572 research outputs found
Structural Properties of Planar Graphs of Urban Street Patterns
Recent theoretical and empirical studies have focused on the structural
properties of complex relational networks in social, biological and
technological systems. Here we study the basic properties of twenty
1-square-mile samples of street patterns of different world cities. Samples are
represented by spatial (planar) graphs, i.e. valued graphs defined by metric
rather than topologic distance and where street intersections are turned into
nodes and streets into edges. We study the distribution of nodes in the
2-dimensional plane. We then evaluate the local properties of the graphs by
measuring the meshedness coefficient and counting short cycles (of three, four
and five edges), and the global properties by measuring global efficiency and
cost. As normalization graphs, we consider both minimal spanning trees (MST)
and greedy triangulations (GT) induced by the same spatial distribution of
nodes. The results indicate that most of the cities have evolved into networks
as efficienct as GT, although their cost is closer to the one of a tree. An
analysis based on relative efficiency and cost is able to characterize
different classes of cities.Comment: 7 pages, 3 figures, 3 table
Apollo asteroids (1566) Icarus and 2007 MK6: Icarus family members?
Although it is more complicated to search for near-Earth object (NEO)
families than main belt asteroid (MBA) families, since differential orbital
evolution within a NEO family can cause current orbital elements to drastically
differ from each other, we have found that Apollo asteroids (1566) Icarus and
the newly discovered 2007 MK6 are almost certainly related. Specifically, their
orbital evolutions show a similar profile, time shifted by only ~1000 yr, based
on our time-lag theory. The dynamical relationship between Icarus and 2007 MK6
along with a possible dust band, the Taurid-Perseid meteor swarm, implies the
first detection of an asteroidal NEO family, namely the "Icarus asteroid
family".Comment: 11 pages, 1 figure, to appear on Astrophysical Journal Letters
(journal info added
Photoexcitation of a Dipole-Forbidden Resonance in Helium
We have observed photoexcitation of the dipole-forbidden 1s21S0→2p21D2 resonance in helium by measuring the nondipolar forward-backward asymmetry of photoelectron angular distributions in the 2ℓ2ℓ′ autoionizing region. By exploiting the electric dipole-quadrupole interference in the excitation of both the 2s2p1P1 and 2p21D2 levels, we have observed the quadrupole resonance in photoabsorption and extracted its Fano line shape parameters and the relative phase of the 1sEp and 1sEd continua. We find the quadrupole line profile index q2 to be markedly different from theoretical expectations
Thermal emission from WASP-24b at 3.6 and 4.5 {\mu}m
Aims. We observe occultations of WASP-24b to measure brightness temperatures
and to determine whether or not its atmosphere exhibits a thermal inversion
(stratosphere). Methods. We observed occultations of WASP-24b at 3.6 and 4.5
{\mu}m using the Spitzer Space Telescope. It has been suggested that there is a
correlation between stellar activity and the presence of inversions, so we
analysed existing HARPS spectra in order to calculate log R'HK for WASP-24 and
thus determine whether or not the star is chromospherically active. We also
observed a transit of WASP-24b in the Str\"{o}mgren u and y bands, with the
CAHA 2.2-m telescope. Results. We measure occultation depths of 0.159 \pm 0.013
per cent at 3.6 {\mu}m and 0.202 \pm 0.018 per cent at 4.5 {\mu}m. The
corresponding planetary brightness temperatures are 1974 \pm 71 K and 1944 \pm
85 K respectively. Atmosphere models with and without a thermal inversion fit
the data equally well; we are unable to constrain the presence of an inversion
without additional occultation measurements in the near-IR. We find log R'HK =
-4.98 \pm 0.12, indicating that WASP-24 is not a chromospherically active star.
Our global analysis of new and previously-published data has refined the system
parameters, and we find no evidence that the orbit of WASP-24b is non-circular.
