546 research outputs found
Mesoscopic approach to progressive breakdown in ultrathin SiO2 layers
The opening of a breakdown path across the ultrathin oxide layer in a metal-oxide-semiconductor structure caused by the application of electrical stress can be analyzed within the framework of the physics of mesoscopic conductors. Using the Landauer formula for a quantum point contact, the author is able to show that the saturation of the gate leakage current is linked to the progressive evolution of the constriction's conductance toward the ballistic transport regime. The possible physical mechanisms responsible for energy dissipation inside the breakdown path as well as the limitations of the proposed approach are discussed
Eclipsing Binaries with Possible Light-Time Effect
The period changes of six eclipsing binaries have been studied with focus on
the light-time effect. With the least squares method we also calculated
parameters of such an effect and properties of the unresolved body in these
systems. With these results we discussed the probability of presence of such
bodies in the systems with respect to possible confirmation by another method.
In two systems we also suggested the hypothesis of fourth body or magnetic
activity for explanation of the "second-order variability" after subtraction of
the light-time effect of the third body.Comment: 4 pages, 1 figure, 2 tables, conference proceeding
On the equilibrium rotation of Earth-like extra-solar planets
The equilibrium rotation of tidally evolved "Earth-like" extra-solar planets
is often assumed to be synchronous with their orbital mean motion. The same
assumption persisted for Mercury and Venus until radar observations revealed
their true spin rates. As many of these planets follow eccentric orbits and are
believed to host dense atmospheres, we expect the equilibrium rotation to
differ from the synchronous motion. Here we provide a general description of
the allowed final equilibrium rotation states of these planets, and apply this
to already discovered cases in which the mass is lower than twelve
Earth-masses. At low obliquity and moderate eccentricity, it is shown that
there are at most four distinct equilibrium possibilities, one of which can be
retrograde. Because most presently known "Earth-like" planets present eccentric
orbits, their equilibrium rotation is unlikely to be synchronous.Comment: 4 pages, 2 figures. accepted for publication in Astronomy and
Astrophysics. to be published in Astronomy and Astrophysic
Mass-luminosity relation for FGK main sequence stars: metallicity and age contributions
The stellar mass-luminosity relation (MLR) is one of the most famous
empirical "laws", discovered in the beginning of the 20th century. MLR is still
used to estimate stellar masses for nearby stars, particularly for those that
are not binary systems, hence the mass cannot be derived directly from the
observations. It's well known that the MLR has a statistical dispersion which
cannot be explained exclusively due to the observational errors in luminosity
(or mass). It is an intrinsic dispersion caused by the differences in age and
chemical composition from star to star. In this work we discuss the impact of
age and metallicity on the MLR. Using the recent data on mass, luminosity,
metallicity, and age for 26 FGK stars (all members of binary systems, with
observational mass-errors <= 3%), including the Sun, we derive the MLR taking
into account, separately, mass-luminosity, mass-luminosity-metallicity, and
mass-luminosity-metallicity-age. Our results show that the inclusion of age and
metallicity in the MLR, for FGK stars, improves the individual mass estimation
by 5% to 15%.Comment: 7 pages, 4 figures, 1 table, accepted in Astrophysics and Space
Scienc
Close Binary System GO Cyg
In this study, we present long term photometric variations of the close
binary system \astrobj{GO Cyg}. Modelling of the system shows that the primary
is filling Roche lobe and the secondary of the system is almost filling its
Roche lobe. The physical parameters of the system are , , , , , , and . Our results show that \astrobj{GO
Cyg} is the most massive system near contact binary (NCB). Analysis of times of
the minima shows a sinusoidal variation with a period of years due
to a third body whose mass is less than 2.3. Finally a period
variation rate of d/yr has been determined using all
available light curves.Comment: Accepted for publication in New Astronomy, 18 pages, 4 figures, 7
table
The Empirical Mass-Luminosity Relation for Low Mass Stars
This work is devoted to improving empirical mass-luminosity relations and
mass-metallicity-luminosity relation for low mass stars. For these stars,
observational data in the mass-luminosity plane or the
mass-metallicity-luminosity space subject to non-negligible errors in all
coordinates with different dimensions. Thus a reasonable weight assigning
scheme is needed for obtaining more reliable results. Such a scheme is
developed, with which each data point can have its own due contribution.
