894 research outputs found
A post-Newtonian diagnosis of quasiequilibrium configurations of neutron star-neutron star and neutron star-black hole binaries
We use a post-Newtonian diagnostic tool to examine numerically generated
quasiequilibrium initial data sets for non-spinning double neutron star and
neutron star-black hole binary systems. The PN equations include the effects of
tidal interactions, parametrized by the compactness of the neutron stars and by
suitable values of ``apsidal'' constants, which measure the degree of
distortion of stars subjected to tidal forces. We find that the post-Newtonian
diagnostic agrees well with the double neutron star initial data, typically to
better than half a percent except where tidal distortions are becoming extreme.
We show that the differences could be interpreted as representing small
residual eccentricity in the initial orbits. In comparing the diagnostic with
preliminary numerical data on neutron star-black hole binaries, we find less
agreement.Comment: 17 pages, 6 tables, 8 figure
Hydrodynamic simulations of irradiated secondaries in dwarf novae
We investigate numerically the surface flow on the secondary star during
outbursts. We use a simple model for the irradiation and the geometry of the
secondary star: the irradiation temperature is treated as a free parameter and
the secondary is replaced by a spherical star with a space-dependent Coriolis
force that mimics the effect of the Roche geometry. The Euler equations are
solved in spherical coordinates with the TVD-MacCormack scheme. We show that
the Coriolis force leads to the formation of a circulation flow from high
latitude region to the close vicinity of the point. However no heat can
be efficiently transported to the region due to the rapid radiative
cooling of the hot material as it enters the equatorial belt shadowed from
irradiation. Under the assumption of hydrostatic equilibrium, the Coriolis
force could lead to a moderate increase of the mass transfer rate by pushing
the gas in the vertical direction at the point, but only during the
initial phases of the outburst (about 15 -- 20 orbital periods). We conclude
that the Coriolis force does not prevent a flow from the heated regions of the
secondary towards the region, at least during the initial phase of an
outburst, but the resulting increase of the mass transfer rate is moderate, and
it is unlikely to be able to account for the duration of long outbursts.Comment: 11 pages, 11 figures, accepted for publication in A&
Tidal torque induced by orbital decay in compact object binaries
As we observe in the moon-earth system, tidal interactions in binary systems
can lead to angular momentum exchange. The presence of viscosity is generally
regarded as the condition for such transfer to happen. In this paper, we show
how the orbital evolution can cause a persistent torque between the binary
components, even for inviscid bodies. This preferentially occurs at the final
stage of coalescence of compact binaries, when the orbit shrinks successively
by gravitational waves and plunging on a timescale shorter than the viscous
timescale. The total orbital energy transferred to the secondary by this torque
is ~0.01 of its binding energy. We further show that this persistent torque
induces a differentially rotating quadrupole perturbation. Specializing to the
case of a secondary neutron star, we find that this non equilibrium state has
an associated free energy of 10^47-10^48 erg, just prior to coalescence. This
energy is likely stored in internal fluid motions, with a sizable amount of
differential rotation. By tapping this free energy reservoir, a preexisting
weak magnetic field could be amplified up to a strength of ~10^15 Gauss. Such a
dynamically driven tidal torque can thus recycle an old neutron star into a
highly magnetized neutron star, with possible observational consequences at
merger.Comment: 14 pages, 8 figures, new sections added, accepted on Sept.19,
published on MNRA
Very large dielectric response of thin ferroelectric films with the dead layers
We study the dielectric response of ferroelectric (FE) thin films with "dead"
dielectric layer at the interface with electrodes. The domain structure
inevitably forms in the FE film in presence of the dead layer. As a result, the
effective dielectric constant of the capacitor increases
abruptly when the dead layer is thin and, consequently, the pattern of
180-degree domains becomes "soft". We compare the exact results for this
problem with the description in terms of a popular "capacitor" model, which is
shown to give qualitatively incorrect results. We relate the present results to
fatigue observed in thin ferroelectric films.Comment: 5 pages, REVTeX 3.1 with one eps-figure. A note added that the linear
response is not changed by electromechanical effect. To appear in Phys. Rev.
Electromagnetic Modes in Deformed Nuclei
A strength function method is adopted to describe a coupling between electric
and magnetic modes of different multipolarity. The collective vibrations are
analysed for a separable residual interaction in the framework of the
random-phase approximation. The coupling between and giant resonances
is considered as an illustrative example.Comment: 7 pages (latex), 1 figure (ps file), an invited talk at the workshop
"Symmetries and Spin - Praha 98", to be published in Czech.J.Phys., 199
The Rossiter-McLaughlin effect and analytic radial velocity curves for transiting extrasolar planetary systems
A transiting extrasolar planet sequentially blocks off the light coming from
the different parts of the disk of the host star in a time dependent manner.
