11,902 research outputs found
Statistical properties of fractures in damaged materials
We introduce a model for the dynamics of mud cracking in the limit of of
extremely thin layers. In this model the growth of fracture proceeds by
selecting the part of the material with the smallest (quenched) breaking
threshold. In addition, weakening affects the area of the sample neighbour to
the crack. Due to the simplicity of the model, it is possible to derive some
analytical results. In particular, we find that the total time to break down
the sample grows with the dimension L of the lattice as L^2 even though the
percolating cluster has a non trivial fractal dimension. Furthermore, we obtain
a formula for the mean weakening with time of the whole sample.Comment: 5 pages, 4 figures, to be published in Europhysics Letter
Star Formation and AGN in the Core of the Shapley Supercluster: A VLA Survey of A3556, A3558, SC1327-312, SC1329-313, and A3562
The core of the Shapley supercluster (A3556, A3558, SC1327-312, SC1329-313,
and A3562) is an ideal region in which to study the effects of cluster mergers
on the activity of individual galaxies. This paper presents the most
comprehensive radio continuum investigation of the region, relying on a
63-pointing mosaic obtained with the Very Large Array yielding an areal
coverage of nearly 7 square degrees. The mosaic provides a typical sensitivity
of about 80 uJy at a resolution of 16", enabling detection of galaxies with
star formation rates as low as 1 solar mass per year. The radio data are
complemented by optical imaging in B and R, producing a catalog of 210
radio-detected galaxies with m_R <= 17.36 (M_R <= -19). At least 104 of these
radio-detected galaxies are members of the supercluster on the basis of public
velocity measurements. Across the entire core of the supercluster, there
appears to be a significant deficit of radio galaxies at intermediate optical
magnitudes (M_R between -21 and -22). This deficit is offset somewhat by an
increase in the frequency with which brighter galaxies (M_R less than -22) host
radio sources. More dramatic is the highly significant increase in the
probability for fainter galaxies (M_R between -20 and -21) in the vicinity of
A3562 and SC1329-313 to be associated with radio emission. The radio and
optical data for these sources strongly suggest that these active galaxies are
powered by star formation. In conjunction with recent X-ray analysis, this is
interpreted as young starbursts related to the recent merger of SC1329-313 with
A3562 and the rest of the supercluster.Comment: Accepted by AJ; 50 pages, including 16 figures (for full resolution
PDF, see http://mywebpages.comcast.net/nealamiller2/Shapley_pp.pdf
Emergency escape system uses self-braking mechanism on fixed cable
Slide-wire system with a twist level slide device incorporates automatic descent and braking for the safe and rapid evacuation of personnel from tall structures. This device is used on any tall structure that might require emergency evacuation. It is also used to transfer materials and equipment
The time to extinction for an SIS-household-epidemic model
We analyse a stochastic SIS epidemic amongst a finite population partitioned
into households. Since the population is finite, the epidemic will eventually
go extinct, i.e., have no more infectives in the population. We study the
effects of population size and within household transmission upon the time to
extinction. This is done through two approximations. The first approximation is
suitable for all levels of within household transmission and is based upon an
Ornstein-Uhlenbeck process approximation for the diseases fluctuations about an
endemic level relying on a large population. The second approximation is
suitable for high levels of within household transmission and approximates the
number of infectious households by a simple homogeneously mixing SIS model with
the households replaced by individuals. The analysis, supported by a simulation
study, shows that the mean time to extinction is minimized by moderate levels
of within household transmission
Evidence for J and H-band excess in classical T Tauri stars and the implications for disk structure and estimated ages
We argue that classical T Tauri stars (cTTs) possess significant non-
photospheric excess in the J and H bands. We first show that normalizing the
spectral energy distributions (SEDs) of cTTs to the J-band leads to a poor fit
of the optical fluxes, while normalizing the SEDs to the Ic-band produces a
better fit to the optical bands and in many cases reveals the presence of a
considerable excess at J and H. NIR spectroscopic veiling measurements from the
literature support this result. We find that J and H-band excesses correlate
well with the K-band excess, and that the J-K and H-K colors of the excess
emission are consistent with that of a black body at the dust sublimation
temperature (~ 1500-2000 K). We propose that this near-IR excess originates at
a hot inner rim, analogous to those suggested to explain the near-IR bump in
the SEDs of Herbig Ae/Be stars. To test our hypothesis, we use the model
presented by Dullemond et al. (2001) to fit the photometry data between 0.5 um
and 24 um of 10 cTTs associated with the Chamaeleon II molecular cloud. The
models that best fit the data are those where the inner radius of the disk is
larger than expected for a rim in thermal equilibrium with the photospheric
radiation field alone. In particular, we find that large inner rims are
necessary to account for the mid infrared fluxes (3.6-8.0 um) obtained by the
Spitzer Space Telescope. Finally, we argue that deriving the stellar
luminosities of cTTs by making bolometric corrections to the J-band fluxes
systematically overestimates these luminosities. The overestimated luminosities
translate into underestimated ages when the stars are placed in the H-R
diagram. Thus, the results presented herein have important implications for the
dissipation timescale of inner accretion disks.Comment: 45 pages, 13 figure
Dendritic and axonal targeting patterns of a genetically-specified class of retinal ganglion cells that participate in image-forming circuits.
