3,636 research outputs found
2XMM J083026+524133: The most X-ray luminous cluster at redshift 1
In the distant universe X-ray luminous clusters of galaxies are rare objects.
Large area surveys are therefore needed to probe the high luminosity end of the
cluster population at redshifts z >= 1. We correlated extended X-ray sources
from the second XMM-Newton source catalogue (2XMM) with the SDSS in order to
identify new clusters of galaxies. Distant cluster candidates in empty SDSS
fields were imaged in the R and z bands with the Large Binocular Telescope. We
extracted the X-ray spectra of the cluster candidates and fitted thermal plasma
models to the data. We determined the redshift 0.99 +-0.03 for 2XMM
J083026+524133 from its X-ray spectrum. With a bolometric luminosity of 1.8 x
10^45 erg/sec this is the most X-ray luminous cluster at redshifts z >= 1. We
measured a gas temperature of 8.2 +- 0.9 keV and and estimate a cluster mass
M(500) = 5.6 x 10^14 M(solar). The optical imaging revealed a rich cluster of
galaxies.Comment: New version, as accepted for publication in Astronomy & Astrophysic
Theoretical insights into the RR Lyrae K-band Period-Luminosity relation
Based on updated nonlinear, convective pulsation models computed for several
values of stellar mass, luminosity and metallicity, theoretical constraints on
the K-band Period-Luminosity (PLK) relation of RR Lyrae stars are presented. We
show that for each given metal content the predicted PLK is marginally
dependent on uncertainties of the stellar mass and/or luminosity. Then, by
considering the RR Lyrae masses suggested by evolutionary computations for the
various metallicities, we obtain that the predicted infrared magnitude M_K over
the range 0.0001< Z <0.02 is given by the relation
MK=0.568-2.071logP+0.087logZ-0.778logL/Lo, with a rms scatter of 0.032 mag.
Therefore, by allowing the luminosities of RR Lyrae stars to vary within the
range covered by current evolutionary predictions for metal-deficient (0.0001<
Z <0.006) horizontal branch models, we eventually find that the infrared
Period-Luminosity- Metallicity (PLZK) relation is
MK=0.139-2.071(logP+0.30)+0.167logZ, with a total intrinsic dispersion of 0.037
mag. As a consequence, the use of such a PLZK relation should constrain within
+-0.04 mag the infrared distance modulus of field and cluster RR Lyrae
variables, provided that accurate observations and reliable estimates of the
metal content are available. Moreover, we show that the combination of K and V
measurements can supply independent information on the average luminosity of RR
Lyrae stars, thus yielding tight constraints on the input physics of stellar
evolution computations. Finally, for globular clusters with a sizable sample of
first overtone variables, the reddening can be estimated by using the PLZK
relation together with the predicted MV-logP relation at the blue edge of the
instability strip (Caputo et al. 2000).Comment: 8 pages, including 5 postscript figures, accepted for publication on
MNRA
Harnessing entropy to enhance toughness in reversibly crosslinked polymer networks
Reversible crosslinking is a design paradigm for polymeric materials, wherein
they are microscopically reinforced with chemical species that form transient
crosslinks between the polymer chains. Besides the potential for self-healing,
recent experimental work suggests that freely diffusing reversible crosslinks
in polymer networks, such as gels, can enhance the toughness of the material
without substantial change in elasticity. This presents the opportunity for
making highly elastic materials that can be strained to a large extent before
rupturing. Here, we employ Gaussian chain theory, molecular simulation, and
polymer self-consistent field theory for networks to construct an equilibrium
picture for how reversible crosslinks can toughen a polymer network without
affecting its linear elasticity. Maximisation of polymer entropy drives the
reversible crosslinks to bind preferentially near the permanent crosslinks in
the network, leading to local molecular reinforcement without significant
alteration of the network topology. In equilibrium conditions, permanent
crosslinks share effectively the load with neighbouring reversible crosslinks,
forming multi-functional crosslink points. The network is thereby globally
toughened, while the linear elasticity is left largely unaltered. Practical
guidelines are proposed to optimise this design in experiment, along with a
discussion of key kinetic and timescale considerations
Validation of Turbulence Models in the Beggar Code for Unsteady Flow
Validation of the turbulence models in the CFD code Beggar for unsteady turbulent flow are discussed. Six validation cases of the code are considered, three cases with the intent of validating the code without the turbulence model and three cases to validate the turbulence model itself. Finally, a more realistic simulation of a simplified store is examined. The turbulence models considered are the Baldwin-Lomax, Spalart-Allmaras, and a DES model. The conclusions made deal with necessary prerequisites to properly simulating unsteady turbulent flow and model selection. The prerequisites necessary in the Beggar code are a second order temporal discretization and the calculation of at least three Newton dt-iterations per time step. Baldwin-Lomax overpredicts unsteady effects. The Spalart-Allmaras model was found to not properly simulate unsteady turbulent flow and is possible written incorrectly in Beggar. A recommendation of whether to use the DES model or not cannot be made due to time and computational constraints and temporary problems within the Beggar code
Geometry and the onset of rigidity in a disordered network
Disordered spring networks that are undercoordinated may abruptly rigidify
when sufficient strain is applied. Since the deformation in response to applied
strain does not change the generic quantifiers of network architecture - the
number of nodes and the number of bonds between them - this rigidity transition
must have a geometric origin. Naive, degree-of-freedom based mechanical
