1,804 research outputs found
Curvature suppresses the Rayleigh-Taylor instability
The dynamics of a thin liquid film on the underside of a curved cylindrical
substrate is studied. The evolution of the liquid layer is investigated as the
film thickness and the radius of curvature of the substrate are varied. A
dimensionless parameter (a modified Bond number) that incorporates both
geometric parameters, gravity, and surface tension is identified, and allows
the observations to be classified according to three different flow regimes:
stable films, films with transient growth of perturbations followed by decay,
and unstable films. Experiments and theory confirm that, below a critical value
of the Bond number, curvature of the substrate suppresses the Rayleigh-Taylor
instability
Analytic calculation of radio emission from parametrized extensive air showers:A tool to extract shower parameters
The radio intensity and polarization footprint of a cosmic-ray induced extensive air shower is determined by the time-dependent structure of the current distribution residing in the plasma cloud at the shower front. In turn, the time dependence of the integrated charge-current distribution in the plasma cloud, the longitudinal shower structure, is determined by interesting physics which one would like to extract, such as the location and multiplicity of the primary cosmic-ray collision or the values of electric fields in the atmosphere during thunderstorms. To extract the structure of a shower from its footprint requires solving a complicated inverse problem. For this purposewe have developed a code that semianalytically calculates the radio footprint of an extensive air shower given an arbitrary longitudinal structure. This code can be used in an optimization procedure to extract the optimal longitudinal shower structure given a radio footprint. On the basis of air-shower universality we propose a simple parametrization of the structure of the plasma cloud. This parametrization is based on the results of Monte Carlo shower simulations. Deriving the parametrization also teaches which aspects of the plasma cloud are important for understanding the features seen in the radio-emission footprint. The calculated radio footprints are compared with microscopic CoREAS simulations
Broad Line Emission in Low-Metallicity Blue Compact Dwarf Galaxies: Evidence for Stellar Wind, Supernova and Possible AGN Activity
We present spectra of a large sample of low-metallicity blue compact dwarf
galaxies which exhibit broad components in their strong emission lines, mainly
in Hbeta, [O III]4959, 5007 and Halpha. Twenty-three spectra have been obtained
with the MMT, 14 of which show broad emission. The remaining 21 spectra with
broad emission have been selected from the Data Release 5 of the Sloan Digital
Sky Survey. The most plausible origin of broad line emission is the evolution
of massive stars and their interaction with the circumstellar and interstellar
medium. The broad emission with the lowest H luminosities (10^36 -
10^39 erg/s) is likely produced in circumstellar envelopes around hot Ofp/WN9
and/or LBV stars. The broad emission with the highest Halpha luminosities
(10^40 - 10^42 erg/s) probably arises from type IIp or type IIn supernovae
(SNe). It can also come from active galactic nuclei (AGN) containing
intermediate-mass black holes, although we find no strong evidence for hard
non-thermal radiation in our sample galaxies. The oxygen abundance in the host
galaxies with SN candidates is low and varies in the range 12 + log O/H = 7.36
- 8.31. However, type IIn SN / AGN candidates are found only in galaxies with
12 + log O/H < 7.99. Spectroscopic monitoring of these type IIn SN / AGN
candidates over a time scale of several years is necessary to distinguish
between the two possibilities.Comment: 50 pages, 6 figures. Accepted for publication in the Astrophysical
Journa
Cervical artery dissection: An atypical presentation with Ehlers-Danlos-like collagen pathology?
The authors took skin biopsies of the macroscopically normal skin of seven consecutive patients with spontaneous cervical artery dissection (SCAD). Histologically, alterations of the collagen and elastic fiber networks were found in six patients. In five, the histologic, immunohistochemical, and ultrastructural changes were similar to those usually found in Ehlers-Danlos syndrome (EDS). This suggests that SCAD is frequently associated with the dermal alterations seen in EDS
What Affects Social Attention? Social Presence, Eye Contact and Autistic Traits
Social understanding is facilitated by effectively attending to other people and the subtle social cues they generate. In order to more fully appreciate the nature of social attention and what drives people to attend to social aspects of the world, one must investigate the factors that influence social attention. This is especially important when attempting to create models of disordered social attention, e.g. a model of social attention in autism. Here we analysed participants' viewing behaviour during one-to-one social interactions with an experimenter. Interactions were conducted either live or via video (social presence manipulation). The participant was asked and then required to answer questions. Experimenter eye-contact was either direct or averted. Additionally, the influence of participant self-reported autistic traits was also investigated. We found that regardless of whether the interaction was conducted live or via a video, participants frequently looked at the experimenter's face, and they did this more often when being asked a question than when answering. Critical differences in social attention between the live and video interactions were also observed. Modifications of experimenter eye contact influenced participants' eye movements in the live interaction only; and increased autistic traits were associated with less looking at the experimenter for video interactions only. We conclude that analysing patterns of eye-movements in response to strictly controlled video stimuli and natural real-world stimuli furthers the field's understanding of the factors that influence social attention. © 2013 Freeth et al
Calculation of the Hidden Symmetry Operator in PT-Symmetric Quantum Mechanics
In a recent paper it was shown that if a Hamiltonian H has an unbroken PT
symmetry, then it also possesses a hidden symmetry represented by the linear
operator C. The operator C commutes with both H and PT. The inner product with
respect to CPT is associated with a positive norm and the quantum theory built
on the associated Hilbert space is unitary. In this paper it is shown how to
construct the operator C for the non-Hermitian PT-symmetric Hamiltonian
using perturbative techniques. It
is also shown how to construct the operator C for
using nonperturbative methods
Scaling of folding properties in simple models of proteins
Scaling of folding properties of proteins is studied in a toy system -- the
lattice Go model with various two- and three- dimensional geometries of the
maximally compact native states. Characteristic folding times grow as power
laws with the system size. The corresponding exponents are not universal.
Scaling of the thermodynamic stability also indicates size-related
deterioration of the folding properties.Comment: REVTeX, 4 pages, 4 EPS figures, PRL (in press
Coarse grained description of the protein folding
We consider two- and three-dimensional lattice models of proteins which were
characterized previously. We coarse grain their folding dynamics by reducing it
to transitions between effective states. We consider two methods of selection
of the effective states. The first method is based on the steepest descent
mapping of states to underlying local energy minima and the other involves an
additional projection to maximally compact conformations. Both methods generate
connectivity patterns that allow to distinguish between the good and bad
folders. Connectivity graphs corresponding to the folding funnel have few loops
and are thus tree-like. The Arrhenius law for the median folding time of a
16-monomer sequence is established and the corresponding barrier is related to
easily identifiable kinetic trap states.Comment: REVTeX, 9 pages, 15 EPS figures, to appear in Phys. Rev.
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