13,564 research outputs found
The Complexity of Helly- EPG Graph Recognition
Golumbic, Lipshteyn, and Stern defined in 2009 the class of EPG graphs, the
intersection graph class of edge paths on a grid. An EPG graph is a graph
that admits a representation where its vertices correspond to paths in a grid
, such that two vertices of are adjacent if and only if their
corresponding paths in have a common edge. If the paths in the
representation have at most bends, we say that it is a -EPG
representation. A collection of sets satisfies the Helly property when
every sub-collection of that is pairwise intersecting has at least one
common element. In this paper, we show that given a graph and an integer
, the problem of determining whether admits a -EPG representation
whose edge-intersections of paths satisfy the Helly property, so-called
Helly--EPG representation, is in NP, for every bounded by a polynomial
function of . Moreover, we show that the problem of recognizing
Helly--EPG graphs is NP-complete, and it remains NP-complete even when
restricted to 2-apex and 3-degenerate graphs
Gb/s visible light communications with colloidal quantum dot color converters
This paper reports the utilization of colloidal semiconductor
quantum dots as color converters for Gb/s visible light
communications. We briefly review the design and properties of
colloidal quantum dots and discuss them in the context of fast
color conversion of InGaN light sources, in particular in view of
the effects of self-absorption. This is followed by a description of
a CQD/polymer composite format of color converters. We show
samples of such color-converting composite emitting at green, yellow/orange
and red wavelengths, and combine these with a blueemitting
microsize LED to form hybrid sources for wireless visible
light communication links. In this way data rates up to 1 Gb/s over
distances of a few tens of centimeters have been demonstrated. Finally,
we broaden the discussion by considering the possibility for
wavelength division multiplexing as well as the use of alternative
colloidal semiconductor nanocrystals
On the Age of Stars Harboring Transiting Planets
Results of photometric surveys have brought to light the existence of a
population of giant planets orbiting their host stars even closer than the hot
Jupiters (HJ), with orbital periods below 3 days. The reason why radial
velocity surveys were not able to detect these very-hot Jupiters (VHJ) is under
discussion. A possible explanation is that these close-in planets are
short-lived, being evaporated on short time-scales due to UV flux of their host
stars. In this case, stars hosting transiting VHJ planets would be
systematically younger than those in the radial velocity sample. We have used
the UVES spectrograph (VLT-UT2 telescope) to obtain high resolution spectra of
5 faint stars hosting transiting planets, namely, OGLE-TR-10, 56, 111, 113 and
TrES-1. Previously obtained CORALIE spectra of HD189733, and published data on
the other transiting planet-hosts were also used. The immediate objective is to
estimate ages via Li abundances, using the Ca II activity-age relation, and
from the analysis of the stellar rotational velocity. For the stars for which
we have spectra, Li abundances were computed as in Israelian et al. (2004)
using the stellar parameters derived in Santos et al. (2006). The chromospheric
activity index was built as the ratio of the flux within the core of
the Ca II H & K lines and the flux in two nearby continuum regions. The index
was calibrated to Mount Wilson index allowing the computation
of the Ca II H & K corrected for the photospheric contribution. These values
were then used to derive the ages by means of the Henry et al. (1996)
activity-age relation. Bearing in mind the limitations of the ages derived by
Li abundances, chromospheric activity, and stellar rotational velocities, none
of the stars studied in this paper seem to be younger than 0.5 Gyr.Comment: Accepted for publication in A&
Thermalization of an anisotropic granular particle
We investigate the dynamics of a needle in a two-dimensional bath composed of
thermalized point particles. Collisions between the needle and points are
inelastic and characterized by a normal restitution coefficient . By
using the Enskog-Boltzmann equation, we obtain analytical expressions for the
translational and rotational granular temperatures of the needle and show that
these are, in general, different from the bath temperature. The translational
temperature always exceeds the rotational one, though the difference decreases
with increasing moment of inertia. The predictions of the theory are in very
good agreement with numerical simulations of the model.Comment: 7 pages, 6 Figures, submitted to PRE. Revised version (Fig1, Fig5 and
Fig6 corrected + minor typos
Detailed study of the microwave emission of the supernova remnant 3C 396
We have observed the supernova remnant 3C~396 in the microwave region using
the Parkes 64-m telescope. Observations have been made at 8.4 GHz, 13.5 GHz,
and 18.6 GHz and in polarisation at 21.5 GHz. We have used data from several
other observatories, including previously unpublished observations performed by
the Green Bank Telescope at 31.2 GHz, to investigate the nature of the
microwave emission of 3C 396. Results show a spectral energy distribution
dominated by a single component power law emission with . Data do not favour the presence of anomalous microwave emission coming
from the source. Polarised emission at 21.5 GHz is consistent with
synchrotron-dominated emission. We present microwave maps and correlate them
with infrared (IR) maps in order to characterise the interplay between thermal
dust and microwave emission. IR vs. microwave TT plots reveal poor correlation
between mid-infrared and microwave emission from the core of the source. On the
other hand, a correlation is detected in the tail emission of the outer shell
of 3C 396, which could be ascribed to Galactic contamination.Comment: published in MNRA
Astrometric search for a planet around VB 10
We observed VB 10 in August and September 2009 using the FORS2 camera of the
VLT with the aim of measuring its astrometric motion and of probing the
presence of the announced planet VB 10b. We used the published STEPS
astrometric positions of VB 10 over a time-span of 9 years, which allowed us to
