4,586 research outputs found
Generalized mean-field study of a driven lattice gas
Generalized mean-field analysis has been performed to study the ordering
process in a half-filled square lattice-gas model with repulsive nearest
neighbor interaction under the influence of a uniform electric field. We have
determined the configuration probabilities on 2-, 4-, 5-, and 6-point clusters
excluding the possibility of sublattice ordering. The agreement between the
results of 6-point approximations and Monte Carlo simulations confirms the
absence of phase transition for sufficiently strong fields.Comment: 4 pages (REVTEX) with 4 PS figures (uuencoded
Pushing the limits: K2 observations of the trans-Neptunian objects 2002 GV31 and (278361) 2007 JJ43
We present the first photometric observations of trans-Neptunian objects
(TNOs) taken with the Kepler space telescope, obtained in the course of the K2
ecliptic survey. Two faint objects have been monitored in specifically designed
pixel masks that were centered on the stationary points of the objects, when
their daily motion was the slowest. In the design of the experiment, only the
apparent path of these objects were retrieved from the detectors, i.e. the
costs in terms of Kepler pixels were minimized. Because of the faintness of the
targets we employ specific reduction techniques and co-added images. We measure
rotational periods and amplitudes in the unfiltered Kepler band as follows: for
(278361) 2007 JJ43 and 2002 GV31 we get P_rot=12.097 h and P_rot=29.2 h while
0.10 and 0.35 mag for the total amplitudes, respectively. Future space
missions, like TESS and PLATO are not well suited to this kind of observations.
Therefore, we encourage to include the brightest TNOs around their stationary
points in each observing campaign to exploit this unique capability of the K2
Mission -- and therefore to provide unbiased rotational, shape and albedo
characteristics of many objects.Comment: Accepted for publication in ApJ Letters, 5.2 pages in emulateapj
style, misspelled 2007 JJ43 designation correcte
Point-contact spectroscopy of Al- and C-doped MgB2. Superconducting energy gaps and scattering studies
The two-band/two-gap superconductivity in aluminium and carbon doped MgB
has been addressed by the point-contact spectroscopy. Two gaps are preserved in
all samples with down to 22 K. The evolution of two gaps as a function
of the critical temperature in the doped systems suggest the dominance of the
band-filling effects but for the increased Al-doping the enhanced interband
scattering approaching two gaps must be considered. The magnetic field
dependences of the Andreev reflection excess currents as well as zero-energy
density of states determined from the experimental data are used to analyze the
intraband scattering. It is shown, that while the C-doping increases the
intraband scattering in the -band more rapidly then in the band,
the Al-doping does not change their relative weight.Comment: 8 pages, incl. 6 figure
Mapping a star with transits: orbit precession effects in the Kepler-13 system
Kepler-13b (KOI-13.01) is a most intriguing exoplanet system due to the rapid
precession rate, exhibiting several exotic phenomena. We analyzed
Short Cadence data up to Quarter 14, with a total time-span of 928 days, to
reveal changes in transit duration, depth, asymmetry, and identify the possible
signals of stellar rotation and low-level activity. We investigated long-term
variations of transit light curves, testing for duration, peak depth and
asymmetry. We also performed cluster analysis on quarters. We computed
the autocorrelation function of the out-of-transit light variations. Transit
duration, peak depth, and asymmetry evolve slowly, due to the slowly drifting
transit path through the stellar disk. The detected transit shapes will map the
stellar surface on the time scale of decades. We found a very significant
clustering pattern with 3-orbit period. Its source is very probably the
rotating stellar surface, in the 5:3 spin-orbit resonance reported in a
previous study. The autocorrelation function of the out-of-transit light
variations, filtered to 25.4 hours and harmonics, shows slow variations and a
peak around 300--360 day period, which could be related to the activity cycle
of the host star.Comment: 7 pages, 7 figures, accepted in MNRA
Magnetic phase diagram of an Fe monolayer on W(110) and Ta(110) surfaces based on ab initio calculations
We present detailed investigations of the magnetic properties of an Fe
monolayer on W and Ta (110) surfaces based on the ab initio screened
Korringa-Kohn-Rostoker method. By calculating tensorial exchange coupling
coefficients, the ground states of the systems are determined using atomistic
