2,746 research outputs found
Direct evidence of dust growth in L183 from MIR light scattering
Theoretical arguments suggest that dust grains should grow in the dense cold
parts of molecular clouds. Evidence of larger grains has so far been gathered
in near/mid infrared extinction and millimeter observations. Interpreting the
data is, however, aggravated by the complex interplay of density and dust
properties (as well as temperature for thermal emission). We present new
Spitzer data of L183 in bands that are sensitive and insensitive to PAHs. The
visual extinction AV map derived in a former paper was fitted by a series of 3D
Gaussian distributions. For different dust models, we calculate the scattered
MIR radiation images of structures that agree agree with the AV map and compare
them to the Spitzer data. The Spitzer data of L183 show emission in the 3.6 and
4.5 micron bands, while the 5.8 micron band shows slight absorption. The
emission layer of stochastically heated particles should coincide with the
layer of strongest scattering of optical interstellar radiation, which is seen
as an outer surface on I band images different from the emission region seen in
the Spitzer images. Moreover, PAH emission is expected to strongly increase
from 4.5 to 5.8 micron, which is not seen. Hence, we interpret this emission to
be MIR cloudshine. Scattered light modeling when assuming interstellar medium
dust grains without growth does not reproduce flux measurable by Spitzer. In
contrast, models with grains growing with density yield images with a flux and
pattern comparable to the Spitzer images in the bands 3.6, 4.5, and 8.0 micron.Comment: 13 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
Host Galaxies of low z Radio-loud Quasars: A search of HST archives
We searched the HST archives for unpublished WFPC2 images of low redshift
(z<0.5) radio loud quasars (RLQ). This led to the identification of 11 objects.
We present here the results of the analysis of these images from which we
derive the properties of their host galaxies. All objects are clearly resolved
and their surrounding nebulosity is consistent with an elliptical galaxy model.
These new data, together with previous published HST observations, form a
sample of 34 sources which significantly expands all previous studies of low
redshift RLQ based on HST data. For this full sample we derive the average
absolute magnitude of the host galaxies =-24.01+/-0.48, and the effective
radius =10.5+/-3.7kpc. No significant correlation is found between the
nucleus and the host galaxy luminosity. Using the relationship between black
hole mass (M_BH) and bulge luminosity we investigate the relation between M_BH
and total radio power for RLQ and compare with other classes of radio sources.
The overall distribution of AGN in the plane M_BH-P(radio) exhibits a trend for
increasing M_BH with increasing P(radio) but with a substantial spread. RLQ
occupy the region of most powerful sources and most massive BH. The quasars
appear to emit over a wide range of power with respect to their Eddington
luminosity as deduced by the estimated M_BH.Comment: 23 pages, 8 figures, ApJ in pres
Collisional excitation rate coefficients of N2H+ by He
Using a recoupling technique with close-coupling spin-free calculations de-excitation rate coefficients are obtained among hyperfine transitions for He colliding with N2H+. A recently determined potential energy surface suitable for scattering calculations is used to investigate rate coefficients for temperatures between 5 and 50 K, and for the seven lowest rotational levels of N2H+. Fitting functions are provided for the Maxwellian averaged opacity tensors and for the rotational de-excitation collisional rate coefficients. The fitting functions for the opacity tensors can be used to calculate hyperfine (de)-excitation rate coefficients among elastic and inelastic rotational levels, and among the corresponding magnetic sublevels of the hyperfine structure. Certain dynamical approximations are investigated and found to be invali
On the properties of fractal cloud complexes
We study the physical properties derived from interstellar cloud complexes
having a fractal structure. We first generate fractal clouds with a given
fractal dimension and associate each clump with a maximum in the resulting
density field. Then, we discuss the effect that different criteria for clump
selection has on the derived global properties. We calculate the masses, sizes
and average densities of the clumps as a function of the fractal dimension
(D_f) and the fraction of the total mass in the form of clumps (epsilon). In
general, clump mass does not fulfill a simple power law with size of the type
M_cl ~ (R_cl)**(gamma), instead the power changes, from gamma ~ 3 at small
sizes to gamma<3 at larger sizes. The number of clumps per logarithmic mass
interval can be fitted to a power law N_cl ~ (M_cl)**(-alpha_M) in the range of
relatively large masses, and the corresponding size distribution is N_cl ~
(R_cl)**(-alpha_R) at large sizes. When all the mass is forming clumps
(epsilon=1) we obtain that as D_f increases from 2 to 3 alpha_M increases from
~0.3 to ~0.6 and alpha_R increases from ~1.0 to ~2.1. Comparison with
observations suggests that D_f ~ 2.6 is roughly consistent with the average
properties of the ISM. On the other hand, as the fraction of mass in clumps
decreases (epsilon<1) alpha_M increases and alpha_R decreases. When only ~10%
of the complex mass is in the form of dense clumps we obtain alpha_M ~ 1.2 for
D_f=2.6 (not very different from the Salpeter value 1.35), suggesting this a
likely link between the stellar initial mass function and the internal
structure of molecular cloud complexes.Comment: 32 pages, 13 figures, 1 table. Accepted for publication in Ap
Swift/XRT monitoring of the Supergiant Fast X-ray Transient IGR J18483-0311 for an entire orbital period
IGR J18483-0311 is an X-ray pulsar with transient X-ray activity, belonging
to the new class of High Mass X-ray Binaries called Supergiant Fast X-ray
Transients. This system is one of the two members of this class, together with
