15,495 research outputs found
Photometry of the eclipsing cataclysmic variable SDSS J152419.33+220920.0
Aims. We present new photometry of the faint and poorly studied cataclysmic
variable SDSS J152419.33+220920.0, analyze its light curve and provide an
accurate ephemeris for this system. Methods. Time-resolved CCD differential
photometry was carried out using the 1.5m and 0.84m telescopes at the
Observatorio Astronomico Nacional at San Pedro Martir. Results. From
time-resolved photometry of the system obtained during six nights (covering
more than twenty primary eclipse cycles in more than three years), we show that
this binary presents a strong primary and a weak secondary modulation. Our
light curve analysis shows that only two fundamental frequencies are present,
corresponding to the orbital period and a modulation with twice this frequency.
We determine the accurate ephemeris of the system to be HJD(eclipse)=
2454967.6750(1) + 0.06531866661(1) E. A double-hump orbital period modulation,
a standing feature in several bounce-back systems at quiescence, is present at
several epochs. However, we found no other evidence to support the hypothesis
that this system belongs to the post-minimum orbital-period systems
A Radial Velocity Study of the Intermediate Polar EX Hydrae
A study on the intermediate polar EX Hya is presented, based on simultaneous
photometry and high dispersion spectroscopic observations, during four
consecutive nights. The strong photometric modulation related to with the
67-min spin period of the primary star is clearly present, as well as the
narrow eclipses associated to the orbital modulation. Since our eclipse timings
have been obtained almost 91,000 cycles since the last reported observations,
we present new linear ephemeris, although we cannot rule out a sinusoidal
variation suggested by previous authors. The system mainly shows double-peaked
H, H and HeI 5876 \AA emission lines. From the profile
of the H line, we find two components; one with a steep rise and
velocities not larger than 1000 km s and another broader component
extending up to 2000 km s, which we interpret as coming mainly
from the inner disc. A strong and variable hotspot is found and a stream-like
structure is seen at times. We show that the best solution correspond to km s from H, from the two emission components, which
are both in phase with the orbital modulation. We remark on a peculiar effect
in the radial velocity curve around phase zero, which could be interpreted as a
Rositter-MacLaughlin-like effect, which has been taken into account before
deriving . This value is compatible with the values found in
high-resolution both in the ultraviolet and X-ray. We find: M, M and
R. Doppler Tomography has been applied, to construct six Doppler
tomograms for single orbital cycles spanning the four days of observations to
support our conclusions. Our results indicate that EX Hya has a well formed
disc and that the magnetosphere should extend only to about
.Comment: 16 pages, 14 figures, accepted for publication in MNRA
Structure of Exotic Nuclei
The progress in the modeling of exotic nuclei with an extreme
neutron-to-proton ratio is discussed. Two topics are emphasized: (i) the quest
for the universal microscopic nuclear energy density functional and (ii) the
progress in the continuum shell model.Comment: 10 pages, 2 figures, presented by W. Nazarewicz at the International
Symposium on "A New Era of Nuclear Structure Physics", Kurokawa Village
(Niigata Pref.), Japan, Nov. 19-22, 200
Crystal Structures of Polymerized Fullerides AC60, A=K, Rb, Cs and Alkali-mediated Interactions
Starting from a model of rigid interacting C60 polymer chains on an
orthorhombic lattice, we study the mutual orientation of the chains and the
stability of the crystalline structures Pmnn and I2/m. We take into account i)
van der Waals interactions and electric quadrupole interactions between C60
monomers on different chains as well as ii) interactions of the monomers with
the surrounding alkali atoms. The direct interactions i) always lead to an
antiferrorotational structure Pmnn with alternate orientation of the C60 chains
in planes (001). The interactions ii) with the alkalis consist of two parts:
translation-rotation (TR) coupling where the orientations of the chains
interact with displacements of the alkalis, and quadrupolar electronic
polarizability (ep) coupling, where the electric quadrupoles on the C60
monomers interact with induced quadrupoles due to excited electronic d states
of the alkalis. Both interactions ii) lead to an effective
orientation-orientation interaction between the C60 chains and always favor the
ferrorotational structure I2/m where C60 chains have a same orientation. The
structures Pmnn for KC60 and I2/m for Rb- and CsC60 are the result of a
competition between the direct interaction i) and the alkali-mediated
interactions ii). In Rb- and CsC60 the latter are found to be dominant, the
preponderant role being played by the quadrupolar electronic polarizability of
the alkali ions.Comment: J.Chem.Phys., in press, 14 pages, 3 figures, 8 table
New insights in particle dynamics from group cohomology
The dynamics of a particle moving in background electromagnetic and
gravitational fields is revisited from a Lie group cohomological perspective.
