42,798 research outputs found
Comprehensive theory of the relative phase in atom-field interactions
We explore the role played by the quantum relative phase in a well-known
model of atom-field interaction, namely, the Dicke model. We introduce an
appropriate polar decomposition of the atom-field relative amplitudes that
leads to a truly Hermitian relative-phase operator, whose eigenstates correctly
describe the phase properties, as we demonstrate by studying the positive
operator-valued measure derived from it. We find the probability distribution
for this relative phase and, by resorting to a numerical procedure, we study
its time evolution.Comment: 20 pages, 4 figures, submitted to Phys. Rev.
Metallicity inhomogeneities in local star-forming galaxies as sign of recent metal-poor gas accretion
We measure the oxygen metallicity of the ionized gas along the major axis of
seven dwarf star-forming galaxies. Two of them, SDSSJ1647+21 and SDSSJ2238+14,
show 0.5 dex metallicity decrements in inner regions with enhanced
star-formation activity. This behavior is similar to the metallicity drop
observed in a number of local tadpole galaxies by Sanchez Almeida et al. (2013)
and interpreted as showing early stages of assembling in disk galaxies, with
the star formation sustained by external metal-poor gas accretion. The
agreement with tadpoles has several implications: (1) it proves that galaxies
other than the local tadpoles present the same unusual metallicity pattern. (2)
Our metallicity inhomogeneities were inferred using the direct method, thus
discarding systematic errors usually attributed to other methods. (3) Taken
together with the tadpole data, our findings suggest a threshold around one
tenth the solar value for the metallicity drops to show up. Although galaxies
with clear metallicity drops are rare, the physical mechanism responsible for
them may sustain a significant part of the star-formation activity in the local
Universe. We argue that the star-formation dependence of the mass-metallicity
relationship, as well as other general properties followed by most local disk
galaxies, are naturally interpreted as side effects of pristine gas infall.
Alternatives to the metal poor gas accretion are examined too.Comment: Accepted for publication in ApJ. 10 pages. 5 Fig
Immediate effects of diamond burr debridement in patients with spontaneous chronic corneal epithelial defects, light and electron microscopic evaluation
Operational and Technical Updates to the Object Reentry Survival Analysis Tool
The Object Reentry Survival Analysis Tool (ORSAT) has been used in the NASA Orbital Debris Program Office for over 25 years to estimate risk due to uncontrolled reentry of spacecraft and rocket bodies. Development over the last 3 years has included: a major change to the treatment of carbon fiber- and glass fiber-reinforced plastics (CFRP and GFRP, respectively); an updated atmospheric model; a new model for computing casualty area around an impacting debris object; and a newly-implemented scheme to determine the breakup altitude of a reentry object. Software also was written to automatically perform parameter sweeps in ORSAT to allow for uncertainty quantification and sensitivity analysis for components with borderline demisability. These updates have improved the speed and fidelity of the reentry analysis performed using ORSAT, and have allowed for improved engineering understanding by estimating the uncertainty for each components survivability. A statistical model for initial conditions captures the latitude bias in population density, a large improvement over the previous inclination-based latitude-averaged models. A sample spacecraft has been analyzed with standard techniques using ORSAT 6.2.1 and again using all the updated models; we will demonstrate the variation in the total debris casualty area and overall expectation of casualty
On the magnetic structure of the solar transition region
We examine the hypothesis that ``cool loops'' dominate emission from solar
transition region plasma below temperatures of K. We compare
published VAULT images of H L, a lower transition region line, with
near-contemporaneous magnetograms from Kitt Peak, obtained during the second
flight (VAULT-2) on 14 June 2002. The measured surface fields and potential
extrapolations suggest that there are too few short loops, and that L
emission is associated with the base regions of longer, coronal loops. VAULT-2
data of network boundaries have an asymmetry on scales larger than
supergranules, also indicating an association with long loops. We complement
the Kitt Peak data with very sensitive vector polarimetric data from the
Spectro-Polarimeter on board Hinode, to determine the influence of very small
magnetic concentrations on our analysis. From these data two classes of
behavior are found: within the cores of strong magnetic flux concentrations ( Mx) associated with active network and plage, small-scale mixed
fields are absent and any short loops can connect just the peripheries of the
flux to cell interiors. Core fields return to the surface via longer, most
likely coronal, loops. In weaker concentrations, short loops can connect
between concentrations and produce mixed fields within network boundaries as
suggested by Dowdy and colleagues. The VAULT-2 data which we examined are
associated with strong concentrations. We conclude that the cool loop model
applies only to a small fraction of the VAULT-2 emission, but we cannot
discount a significant role for cool loops in quieter regions. We suggest a
physical picture for how network L emission may occur through the
cross-field diffusion of neutral atoms from chromospheric into coronal plasma.Comment: Accepted by ApJ, 9 May 200
Time-Multiplexed Measurements of Nonclassical Light at Telecom Wavelengths
We report the experimental reconstruction of the statistical properties of an
ultrafast pulsed type-II parametric down conversion source in a periodically
poled KTP waveguide at telecom wavelengths, with almost perfect photon-number
correlations. We used a photon-number-resolving time-multiplexed detector based
on a fiber-optical setup and a pair of avalanche photodiodes. By resorting to a
germane data-pattern tomography, we assess the properties of the nonclassical
light states states with unprecedented precision.Comment: 4.5 pages, 5 color figues. Comments welcome
NUMERICAL EVALUATION OF THE CORROSION EFFECTS IN PRESTRESSED CONCRETE BEAMS WITHOUT SHEAR REINFORCEMENT
Corrosion of prestressed concrete structures causes size reduction of
strands, degradation of mechanical properties of steel, cracking of the
surrounding concrete and bond decay at steel-to-concrete interface. In
this paper, a numerical approach able to take into account all the effects
involved in the corrosion process by using non-linear finite element
analysis (NLFEA) and membrane or shell elements modelling, is
proposed. Two different strategies are adopted to model strands: the
smeared and the discrete approaches. The results obtained using these
latter strategies are validated by comparing NLFEA results with
experimental measurements of a naturally corroded prestressed beam
tested at the “Instituto de Ciencias de la Construcción Eduardo Torroja”
in Madrid. Finally, pros and cons of the proposed modelling approach are
critically analysed, demonstrating that considering the actual spatial
corrosion distribution is necessary to predict the position where failure
occurs
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