IGR J11215-5952, where both the orbital (18.52d) and spin period (21s) are
known. We report on the first complete monitoring of the X-ray activity along
an entire orbital period of a Supergiant Fast X-ray Transient. These Swift
observations, lasting 28d, cover more than one entire orbital phase
consecutively. They are a unique data-set, which allows us to constrain the
different mechanisms proposed to explain the nature of this new class of X-ray
transients. We applied the new clumpy wind model for blue supergiants developed
by Ducci et al. (2009), to the observed X-ray light curve. Assuming an
eccentricity of e=0.4, the X-ray emission from this source can be explained in
terms of the accretion from a spherically symmetric clumpy wind, composed of
clumps with different masses, ranging from 10^{18}g to 5x 10^{21}g.Comment: Accepted for publication in MNRAS. 7 pages, 7 figures, 2 table
Assessment of layerwise user-elements in Abaqus for static and free vibration analysis of variable stiffness composite laminates
In this work, user-elements (UEL) in Abaqus are taken a step forward into the high-order layerwise modelling of variable stiffness composite laminates with curvilinear fibre paths, extending the limited number of available literature on refined multilayered UEL models. Two layerwise UEL models with three discrete layers are here proposed, assigning to each layer the displacements of the first- and third-order shear deformation theories, thus named UEL1 and UEL3, respectively. A complete assessment of the models predictive capabilities is carried out by a comparison with available static and free vibration solutions in the literature – either for constant or variable stiffness laminates – considering various boundary and loading conditions, as well as thin and moderately thick plates. Numerical results demonstrate that the developed models are capable to render fairly accurate and computationally efficient results, with particular emphasis on the higher-order model for predicting the global–local response behaviour of moderately thick plates
Analytical modeling of panel flutter and active control in supersonic variable stiffness composite laminates
This work focuses on panel flutter and active control in supersonic variable stiffness composite laminates, making progress on the analytical modeling and combined exploration of curvilinear fiber composites tailoring and piezoelectric sensors/actuators, as promising structural design technologies, for aeroelastic control. The Classical Laminated Plate Theory and the First-Order Piston Theory are used as structural and aerodynamic models, respectively. Flutter analyses are carried out for simply supported plates, either purely elastic laminates or piezoelectric composite laminates. The tailor-ability of curvilinear fiber composites and the effect of proportional control are discussed. Ultimately, the presented results provide a comprehensive benchmark for future assessments
Fast hierarchical fusion model based on least squares B-splines approximation
With manufacturing shifting from traditional products to high value products, the complexity and accuracy of the products are increasing in order to reduce energy costs, create friendly environment and better health care. Structured surfaces, freeform surfaces, and other functional engineering surfaces are becoming the core part of high value manufacturing products. However, measurement of these surfaces is becoming very difficult due to instrumental limitations including measurement range, speed, resolution and accuracy. Multi-instruments/sensors measurement are now being developed for freeform and structured surface assessment, which requires the fusion of the data into a unified system to achieve larger dynamic measurements with greater reliability. This paper discusses the process of combining data from several information sources (instruments/sensors) into a common representational format and the surface topography can be reconstructed using Gaussian processes and B-spline techniques. In this paper the Gaussian process model is extended in order to take into account the uncertainty propagation and a new data fusion model based on least squares B-splines that drastically reduce the computational time are presented. The results are validated by two for freeform surface measurements
Tracking advanced persistent threats in critical infrastructures through opinion dynamics
Advanced persistent threats pose a serious issue for modern industrial environments, due to their targeted and complex attack vectors that are difficult to detect. This is especially severe in critical infrastructures that are accelerating the integration of IT technologies. It is then essential to further develop effective monitoring and response systems that ensure the continuity of business to face the arising set of cyber-security threats. In this paper, we study the practical applicability of a novel technique based on opinion dynamics, that permits to trace the attack throughout all its stages along the network by correlating different anomalies measured over time, thereby taking the persistence of threats and the criticality of resources into consideration. The resulting information is of essential importance to monitor the overall health of the control system and cor- respondingly deploy accurate response procedures. Advanced Persistent Threat Detection Traceability Opinion Dynamics.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech
Service Orientation and the Smart Grid state and trends
The energy market is undergoing major changes, the most notable of which is the transition from a hierarchical closed system toward a more open one highly based on a “smart” information-rich infrastructure. This transition calls for new information and communication technologies infrastructures and standards to support it. In this paper, we review the current state of affairs and the actual technologies with respect to such transition. Additionally, we highlight the contact points between the needs of the future grid and the advantages brought by service-oriented architectures.
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