compare the expected motion of VB 10 due to parallax and proper motion with the
observed motion and to compute precise deviations. The achieved single-epoch
precisions of our observations are about 0.1 mas and the data showed no
significant residual trend, while the presence of the planet should have
induced an apparent proper motion larger than 10 mas/yr. Subtraction of the
predicted orbital motion from the observed data produces a large trend in
position residuals of VB 10. We estimated the probability that this trend is
caused by random noise. Taking all the uncertainties into account and using
Monte-Carlo resampling of the data, we are able to reject the existence of VB
10b with the announced mass of 6.4 M_J with the false alarm probability of only
0.0005. A 3.2 M_J planet is also rejected with a false alarm probability of
0.023.Comment: 6 pages, 6 figures, 2 tables, accepted for publication in A&
Analyzing the heterogeneity of farmers’ preferences for improvements in dairy cow traits using farmer typologies
AbstractGiving consideration to farmers’ preferences for improvements in animal traits when designing genetic selection tools such as selection indexes might increase the uptake of these tools. The increase in use of genetic selection tools will, in turn, assist in the realization of genetic gain in breeding programs. However, the determination of farmers’ preferences is not trivial because of its large heterogeneity. The aim of this study was to quantify Australian dairy farmers’ preferences for cow trait improvements to inform and ultimately direct the choice of traits and selection indexes in the 2014 review of the National Breeding Objective. A specific aim was to analyze the heterogeneity of preferences for cow trait improvements by determining whether there are farmer types that can be identified with specific patterns of preferences. We analyzed whether farmer types differed in farming system, socioeconomic profile, and attitudes toward breeding and genetic evaluation tools. An online survey was developed to explore farmers’ preferences for improvement in 13 cow traits. The pairwise comparisons method was used to derive a ranking of the traits for each respondent. A total of 551 farmers fully completed the survey. A principal component analysis followed by a Ward hierarchical cluster analysis was used to group farmers according to their preferences. Three types of farmers were determined: (1) production-focused farmers, who gave the highest preference of all for improvements in protein yield, lactation persistency, feed efficiency, cow live weight, and milking speed; (2) functionality-focused farmers with the highest preferences of all for improvements in mastitis, lameness, and calving difficulty; and (3) type-focused farmers with the highest preferences of all for mammary system and type. Farmer types differed in their age, their attitudes toward genetic selection, and in the selection criteria they use. Surprisingly, farmer types did not differ for herd size, calving, feeding system, or breed. These results support the idea that preferences for cow trait improvements are intrinsic to farmers and not to production systems or breeds. As a result of this study, and some bioeconomic modeling (not included in this study), the Australian dairy industry has implemented a main index and 2 alternative indexes targeting the different farmer types described here
Discovery of a massive X-ray luminous galaxy cluster at z=1.579
We report on the discovery of a very distant galaxy cluster serendipitously
detected in the archive of the XMM-Newton mission, within the scope of the
XMM-Newton Distant Cluster Project (XDCP). XMMUJ0044.0-2033 was detected at a
high significance level (5sigma) as a compact, but significantly extended
source in the X-ray data, with a soft-band flux f(r<40")=(1.5+-0.3)x10^(-14)
erg/s/cm2. Optical/NIR follow-up observations confirmed the presence of an
overdensity of red galaxies matching the X-ray emission. The cluster was
spectroscopically confirmed to be at z=1.579 using ground-based VLT/FORS2
spectroscopy. The analysis of the I-H colour-magnitude diagram shows a sequence
of red galaxies with a colour range [3.7 < I-H < 4.6] within 1' from the
cluster X-ray emission peak. However, the three spectroscopic members (all with
complex morphology) have significantly bluer colours relative to the observed
red-sequence. In addition, two of the three cluster members have [OII]
emission, indicative of on-going star formation. Using the spectroscopic
redshift we estimated the X-ray bolometric luminosity, Lbol = 5.8x10^44 erg/s,
implying a massive galaxy cluster. This places XMMU J0044.0-2033 at the
forefront of massive distant clusters, closing the gap between lower redshift
systems and recently discovered proto- and low-mass clusters at z >1.6.Comment: letter to appear in A&
Breakdown of Energy Equipartition in a 2D Binary Vibrated Granular Gas
We report experiments on the equipartition of kinetic energy between grains
made of two different materials in a mixture of grains vibrated in 2
dimensions. In general, the two types of grains do not attain the same granular
temperature, Tg = 1/2m v^2. However, the ratio of the two temperatures is
constant in the bulk of the system and independent of the vibration velocity.
The ratio depends strongly on the ratio of mass densities of the grains, but is
not sensitive to the inelasticity of grains. Also, this ratio is insensitive to
compositional variables of the mixture such as the number fraction of each
component and the total number density. We conclude that a single granular
temperature, as traditionally defined, does not characterize a multi-component
mixture.Comment: 4 pages, 5 figures, submitted to Physical Review Letters, updated
reference
Bloch-Like Quantum Multiple Reflections of Atoms
We show that under certain circumstances an atom can follow an oscillatory
motion in a periodic laser profile with a Gaussian envelope. These oscillations
can be well explained by using a model of energetically forbidden spatial
regions. The similarities and differences with Bloch oscillations are
discussed. We demonstrate that the effect exists not only for repulsive but
also for attractive potentials, i.e. quantum multiple reflections are also
possible.Comment: LaTeX, 7 pages, 7 figure
- …