Physical constants characterising the particle appear as central extension
parameters of a group which is obtained from a centrally extended kinematical
group (Poincare or Galilei) by making local some subgroup. The corresponding
dynamics is generated by a vector field inside the kernel of a presymplectic
form which is derived from the canonical left-invariant one-form on the
extended group. A non-relativistic limit is derived from the geodesic motion
via an Inonu-Wigner contraction. A deeper analysis of the cohomological
structure reveals the possibility of a new force associated with a non-trivial
mixing of gravity and electromagnetism leading to in principle testable
predictions.Comment: 8 pages, LaTeX, no figures. To appear in J. Phys. A (Letter to the
editor
Locomotion in Parkinson's disease: neuronal coupling of upper and lower limbs
Quadrupedal limb coordination during human walking was recently shown to be upregulated during obstacle stepping. An anticipatory activity of coupled cervico-thoraco-lumbar interneuronal circuits is followed by an appropriate executory activation of leg and arm muscles during task performance. This mechanism was studied in subjects with Parkinson's disease and age-matched controls walking on a treadmill with a randomly approaching obstacle. Spinal reflex (SR) responses, evoked by tibial nerve stimulation during mid-stance, were present in all arm and leg muscles investigated. They were larger before execution of obstacle avoidance compared with normal steps in both subject groups. The performance of obstacle stepping was slightly worse in Parkinson's disease than in control subjects. The anticipatory SR in the arm muscles prior to normal and obstacle steps was larger in Parkinson's disease compared with age-matched subjects, but smaller in the tibialis anterior. The arm and leg muscle activation was stronger during obstacle compared with normal swing but did not differ between Parkinson's disease and age-matched subjects. These observations indicate that quadrupedal limb coordination is basically preserved in Parkinson's disease subjects. Our data are consistent with the proposal that in Parkinson's disease subjects the enhanced anticipatory spinal neuronal activity (reflected in the SR) in the arm muscles is required to achieve an appropriate muscle activation for the automatic control of body equilibrium during the performance of the task. In the tibialis anterior the SR is attenuated presumably because of a stronger voluntary (i.e. cortical) control of leg movement
Huyghens, Bohr, Riemann and Galois: Phase-Locking
Several mathematical views of phase-locking are developed. The classical
Huyghens approach is generalized to include all harmonic and subharmonic
resonances and is found to be connected to 1/f noise and prime number theory.
Two types of quantum phase-locking operators are defined, one acting on the
rational numbers, the other on the elements of a Galois field. In both cases we
analyse in detail the phase properties and find them related respectively to
the Riemann zeta function and to incomplete Gauss sums.Comment: 18 pages paper written in relation to the ICSSUR'05 conference held
in Besancon, France to be published at a special issue of IJMP
X-ray microanalysis in STEM of short-term physico-chemical reactions at bioactive glass particles / biological fluids interface. Determination of O/Si atomic ratios
Short-term physico-chemical reactions at the interface between bioactive
glass particles and biological fluids are studied and we focus our attention on
the measurements of O/Si atomic ratio. The studied bioactive glass is in the
SiO2-Na2O-CaO-P2O5-K2O-Al2O3-MgO system. The elemental analysis is performed at
the submicrometer scale by STEM associated with EDXS and EELS. We previously
developed an EDXS quantification method based on the ratio method and taking
into account local absorption corrections. In this way, we use EELS data to
determine, by an iterative process, the local mass thickness which is an
essential parameter to correct absorption in EDXS spectra. After different
delays of immersion of bioactive glass particles in a simulated biological
solution, results show the formation of different surface layers at the
bioactive glass periphery. Before one day of immersion, we observe the presence
of an already shown (Si,O,Al) rich layer at the periphery. In this paper, we
demonstrate that a thin electron dense (Si,O) layer is formed on top of the
(Si,O,Al) layer. In this (Si,O) layer, depleted in aluminium, we point out an
increase of oxygen weight concentration which can be interpreted by the
presence of Si(OH)4 groups, that permit the formation of a (Ca,P) layer.
Aluminium plays a role in the glass solubility and may inhibit apatite
nucleation. After the beginning of the (Ca,P) layer formation, the size of the
electron dense (Si,O) layer decreases and tends to disappear. After two days of
immersion, the (Ca,P) layer grows in thickness and leads to apatite
precipitatio
Comparative Measurements of Inverse Spin Hall and Magnetoresistance in YIG|Pt and YIG|Ta
We report on a comparative study of spin Hall related effects and
magnetoresistance in YIG|Pt and YIG|Ta bilayers. These combined measurements
allow to estimate the characteristic transport parameters of both Pt and Ta
layers juxtaposed to YIG: the spin mixing conductance
at the YIGnormal metal interface, the spin Hall angle , and the
spin diffusion length in the normal metal. The inverse spin Hall
voltages generated in Pt and Ta by the pure spin current pumped from YIG
excited at resonance confirm the opposite signs of spin Hall angles in these
two materials. Moreover, from the dependence of the inverse spin Hall voltage
on the Ta thickness, we extract the spin diffusion length in Ta, found to be
nm. Both the YIG|Pt and YIG|Ta systems
display a similar variation of resistance upon magnetic field orientation,
which can be explained in the recently developed framework of spin Hall
magnetoresistance.Comment: 8 pages, 5 figures, 1 tabl
Detection of the microwave spin pumping using the inverse spin Hall effect
We report electrical detection of the dynamical part of the spin pumping
current emitted during ferromagnetic resonance (FMR) using the inverse Spin
Hall Effect (ISHE). The experiment is performed on a YIGPt bilayer. The
choice of YIG, a magnetic insulator, ensures that no charge current flows
between the two layers and only pure spin current produced by the magnetization
dynamics are transferred into the adjacent strong spin-orbit Pt layer via spin
pumping. To avoid measuring the parasitic eddy currents induced at the
frequency of the microwave source, a resonance at half the frequency is induced
using parametric excitation in the parallel geometry. Triggering this nonlinear
effect allows to directly detect on a spectrum analyzer the microwave component
of the ISHE voltage. Signals as large as 30 V are measured for precession
angles of a couple of degrees. This direct detection provides a novel efficient
means to study magnetization dynamics on a very wide frequency range with great
sensitivity
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