3,516 research outputs found
Filtenna Integration Achieving Ideal Chebyshev Return Losses
This paper demonstrates that it is possible to find an ideal filter response (Chebyshew, Butterworth,..) considering the antenna as the last resonator of a filter under certain circumstances related with the antenna performance and the bandwidth of the filtenna device. If these circumstances are not accomplished, we can achieve excellent performance as well, by means of an iterative process the goal of which is defined by either a filter mask or a classical filter function itself. The methodology is based on the conventional coupling matrix technique for filter design and has been validated by fabricating a microstrip prototype using hairpin resonators and a rectangular patch antenna
Searching for tidal tails around Centauri using RR Lyrae Stars
We present a survey for RR Lyrae stars in an area of 50 deg around the
globular cluster Centauri, aimed to detect debris material from the
alleged progenitor galaxy of the cluster. We detected 48 RR Lyrae stars of
which only 11 have been previously reported. Ten among the eleven previously
known stars were found inside the tidal radius of the cluster. The rest were
located outside the tidal radius up to distances of degrees from the
center of the cluster. Several of those stars are located at distances similar
to that of Centauri. We investigated the probability that those stars
may have been stripped off the cluster by studying their properties (mean
periods), calculating the expected halo/thick disk population of RR Lyrae stars
in this part of the sky, analyzing the radial velocity of a sub-sample of the
RR Lyrae stars, and finally, studying the probable orbits of this sub-sample
around the Galaxy. None of these investigations support the scenario that there
is significant tidal debris around Centauri, confirming previous
studies in the region. It is puzzling that tidal debris have been found
elsewhere but not near the cluster itself.Comment: 11 pages, 11 figures, Accepte
Possible ring material around centaur (2060) Chiron
We propose that several short duration events observed in past stellar
occultations by Chiron were produced by rings material. From a reanalysis of
the stellar occultation data in the literature we determined two possible
orientations of the pole of Chiron's rings, with ecliptic coordinates
l=(352+/-10) deg, b=(37+/-10) deg or l=(144+/-10) deg, b=(24+/-10) deg . The
mean radius of the rings is (324 +/- 10) km. One can use the rotational
lightcurve amplitude of Chiron at different epochs to distinguish between the
two solutions for the pole. Both imply lower lightcurve amplitude in 2013 than
in 1988, when the rotational lightcurve was first determined. We derived
Chiron's rotational lightcurve in 2013 from observations at the 1.23-m CAHA
telescope and indeed its amplitude is smaller than in 1988. We also present a
rotational lightcurve in 2000 from images taken at CASLEO 2.15-m telescope that
is consistent with our predictions. Out of the two poles the l=(144+/-10) deg,
b=(24+/-10) deg solution provides a better match to a compilation of rotational
lightcurve amplitudes from the literature and those presented here. We also
show that using this preferred pole, Chiron's long term brightness variations
are compatible with a simple model that incorporates the changing brightness of
the rings as the tilt angle with respect to the Earth changes with time. Also,
the variability of the water ice band in Chiron's spectra in the literature can
be explained to a large degree by an icy ring system whose tilt angle changes
with time and whose composition includes water ice, analogously to the case of
Chariklo. We present several possible formation scenarios for the rings from
qualitative points of view and speculate on the reasons why rings might be
common in centaurs. We speculate on whether the known bimodal color
distribution of centaurs could be due to presence of rings and lack of them
Close encounters involving RAVE stars beyond the 47 Tucanae tidal radius
The most accurate 6D phase-space information from the Radial Velocity
Experiment (RAVE) was used to integrate the orbits of 105 stars around the
galactic globular cluster 47 Tucanae, to look for close encounters between them
in the past, with a minimum distance approach less than the cluster tidal
radius. The stars are currently over the distance range 3.0 kpc d 5.5
kpc. Using the uncertainties in the current position and velocity vector for
both, star and cluster, 105 pairs of star-cluster orbits were generated in a
Monte Carlo numerical scheme, integrated over 2 Gyr and considering an
axisymmetric and non-axisymmetric Milky-Way-like Galactic potential,
respectively. In this scheme, we identified 20 potential cluster members that
had close encounters with the globular cluster 47 Tucanae, all of which have a
relative velocity distribution (V) less than 200 km s at the
minimum distance approach. Among these potential members, 9 had close
encounters with the cluster with velocities less than the escape velocity of 47
Tucanae, therefore a scenario of tidal stripping seems likely. These stars have
been classified with a 93\% confidence level, leading to the identification of