Due to the spin of the star, this produces an asymmetric distortion in the line
profiles of the stellar spectrum, leading to an apparent anomaly of the radial
velocity curves, known as the Rossiter - McLaughlin effect. Here, we derive
approximate but accurate analytic formulae for the anomaly of radial velocity
curves taking account of the stellar limb darkening. The formulae are
particularly useful in extracting information of the projected angle between
the planetary orbit axis and the stellar spin axis, \lambda, and the projected
stellar spin velocity, V sin I_s. We create mock samples for the radial curves
for the transiting extrasolar system HD209458, and demonstrate that constraints
on the spin parameters (V sin I_s, \lambda) may be significantly improved by
combining our analytic template formulae and the precision velocity curves from
high-resolution spectroscopic observations with 8-10 m class telescopes. Thus
future observational exploration of transiting systems using the Rossiter -
McLaughlin effect is one of the most important probes to better understanding
of the origin of extrasolar planetary systems, especially the origin of their
angular momentum.Comment: 39 pages, 16 figures, Accepted to ApJ. To match the published version
(ApJ 623, April 10 issue
Permutation Entropy and Signal Energy Increase the Accuracy of Neuropathic Change Detection in Needle EMG
Background and Objective. Needle electromyography can be used to detect the number of changes and morphological changes in motor unit potentials of patients with axonal neuropathy. General mathematical methods of pattern recognition and signal analysis were applied to recognize neuropathic changes. This study validates the possibility of extending and refining turns-amplitude analysis using permutation entropy and signal energy. Methods. In this study, we examined needle electromyography in 40 neuropathic individuals and 40 controls. The number of turns, amplitude between turns, signal energy, and “permutation entropy” were used as features for support vector machine classification. Results. The obtained results proved the superior classification performance of the combinations of all of the above-mentioned features compared to the combinations of fewer features. The lowest accuracy from the tested combinations of features had peak-ratio analysis. Conclusion. Using the combination of permutation entropy with signal energy, number of turns and mean amplitude in SVM classification can be used to refine the diagnosis of polyneuropathies examined by needle electromyography
Modeling Multi-Wavelength Stellar Astrometry. II. Determining Absolute Inclinations, Gravity Darkening Coefficients, and Spot Parameters of Single Stars with SIM Lite
We present a novel technique to determine the absolute inclination of single
stars using multi-wavelength sub-milliarcsecond astrometry. The technique
exploits the effect of gravity darkening, which causes a wavelength-dependent
astrometric displacement parallel to a star's projected rotation axis. We find
this effect is clearly detectable using SIM Lite for various giant stars and
rapid rotators, and present detailed models for multiple systems using the
REFLUX code. We also explore the multi-wavelength astrometric reflex motion
induced by spots on single stars. We find that it should be possible to
determine spot size, relative temperature, and some positional information for
both giant and nearby main-sequence stars utilizing multi-wavelength SIM Lite
data. This data will be extremely useful in stellar and exoplanet astrophysics,
as well as supporting the primary SIM Lite mission through proper
multi-wavelength calibration of the giant star astrometric reference frame, and
reduction of noise introduced by starspots when searching for extrasolar
planets.Comment: 8 pages, 7 figures, 4 tables. Accepted for publication in the
Astrophysical Journa
Two New Tidally Distorted White Dwarfs
We identify two new tidally distorted white dwarfs (WDs), SDSS
J174140.49+652638.7 and J211921.96-001825.8 (hereafter J1741 and J2119). Both
stars are extremely low mass (ELM, < 0.2 Msun) WDs in short-period, detached
binary systems. High-speed photometric observations obtained at the McDonald
Observatory reveal ellipsoidal variations and Doppler beaming in both systems;
J1741, with a minimum companion mass of 1.1 Msun, has one of the strongest
Doppler beaming signals ever observed in a binary system (0.59 \pm 0.06%
amplitude). We use the observed ellipsoidal variations to constrain the radius
of each WD. For J1741, the star's radius must exceed 0.074 Rsun. For J2119, the
radius exceeds 0.10 Rsun. These indirect radius measurements are comparable to
the radius measurements for the bloated WD companions to A-stars found by the
Kepler spacecraft, and they constitute some of the largest radii inferred for
any WD. Surprisingly, J1741 also appears to show a 0.23 \pm 0.06% reflection
effect, and we discuss possible sources for this excess heating. Both J1741 and
J2119 are strong gravitational wave sources, and the time-of-minimum of the
ellipsoidal variations can be used to detect the orbital period decay. This may
be possible on a timescale of a decade or less.Comment: 6 pages, 4 figures, accepted for publication in the Astrophysical
Journa
Abrupt appearance of the domain pattern and fatigue of thin ferroelectric films
We study the domain structure in ferroelectric thin films with a `passive'
layer (material with damaged ferroelectric properties) at the interface between
the film and electrodes within a continuous medium approximation. An abrupt
transition from a monodomain to a polydomain state has been found with the
increase of the `passive' layer thickness . The domain width changes very
quickly at the transition (exponentially with ). We have estimated the
dielectric response (the slope of the hysteresis loop) in the
`fatigued' multidomain state and found that it is in agreement with experiment,
assuming realistic parameters of the layer. We derive a simple universal
relation for the dielectric response, which scales as , involving only the
properties of the passive layer. This relation qualitatively reproduces the
evolution of the hysteresis loop in fatigued samples and it could be tested
with controlled experiments. It is expected that the coercive field should
increase with decreasing lateral size of the film. We believe that specific
properties of the domain structure under bias voltage in ferroelectrics with a
passive layer can resolve the long-standing `paradox of the coercive field'.Comment: 5 pages, REVTeX 3.1 with two eps-figures. Minor amendments. To appear
in Phys. Rev. Letter
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