spin dynamics simulations. Different types of ground states are found in the
systems as a function of relaxation of the Fe layer. In case of W(110)
substrate this is reflected in a reorientation of the easy axis from in-plane
to out-of-plane. For Ta(110) a switching appears from the ferromagnetic state
to a cycloidal spin spiral state, then to another spin spiral state with a
larger wave vector and, for large relaxations, a rotation of the normal vector
of the spin spiral is found. Classical Monte Carlo simulations indicate
temperature-induced transitions between the different magnetic phases observed
in the Fe/Ta(110) system. These phase transitions are analyzed both
quantitatively and qualitatively by finite-temperature spin wave theory.Comment: 18 pages, 11 figure
Determination of the size, mass, and density of "exomoons" from photometric transit timing variations
Precise photometric measurements of the upcoming space missions allow the
size, mass, and density of satellites of exoplanets to be determined. Here we
present such an analysis using the photometric transit timing variation
(). We examined the light curve effects of both the transiting planet
and its satellite. We define the photometric central time of the transit that
is equivalent to the transit of a fixed photocenter. This point orbits the
barycenter, and leads to the photometric transit timing variations. The exact
value of depends on the ratio of the density, the mass, and the size of
the satellite and the planet. Since two of those parameters are independent, a
reliable estimation of the density ratio leads to an estimation of the size and
the mass of the exomoon. Upper estimations of the parameters are possible in
the case when an upper limit of is known. In case the density ratio
cannot be estimated reliably, we propose an approximation with assuming equal
densities. The presented photocenter analysis predicts the size of the
satellite better than the mass. We simulated transits of the Earth-Moon system
in front of the Sun. The estimated size and mass of the Moon are 0.020
Earth-mass and 0.274 Earth-size if equal densities are assumed. This result is
comparable to the real values within a factor of 2. If we include the real
density ratio (about 0.6), the results are 0.010 Earth-Mass and 0.253
Earth-size, which agree with the real values within 20%.Comment: 6 pages, 5 figures, to appear in Astronomy and Astrophysic
Phase transition and selection in a four-species cyclic Lotka-Volterra model
We study a four species ecological system with cyclic dominance whose
individuals are distributed on a square lattice. Randomly chosen individuals
migrate to one of the neighboring sites if it is empty or invade this site if
occupied by their prey. The cyclic dominance maintains the coexistence of all
the four species if the concentration of vacant sites is lower than a threshold
value. Above the treshold, a symmetry breaking ordering occurs via growing
domains containing only two neutral species inside. These two neutral species
can protect each other from the external invaders (predators) and extend their
common territory. According to our Monte Carlo simulations the observed phase
transition is equivalent to those found in spreading models with two equivalent
absorbing states although the present model has continuous sets of absorbing
states with different portions of the two neutral species. The selection
mechanism yielding symmetric phases is related to the domain growth process
whith wide boundaries where the four species coexist.Comment: 4 pages, 5 figure
Main-Belt Asteroids in the K2 Engineering Field of View
Unlike NASA's original Kepler Discovery Mission, the renewed K2 Mission will
stare at the plane of the Ecliptic, observing each field for approximately 75
days. This will bring new opportunities and challenges, in particular the
presence of a large number of main-belt asteroids that will contaminate the
photometry. The large pixel size makes K2 data susceptible to the effect of
apparent minor planet encounters. Here we investigate the effects of asteroid
encounters on photometric precision using a sub-sample of the K2 Engineering
data taken in February, 2014. We show examples of asteroid contamination to
facilitate their recognition and distinguish these events from other error
sources. We conclude that main-belt asteroids will have considerable effects on
K2 photometry of a large number of photometric targets during the Mission, that
will have to be taken into account. These results will be readily applicable
for future space photometric missions applying large-format CCDs, such as TESS
and PLATO.Comment: accepted for publication in AJ, 6 page
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