extratidal cluster stars. For the other 11 stars, V exceeds the escape
velocity of the cluster, therefore they were likely ejected or are unassociated
interlopers.Comment: 10 pages, 6 figures, 2 table, Accepted for publication in MNRA
Optical Response for the d-density wave model
We have calculated the optical conductivity and the Raman response for the
d-density wave model, proposed as a possible explanation for the pseudogap seen
in high Tc cuprates. The total optical spectral weight remains approximately
constant on opening of the pseudogap for fixed temperature. This occurs because
there is a transfer of weight from the Drude peak to interband transitions
across the pseudogap. The interband peak in the optical conductivity is
prominent but becomes progressively reduced with increasing temperature, with
impurity scattering, which distributes it over a larger energy range, and with
ineleastic scattering which can also shift its position, making it difficult to
have a direct determination of the value of the pseudogap. Corresponding
structure is seen in the optical scattering rate, but not necessarily at the
same energies as in the conductivity.Comment: 14 pages, 15 figures, final revised version published in PR
Halo properties and secular evolution in barred galaxies
The halo plays a crucial role in the evolution of barred galaxies. Its
near-resonant material absorbs angular momentum emitted from some of the disc
particles and helps the bar become stronger. As a result, a bar (oval) forms in
the inner parts of the halo of strongly barred disc galaxies. It is thinner in
the inner parts (but still considerably fatter than the disc bar) and tends to
spherical at larger radii. Its length increases with time, while always staying
shorter than the disc bar. It is roughly aligned with the disc bar, which it
trails only slightly, and it turns with roughly the same pattern speed. The
bi-symmetric component of the halo density continues well outside the halo bar,
where it clearly trails behind the disc bar. The length and strength of the
disc and halo bars correlate; the former being always much stronger than the
latter. If the halo is composed of weakly interacting massive particles, then
the formation of the halo bar, by redistributing the matter in the halo and
changing its shape, could influence the expected annihilation signal. This is
indeed found to be the case if the halo has a core, but not if it has a steep
cusp. The formation and evolution of the bar strongly affect the halo orbits. A
fraction of them becomes near-resonant, similar to the disc near-resonant
orbits at the same resonance, while another fraction becomes chaotic. Finally,
a massive and responsive halo makes it harder for a central mass concentration
to destroy the disc bar.Comment: 6 pages, 3 figures, to appear in "Island Universes - Structure and
Evolution of Disk Galaxies" ed. R. S. de Jon
Charge and Spin Currents Generated by Dynamical Spins
We demonstrate theoretically that a charge current and a spin current are
generated by spin dynamics in the presence of spin-orbit interaction in the
perturbative regime. We consider a general spin-orbit interaction including the
spatially inhomogeneous case. Spin current due to spin damping is identified as
one origin of generated charge current, but other contributions exist, such as
the one due to an induced conservative field and the one arising from the
inhomogeneity of spin-orbit interaction.Comment: 14 pages, 4 figure
Data Portraits and Intermediary Topics: Encouraging Exploration of Politically Diverse Profiles
In micro-blogging platforms, people connect and interact with others.
However, due to cognitive biases, they tend to interact with like-minded people
and read agreeable information only. Many efforts to make people connect with
those who think differently have not worked well. In this paper, we
hypothesize, first, that previous approaches have not worked because they have
been direct -- they have tried to explicitly connect people with those having
opposing views on sensitive issues. Second, that neither recommendation or
presentation of information by themselves are enough to encourage behavioral
change. We propose a platform that mixes a recommender algorithm and a
visualization-based user interface to explore recommendations. It recommends
politically diverse profiles in terms of distance of latent topics, and
displays those recommendations in a visual representation of each user's
personal content. We performed an "in the wild" evaluation of this platform,
and found that people explored more recommendations when using a biased
algorithm instead of ours. In line with our hypothesis, we also found that the
mixture of our recommender algorithm and our user interface, allowed
politically interested users to exhibit an unbiased exploration of the
recommended profiles. Finally, our results contribute insights in two aspects:
first, which individual differences are important when designing platforms
aimed at behavioral change; and second, which algorithms and user interfaces
should be mixed to help users avoid cognitive mechanisms that lead to biased
behavior.Comment: 12 pages, 7 figures. To be presented at ACM Intelligent User
Interfaces 201
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