analyses such as the Maxwell-Calladine count or the pebble game algorithm
overlook such geometric rigidity transitions and offer no means of predicting
or characterizing them. We apply tools that were developed for the topological
analysis of zero modes and states of self-stress on regular lattices to
two-dimensional random spring networks, and demonstrate that the onset of
rigidity, at a finite simple shear strain , coincides with the
appearance of a single state of self stress, accompanied by a single floppy
mode. The process conserves the topologically invariant difference between the
number of zero modes and the number of states of self stress, but imparts a
finite shear modulus to the spring network. Beyond the critical shear, we
confirm previously reported critical scaling of the modulus. In the
sub-critical regime, a singular value decomposition of the network's
compatibility matrix foreshadows the onset of rigidity by way of a continuously
vanishing singular value corresponding to nascent state of self stress.Comment: 6 pages, 6 figue
Direct Distances to Cepheids in the Large Magellanic Cloud: Evidence for a Universal Slope of the Period-Luminosity Relation up to Solar Abundance
We have applied the infrared surface brightness (ISB) technique to derive
distances to 13 Cepheids in the LMC which span a period range from 3 to 42
days. From the absolute magnitudes of the variables calculated from these
distances, we find that the LMC Cepheids define tight period-luminosity
relations in the V, I, W,
J and K bands which agree exceedingly well with the corresponding Galactic PL
relations derived from the same technique, and are significantly steeper than
the LMC PL relations in these bands observed by the OGLE-II Project in V, I and
W, and by Persson et al. in J and K. We find that the tilt-corrected true
distance moduli of the LMC Cepheids show a significant dependence on period,
which hints at a systematic error in the ISB technique related to the period of
the stars. We identify as the most likely culprit the p-factor which converts
the radial into pulsational velocities; our data imply a much steeper period
dependence of the p-factor than previously thought, and we derive p=1.58
(+/-0.02) -0.15 (+/-0.05) logP as the best fit from our data, with a zero point
tied to the Milky Way open cluster Cepheids. Using this revised p-factor law,
the period dependence of the LMC Cepheid distance moduli disappears, and at the
same time the Milky Way and LMC PL relations agree among themselves, and with
the directly observed LMC PL relations, within the 1 sigma uncertainties. Our
main conclusion is that the previous, steeper Galactic PL relations were caused
by an erroneous calibration of the p-factor law, and that there is now evidence
that the slope of the Cepheid PL relation is independent of metallicity up to
solar metallicity, in both optical, and near-infrared bands.Comment: ApJ accepte
Distances to six Cepheids in the LMC cluster NGC1866 from the near-IR surface-brightness method
We derive individual distances to six Cepheids in the young populous star
cluster NGC1866 in the Large Magellanic Cloud employing the near-IR surface
brightness technique. With six stars available at the exact same distance we
can directly measure the intrinsic uncertainty of the method. We find a
standard deviation of 0.11 mag, two to three times larger than the error
estimates and more in line with the estimates from Bayesian statistical
analysis by Barnes et al. (2005). Using all six distance estimates we determine
an unweighted mean cluster distance of 18.30+-0.05. The observations indicate
that NGC1866 is close to be at the same distance as the main body of the LMC.
If we use the stronger dependence of the p-factor on the period as suggested by
Gieren et al. (2005) we find a distance of 18.50+-0.05 (internal error) and the
PL relations for Galactic and MC Cepheids are in very good agreement.Comment: Presented at the conference "Stellar Pulsation and Evolution" in
Monte Porzio Catone, June 2005. To appear in Mem. Soc. Ast. It. 76/
Efficacy of pheromone-based control system, Exosex™ SPTab, against moth pests in European food processing facilities
The Exosex™ SPTab auto-confusion system is a novel pheromone-based method for control of stored product moth pests in both food and tobacco processing and storage facilities. The method uses a femaleproduced sex pheromone, (Z,E)-9,12-tetradecadienyl acetate, combined with a patented electrostatic powder delivery system known as Entostat™ to disrupt mating and interrupt the lifecycle of several important moth pests: Plodia interpunctella, Ephestia kuehniella, Ephestia cautella and Ephestia elutella. Male moths are attracted to a compressed tablet of the powder, which contains the sex pheromone. The powder releases pheromone at a slow enough rate to attract males to make contact with the tablet. The powder adheres to the moth cuticle via electrostatic attraction and the moth leaves the tablet coated in female sex pheromone. Flight tunnel studies have shown that this disrupts their ability to locate female moths and they become attractive sources for other males. Here we will present findings from full scale, long term trials that were conducted under real conditions at commercial food processing facilities across Europe. Populations of target moth species were monitored alongside deployment of the SPTab system and compared with untreated control areas and historical data from the test areas in the years prior to deployment. In all cases populations were reduced compared to the same area in the previous year and compared to untreated control areas under local pest control practices. The SPTab auto-confusion system could offer the opportunity to actively reduce the use of pesticides and its use as an integrated pest management tool within the food and tobacco processing and storage industry is discussed. Keywords: Mating disruption, Plodia interpunctella, Ephestia kuehniella, SPTab, Sex pheromone
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