BackgroundThere are numerous functional types of retinal ganglion cells (RGCs), each participating in circuits that encode a specific aspect of the visual scene. This functional specificity is derived from distinct RGC morphologies and selective synapse formation with other retinal cell types; yet, how these properties are established during development remains unclear. Islet2 (Isl2) is a LIM-homeodomain transcription factor expressed in the developing retina, including approximately 40% of all RGCs, and has previously been implicated in the subtype specification of spinal motor neurons. Based on this, we hypothesized that Isl2+ RGCs represent a related subset that share a common function.ResultsWe morphologically and molecularly characterized Isl2+ RGCs using a transgenic mouse line that expresses GFP in the cell bodies, dendrites and axons of Isl2+ cells (Isl2-GFP). Isl2-GFP RGCs have distinct morphologies and dendritic stratification patterns within the inner plexiform layer and project to selective visual nuclei. Targeted filling of individual cells reveals that the majority of Isl2-GFP RGCs have dendrites that are monostratified in layer S3 of the IPL, suggesting they are not ON-OFF direction-selective ganglion cells. Molecular analysis shows that most alpha-RGCs, indicated by expression of SMI-32, are also Isl2-GFP RGCs. Isl2-GFP RGCs project to most retino-recipient nuclei during early development, but specifically innervate the dorsal lateral geniculate nucleus and superior colliculus (SC) at eye opening. Finally, we show that the segregation of Isl2+ and Isl2- RGC axons in the SC leads to the segregation of functional RGC types.ConclusionsTaken together, these data suggest that Isl2+ RGCs comprise a distinct class and support a role for Isl2 as an important component of a transcription factor code specifying functional visual circuits. Furthermore, this study describes a novel genetically-labeled mouse line that will be a valuable resource in future investigations of the molecular mechanisms of visual circuit formation
Preliminary Results from an Experimental Assessment of a Natural Laminar Flow Design Method
A 5.2% scale semispan model of the new Common Research Model with Natural Laminar Flow (CRM-NLF) was tested in the National Transonic Facility (NTF) at the NASA Langley Research Center. The model was tested at transonic cruise flight conditions with Reynolds numbers based on mean aerodynamic chord ranging from 10 to 30 million. The goal of the test was to experimentally validate a new design method, referred to as Crossflow Attenuated NLF (CATNLF), which shapes airfoils to have pressure distributions that delay transition on wings with high sweep and Reynolds numbers. Additionally, the test aimed to characterize the NTF laminar flow testing capabilities, as well as establish best practices for laminar flow wind tunnel testing. Preliminary results regarding the first goal of validating the new design method are presented in this paper. Experimental data analyzed in this assessment include surface pressure data and transition images. The surface pressure data acquired during the test agree well with computational fluid dynamics (CFD) results. Transition images at a variety of Reynolds numbers and angles of attack are presented and compared to computational transition predictions. The experimental data are used to assess transition due to a turbulent attachment line, as well as crossflow and Tollmien-Schlichting modal instabilities. Preliminary results suggest the CATNLF design method is successful at delaying transition on wings with high sweep. Initial analysis of the transition front images showed transition Reynolds numbers that exceed historic experimental values at similar sweep angles.
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The imprints of primordial non-gaussianities on large-scale structure: scale dependent bias and abundance of virialized objects
We study the effect of primordial nongaussianity on large-scale structure,
focusing upon the most massive virialized objects. Using analytic arguments and
N-body simulations, we calculate the mass function and clustering of dark
matter halos across a range of redshifts and levels of nongaussianity. We
propose a simple fitting function for the mass function valid across the entire
range of our simulations. We find pronounced effects of nongaussianity on the
clustering of dark matter halos, leading to strongly scale-dependent bias. This
suggests that the large-scale clustering of rare objects may provide a
sensitive probe of primordial nongaussianity. We very roughly estimate that
upcoming surveys can constrain nongaussianity at the level |fNL| <~ 10,
competitive with forecasted constraints from the microwave background.Comment: 16 pages, color figures, revtex4. v2: added references and an
equation. submitted to PRD. v3: simplified derivation, additional reference
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