512 research outputs found
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
Numerical investigation of the late-time Kerr tails
The late-time behavior of a scalar field on fixed Kerr background is examined
in a numerical framework incorporating the techniques of conformal
compactification and hyperbolic initial value formulation. The applied code is
1+(1+2) as it is based on the use of the spectral method in the angular
directions while in the time-radial section fourth order finite differencing,
along with the method of lines, is applied. The evolution of various types of
stationary and non-stationary pure multipole initial states are investigated.
The asymptotic decay rates are determined not only in the domain of outer
communication but along the event horizon and at future null infinity as well.
The decay rates are found to be different for stationary and non-stationary
initial data, and they also depend on the fall off properties of the initial
data toward future null infinity. The energy and angular momentum transfers are
found to show significantly different behavior in the initial phase of the time
evolution. The quasinormal ringing phase and the tail phase are also
investigated. In the tail phase, the decay exponents for the energy and angular
momentum losses at future null infinity are found to be smaller than at the
horizon which is in accordance with the behavior of the field itself and it
means that at late times the energy and angular momentum falling into the black
hole become negligible in comparison with the energy and angular momentum
radiated toward future null infinity. The energy and angular momentum balances
are used as additional verifications of the reliability of our numerical
method.Comment: 33 pages, 12 figure
Casimir effect in the nonequilibrium steady-state of a quantum spin chain
We present a fully microscopics-based calculation of the Casimir effect in a
nonequilibrium system, namely an energy flux driven quantum XX chain. The force
between the walls (transverse-field impurities) is calculated in a
nonequilibrium steady state which is prepared by letting the system evolve from
an initial state with the two halves of the chain prepared at equilibrium at
different temperatures. The steady state emerging in the large-time limit is
homogeneous but carries an energy flux. The Casimir force in this
nonequilibrium state is calculated analytically in the limit when the
transverse fields are small. We find that the the Casimir force range is
reduced compared to the equilibrium case, and suggest that the reason for this
is the reduction of fluctuations in the flux carrying steady state.Comment: 11 page
Dynamic scaling of fronts in the quantum XX chain
The dynamics of the transverse magnetization in the zero-temperature XX chain
is studied with emphasis on fronts emerging from steplike initial magnetization
profiles. The fronts move with fixed velocity and display a staircase like
internal structure whose dynamic scaling is explored both analytically and
numerically. The front region is found to spread with time sub-diffusively with
the height and the width of the staircase steps scaling as t^(-1/3) and t^1/3,
respectively. The areas under the steps are independent of time, thus the
magnetization relaxes in quantized "steps" of spin-flips.Comment: 4 pages, 3 eps figures, RevTe
Ellipsoidal shapes in general relativity: general definitions and an application
A generalization of the notion of ellipsoids to curved Riemannian spaces is
given and the possibility to use it in describing the shapes of rotating bodies
in general relativity is examined. As an illustrative example, stationary,
axisymmetric perfect-fluid spacetimes with a so-called confocal inside
ellipsoidal symmetry are investigated in detail under the assumption that the
4-velocity of the fluid is parallel to a time-like Killing vector field. A
class of perfect-fluid metrics representing interior NUT-spacetimes is obtained
along with a vacuum solution with a non-zero cosmological constant.Comment: Latex, 22 pages, Revised version accepted in Class. Quantum. Grav.,
references adde
Analysis of chosen growth and reproductive traits of manna ash (Fraxinus ornus) with regards to its expansive properties
Manna ash (Fraxinus ornus L.) is a Mediterranean shrub or a small tree with the northern limit of its natural range in central Europe. At the same time, it is also a tree species which, due to its considerable tolerance to high temperatures and lack of moisture, as well as some expansive properties, could be expected to increase its range in the coming decades as a result of the changing climate and the associated spontaneous spread. The paper summarizes the results of several years of our research, during which we evaluated the following growth and reproductive characteristics: (1) phenological traits and the length of the growing season, (2) the intensity and evenness of fruiting, and (3) the numbers of individuals and stems according to the height categories. In addition, the species composition of vegetation cover was analysed on the sites with the occurrence of manna ash. Observations were made in two autochthonous populations of manna ash in Southern Slovakia and one allochthonous population in its central region. The onset of spring vegetative and generative phenophases showed a noticeable latitudinal trend during our observations (2015‒2019). In Central Slovakia, flowering and leafing of manna ash occurred on average more than two weeks later than in its southernmost region. The “Central Slovakian” population of manna ash was also characterised by the earliest onset of autumn vegetative phenophases and the lowest total length of the growing season. Despite these trends, flowering and fruiting in this population were relatively regular ‒ in contrast to the other two sites, it was observed every year. In this locality, we also observed the highest total numbers of individuals and stems in all the height categories and the markedly decreased occurrence of competing species. The achieved results point out to the considerable vigour and high reproductive ability of the allochthonous population of manna ash outside its natural range and the natural range of its most important competitors
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