Previous studies have shown that there exists a plateau feature in the
mass-luminosity relation. Taking into account the constraints from the
observational luminosity function, we find by fitting the observational data
using our weight assigning scheme that the plateau spans from 0.28 to 0.50
solar mass. Three-piecewise continuous improved mass-luminosity relations in K,
J, H and V bands, respectively, are obtained. The visual
mass-metallicity-luminosity relation is also improved based on our K band
mass-luminosity relation and the available observational metallicity data.Comment: 8 pages, 2 figures. Accepted for publication in Astrophysics & Space
Scienc
Reanalysis of two eclipsing binaries: EE Aqr and Z Vul
We study the radial-velocity and light curves of the two eclipsing binaries
EE Aqr and Z Vul. Using the latest version of the Wilson & Van Hamme (2003)
model, absolute parameters for the systems are determined. We find that EE Aqr
and Z Vul are near-contact and semi-detached systems, respectively. The primary
component of EE Aqr fills about 96% of its 'Roche lobe', while its secondary
one appears close to completely filling this limiting volume. In a similar way,
we find fill-out proportions of about 72 and 100% of these volumes for the
primary and secondary components of Z Vul respectively. We compare our results
with those of previous authors.Comment: 13 pages, 8 figures, 10 table
The large-scale ionised outflow of CH Cygni
HST and ground-based [OII} and [NII] images obtained from 1996 to 1999 reveal
the existence of a ionised optical nebula around the symbiotic binary CH Cyg
extending out to 5000 A.U. from the central stars. The observed velocity range
of the nebula, derived from long-slit echelle spectra, is of 130 km/s. In spite
of its complex appearence, the velocity data show that the basic morphology of
the inner regions of the optical nebula is that of a bipolar (or conical)
outflow extending nearly along the plane of the sky out to some 2000 A.U. from
the centre. Even if the extension of this bipolar outflow and its position
angle are consistent with those of the radio jet produced in 1984 (extrapolated
to the time of our optical imagery), no obvious counterpart is visible of the
original, dense radio bullets ejected by the system. We speculate that the
optical bipolar outflow might be the remannt of the interaction of the bullets
with a relatively dense circumstellar medium.Comment: 8 text pages + 3 figures (jpeg). ApJ in press. For a full PostScript
version with figures inline see
ftp://ftp.ll.iac.es/pub/research/preprints/PP252001.ps.g
Spacings of Quarkonium Levels with the Same Principal Quantum Number
The spacings between bound-state levels of the Schr\"odinger equation with
the same principal quantum number but orbital angular momenta
differing by unity are found to be nearly equal for a wide range of power
potentials , with . Semiclassical approximations are in accord with this behavior. The
result is applied to estimates of masses for quarkonium levels which have not
yet been observed, including the 2P states and the 1D
states.Comment: 20 pages, latex, 3 uuencoded figures submitted separately (process
using psfig.sty
Measurement of Aerosols at the Pierre Auger Observatory
The air fluorescence detectors (FDs) of the Pierre Auger Observatory are
vital for the determination of the air shower energy scale. To compensate for
variations in atmospheric conditions that affect the energy measurement, the
Observatory operates an array of monitoring instruments to record hourly
atmospheric conditions across the detector site, an area exceeding 3,000 square
km. This paper presents results from four instruments used to characterize the
aerosol component of the atmosphere: the Central Laser Facility (CLF), which
provides the FDs with calibrated laser shots; the scanning backscatter lidars,
which operate at three FD sites; the Aerosol Phase Function monitors (APFs),
which measure the aerosol scattering cross section at two FD locations; and the
Horizontal Attenuation Monitor (HAM), which measures the wavelength dependence
of aerosol attenuation.Comment: Contribution to the 30th International Cosmic Ray Conference, Merida
Mexico, July 2007; 4 pages, 4 figure
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