Conclusions. These results emphasise the importance of complementing Spitzer
measurements with observations at shorter wavelengths to gain a full
understanding of hot Jupiter atmospheres.Comment: 7 pages, 4 figures, 3 tables. Accepted for publication in A&
E1-E2 Interference in the VUV Photoionization of He
We have measured the forward-backward asymmetry of photoelectron angular distributions produced in the vacuum ultraviolet photoionization of helium. This asymmetry, a consequence of the breakdown of the dipole approximation, measures the real part of the ratio of the quadrupole and dipole matrix elements. In the autoionization region, the strong energy dependence of the asymmetry permits an experimental separation of the ratio of those magnitudes from their phase difference. We experimentally determined the Fano parameters of the 2p21D2 quadrupole resonance, and report improved values of the width Γ and line profile parameter q from those previously available from electron scattering. Off resonance, the smooth energy dependence of the asymmetry is found to agree well with the theoretical treatment presented here which incorporates higher multipole effects
Multi-site campaign for transit timing variations of WASP-12 b: possible detection of a long-period signal of planetary origin
The transiting planet WASP-12 b was identified as a potential target for
transit timing studies because a departure from a linear ephemeris was reported
in the literature. Such deviations could be caused by an additional planet in
the system. We attempt to confirm the existence of claimed variations in
transit timing and interpret its origin. We organised a multi-site campaign to
observe transits by WASP-12 b in three observing seasons, using 0.5-2.6-metre
telescopes. We obtained 61 transit light curves, many of them with
sub-millimagnitude precision. The simultaneous analysis of the best-quality
datasets allowed us to obtain refined system parameters, which agree with
values reported in previous studies. The residuals versus a linear ephemeris
reveal a possible periodic signal that may be approximated by a sinusoid with
an amplitude of 0.00068+/-0.00013 d and period of 500+/-20 orbital periods of
WASP-12 b. The joint analysis of timing data and published radial velocity
measurements results in a two-planet model which better explains observations
than single-planet scenarios. We hypothesize that WASP-12 b might be not the
only planet in the system and there might be the additional 0.1 M_Jup body on a
3.6-d eccentric orbit. A dynamical analysis indicates that the proposed
two-planet system is stable over long timescales.Comment: Accepted for publication in A&
Establishing the accuracy of asteroseismic mass and radius estimates of giant stars. I. Three eclipsing systems at [Fe/H]~ -0.3 and the need for a large high-precision sample
We aim to establish and improve the accuracy level of asteroseismic estimates
of mass, radius, and age of giant stars. This can be achieved by measuring
independent, accurate, and precise masses, radii, effective temperatures and
metallicities of long period eclipsing binary stars with a red giant component
that displays solar-like oscillations. We measured precise properties of the
three eclipsing binary systems KIC 7037405, KIC 9540226, and KIC 9970396 and
estimated their ages be , , and Gyr. The
measurements of the giant stars were compared to corresponding measurements of
mass, radius, and age using asteroseismic scaling relations and grid modeling.
We found that asteroseismic scaling relations without corrections to
systematically overestimate the masses of the three red giants by
11.7%, 13.7%, and 18.9%, respectively. However, by applying theoretical
correction factors according to Rodrigues et al. (2017), we
reached general agreement between dynamical and asteroseismic mass estimates,
and no indications of systematic differences at the precision level of the
asteroseismic measurements. The larger sample investigated by Gaulme et al.
(2016) showed a much more complicated situation, where some stars show
agreement between the dynamical and corrected asteroseismic measures while
others suggest significant overestimates of the asteroseismic measures. We
found no simple explanation for this, but indications of several potential
problems, some theoretical, others observational. Therefore, an extension of
the present precision study to a larger sample of eclipsing systems is crucial
for establishing and improving the accuracy of asteroseismology of giant stars.Comment: Accepted for publication in MNRAS, 17 page
Inhomogeneous superconductivity in organic conductors: role of disorder and magnetic field
Several experimental studies have shown the presence of spatially
inhomogeneous phase coexistence of superconducting and non superconducting
domains in low dimensional organic superconductors. The superconducting
properties of these systems are found to be strongly dependent on the amount of
disorder introduced in the sample regardless of its origin. The suppression of
the superconducting transition temperature shows clear discrepancy with
the result expected from the Abrikosov-Gor'kov law giving the behavior of
with impurities. Based on the time dependent Ginzburg-Landau theory, we derive
a model to account for the striking feature of in organic superconductors
for different types of disorder by considering the segregated texture of the
system. We show that the calculated quantitatively agrees with
experiments. We also focus on the role of superconducting fluctuations on the
upper critical fields of layered superconductors showing slab
structure where superconducting domains are sandwiched by non-superconducting
regions. We found that may be strongly enhanced by such fluctuations.Comment: to appear in Journal of Physics: Condensed Matte
The Transiting System GJ1214: High-Precision Defocused Transit Observations and a Search for Evidence of Transit Timing Variation
Aims: We present 11 high-precision photometric transit observations of the
transiting super-Earth planet GJ1214b. Combining these data with observations
from other authors, we investigate the ephemeris for possible signs of transit
timing variations (TTVs) using a Bayesian approach.
Methods: The observations were obtained using telescope-defocusing
techniques, and achieve a high precision with random errors in the photometry
as low as 1mmag per point. To investigate the possibility of TTVs in the light
curve, we calculate the overall probability of a TTV signal using Bayesian
methods.
Results: The observations are used to determine the photometric parameters
and the physical properties of the GJ1214 system. Our results are in good
agreement with published values. Individual times of mid-transit are measured
with uncertainties as low as 10s, allowing us to reduce the uncertainty in the
orbital period by a factor of two.
Conclusions: A Bayesian analysis reveals that it is highly improbable that
the observed transit times is explained by TTV, when compared with the simpler
alternative of a linear ephemeris.Comment: Submitted to A&
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