models of patchy colloidal particles. Differently from these
models, theoretical treatments devised for spherical potentials, e.g. integral
equations and ideal mode coupling theory for the glass transition can be
applied in the present case, opening the way for a deeper understanding of the
thermodynamic and dynamic behavior of low valence molecules and particles.Comment: 11 pages, 11 figure
Modeling Forbidden Line Emission Profiles from Colliding Wind Binaries
This paper presents calculations for forbidden emission line profile shapes
arising from colliding wind binaries. The main application is for systems
involving a Wolf-Rayet (WR) star and an OB star companion. The WR wind is
assumed to dominate the forbidden line emission. The colliding wind interaction
is treated as an archimedean spiral with an inner boundary. Under the
assumptions of the model, the major findings are as follows. (a) The
redistribution of the WR wind as a result of the wind collision is not flux
conservative but typically produces an excess of line emission; however, this
excess is modest at around the 10% level. (b) Deviations from a flat-top
profile shape for a spherical wind are greatest for viewing inclinations that
are more nearly face-on to the orbital plane. At intermediate viewing
inclinations, profiles display only mild deviations from a flat-top shape. (c)
The profile shape can be used to constrain the colliding wind bow shock opening
angle. (d) Structure in the line profile tends to be suppressed in binaries of
shorter periods. (e) Obtaining data for multiple forbidden lines is important
since different lines probe different characteristic radial scales. Our models
are discussed in relation to ISO data for WR 147 and gamma Vel (WR11). The
lines for WR 147 are probably not accurate enough to draw firm conclusions. For
gamma Vel, individual line morphologies are broadly reproducible but not
simultaneously so for the claimed wind and orbital parameters. Overall, the
effort demonstrates how lines that are sensitive to the large-scale wind can
help to deduce binary system properties and provide new tests of numerical
simulations.Comment: to appear in MNRA
Competition of hydrophobic and Coulombic interactions between nano-sized solutes
The solvation of charged, nanometer-sized spherical solutes in water, and the
effective, solvent-induced force between two such solutes are investigated by
constant temperature and pressure Molecular Dynamics simulations of model
solutes carrying various charge patterns. The results for neutral solutes agree
well with earlier findings, and with predictions of simple macroscopic
considerations: substantial hydrophobic attraction may be traced back to strong
depletion (``drying'') of the solvent between the solutes. This hydrophobic
attraction is strongly reduced when the solutes are uniformly charged, and the
total force becomes repulsive at sufficiently high charge; there is a
significant asymmetry between anionic and cationic solute pairs, the latter
experiencing a lesser hydrophobic attraction. The situation becomes more
complex when the solutes carry discrete (rather than uniform) charge patterns.
Due to antagonistic effects of the resulting hydrophilic and hydrophobic
``patches'' on the solvent molecules, water is once more significantly depleted
around the solutes, and the effective interaction reverts to being mainly
attractive, despite the direct electrostatic repulsion between solutes.
Examination of a highly coarse-grained configurational probability density
shows that the relative orientation of the two solutes is very different in
explicit solvent, compared to the prediction of the crude implicit solvent
representation. The present study strongly suggests that a realistic modeling
of the charge distribution on the surface of globular proteins, as well as the
molecular treatment of water are essential prerequisites for any reliable study
of protein aggregation.Comment: 20 pages, 25 figure
Comparison between experiment and theory for the thermal Casimir force
We analyze recent experiments on measuring the thermal Casimir force with
account of possible background effects. Special attention is paid to the
validity of the proximity force approximation (PFA) used in the comparison
between the experimental data and computational results in experiments
employing a sphere-plate geometry. The PFA results are compared with the exact
results where they are available. The possibility to use fitting procedures in
theory-experiment comparison is discussed. On this basis we reconsider
experiments exploiting spherical lenses of centimeter-size radii.Comment: Plenary talk at the 10th International Conference "Quantum Field
Theory Under the Influence of External Conditions" (Benasque, Spain, 2011);
16 pages, 5 figure
Contributions to the Power Spectrum of Cosmic Microwave Background from Fluctuations Caused by Clusters of Galaxies
We estimate the contributions to the cosmic microwave background radiation
(CMBR) power spectrum from the static and kinematic Sunyaev-Zel'dovich (SZ)
effects, and from the moving cluster of galaxies (MCG) effect. We conclude, in
agreement with other studies, that at sufficiently small scales secondary
fluctuations caused by clusters provide important contributions to the CMBR. At
, these secondary fluctuations become important relative to
lensed primordial fluctuations. Gravitational lensing at small angular scales
has been proposed as a way to break the ``geometric degeneracy'' in determining
fundamental cosmological parameters. We show that this method requires the
separation of the static SZ effect, but the kinematic SZ effect and the MCG
effect are less important. The power spectrum of secondary fluctuations caused
by clusters of galaxies, if separated from the spectrum of lensed primordial
fluctuations, might provide an independent constraint on several important
cosmological parameters.Comment: LateX, 41 pages and 10 figures. Accepted for publication in the
Astrophysical Journa
Modelling of Phase Separation in Alloys with Coherent Elastic Misfit
Elastic interactions arising from a difference of lattice spacing between two
coherent phases can have a strong influence on the phase separation
(coarsening) of alloys. If the elastic moduli are different in the two phases,
the elastic interactions may accelerate, slow down or even stop the phase
separation process. If the material is elastically anisotropic, the
precipitates can be shaped like plates or needles instead of spheres and can
form regular precipitate superlattices. Tensions or compressions applied
externally to the specimen may have a strong effect on the shapes and
arrangement of the precipitates. In this paper, we review the main theoretical
approaches that have been used to model these effects and we relate them to
experimental observations. The theoretical approaches considered are (i)
`macroscopic' models treating the two phases as elastic media separated by a
sharp interface (ii) `mesoscopic' models in which the concentration varies
continuously across the interface (iii) `microscopic' models which use the
positions of individual atoms.Comment: 106 pages, in Latex, figures available upon request, e-mail
addresses: [email protected], [email protected],
[email protected], submitted to the Journal of Statistical Physic
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