1,315 research outputs found
Curvature-Induced Instabilities of Shells
Induced by proteins within the cell membrane or by differential growth,
heating, or swelling, spontaneous curvatures can drastically affect the
morphology of thin bodies and induce mechanical instabilities. Yet, the
interaction of spontaneous curvature and geometric frustration in curved shells
remains still poorly understood. Via a combination of precision experiments on
elastomeric spherical bilayer shells, simulations, and theory, we show a
spontaneous curvature-induced rotational symmetry-breaking as well as a
snapping instability reminiscent of the Venus fly trap closure mechanism. The
instabilities and their dependence on geometry are rationalized by reducing the
spontaneous curvature to an effective mechanical load. This formulation reveals
a combined pressurelike bulk term and a torquelike boundary term, allowing
scaling predictions for the instabilities in excellent agreement with
experiments and simulations. Moreover, the effective pressure analogy suggests
a curvature-induced buckling in closed shells. We determine the critical
buckling curvature via a linear stability analysis that accounts for the
combination of residual membrane and bending stresses. The prominent role of
geometry in our findings suggests the applicability of the results over a wide
range of scales.Comment: 12 pages, 9 figures (including Supporting Information
Statistics of Cosmological Black Hole Jet Sources: Blazar Predictions for GLAST
A study of the statistics of cosmological black-hole jet sources is applied
to EGRET blazar data, and predictions are made for GLAST. Black-hole jet
sources are modeled as collimated relativistic plasma outflows with radiation
beamed along the jet axis due to strong Doppler boosting. The comoving rate
density of blazar flares is assumed to follow a blazar formation rate (BFR),
modeled by analytic functions based on astronomical observations and fits to
EGRET data. The redshift and size distributions of gamma-ray blazars observed
with EGRET, separated into BL Lac object (BL) and flat spectrum radio quasar
(FSRQ) distributions, are fit with monoparametric functions for the
distributions of the jet Lorentz factor \Gamma, comoving directional power
l'_e, and spectral slope. A BFR factor ~10 x greater at z ~ 1 than at present
is found to fit the FSRQ data. A smaller comoving rate density and greater
luminosity of BL flares at early times compared to the present epoch fits the
BL data. Based on the EGRET observations, ~1000 blazars consisting of ~800
FSRQs and FR2 radio galaxies and ~200 BL Lacs and FR1 radio galaxies will be
detected with GLAST during the first year of the mission. Additional AGN
classes, such as hard-spectrum BL Lacs that were mostly missed with EGRET,
could add more GLAST sources. The FSRQ and BL contributions to the EGRET
gamma-ray background at 1 GeV are estimated at the level of ~10 - 15% and ~2 -
4%, respectively. EGRET and GLAST sensitivities to blazar flares are considered
in the optimal case, and a GLAST analysis method for blazar detection is
outlined.Comment: 17 pages, 9 figures, ApJ, in press, v.660, May 1, 2007 (minor changes
from previous version
The NATA code: Theory and analysis, volume 1
A computer program for calculating quasi-one-dimensional gas flow in axisymmetric and two-dimensional nozzles and rectangular channels is presented. Flow is assumed to start from a state of thermochemical equilibrium at a high temperature in an upstream reservoir. The program provides solutions based on frozen chemistry, chemical equilibrium, and nonequilibrium flow with finite reaction rates. Electronic nonequilibrium effects can be included using a two-temperature model. An approximate laminar boundary layer calculation is given for the shear and heat flux on the nozzle wall. Boundary layer displacement effects on the inviscid flow are considered also. Chemical equilibrium and transport property calculations are provided by subroutines. The code contains precoded thermochemical, chemical kinetic, and transport cross section data for high-temperature air, CO2-N2-Ar mixtures, helium, and argon. It provides calculations of the stagnation conditions on axisymmetric or two-dimensional models, and of the conditions on the flat surface of a blunt wedge. The primary purpose of the code is to describe the flow conditions and test conditions in electric arc heated wind tunnels
RX J0911+05: A Massive Cluster Lens at z=0.769
We report the detection of a massive high-redshift cluster of galaxies near
the quadruple quasar RX J0911+05, using the LRIS instrument on the Keck-II
telescope. The cluster is found to have a mean redshift of =0.7689+/-0.002
and a velocity dispersion of sigma=836{+180-200} km/s, based on redshift
measurements for 24 member galaxies. This massive high-redshift cluster is the
origin of the unusually large external shear required by lensing models of the
quadruple quasar system. We predict the expected time delay depending on the
exact contribution of the cluster. A measurement of the time delay and further
deep lensing and X-ray observations will unravel useful properties of this
serendipitously discovered high-redshift cluster, and may put interesting
cosmological constraints on H0.Comment: Submitted to ApJL, 7 pages, 5 figure
The NATA code; theory and analysis. Volume 2: User's manual
The NATA code is a computer program for calculating quasi-one-dimensional gas flow in axisymmetric nozzles and rectangular channels, primarily to describe conditions in electric archeated wind tunnels. The program provides solutions based on frozen chemistry, chemical equilibrium, and nonequilibrium flow with finite reaction rates. The shear and heat flux on the nozzle wall are calculated and boundary layer displacement effects on the inviscid flow are taken into account. The program contains compiled-in thermochemical, chemical kinetic and transport cross section data for high-temperature air, CO2-N2-Ar mixtures, helium, and argon. It calculates stagnation conditions on axisymmetric or two-dimensional models and conditions on the flat surface of a blunt wedge. Included in the report are: definitions of the inputs and outputs; precoded data on gas models, reactions, thermodynamic and transport properties of species, and nozzle geometries; explanations of diagnostic outputs and code abort conditions; test problems; and a user's manual for an auxiliary program (NOZFIT) used to set up analytical curvefits to nozzle profiles
The NATA code; theory and analysis. Volume 3: Programmer's manual
The present, third volume of the final report is a programmer's manual for the code. It provides a listing of the FORTRAN 4 source program; a complete glossary of FORTRAN symbols; a discussion of the purpose and method of operation of each subroutine (including mathematical analyses of special algorithms); and a discussion of the operation of the code on IBM/360 and UNIVAC 1108 systems, including required control cards and the overlay structure used to accommodate the code to the limited core size of the 1108. In addition, similar information is provided to document the programming of the NOZFIT code, which is employed to set up nozzle profile curvefits for use in NATA
The QSO evolution derived from the HBQS and other complete QSO surveys
An ESO Key programme dedicated to an Homogeneous Bright QSO Survey (HBQS) has
been completed. 327 QSOs (Mb<-23, 0.3<z<2.2) have been selected over 555 deg^2
with 15<B<18.75. For B<16.4 the QSO surface density turns out to be a factor
2.2 higher than what measured by the PG survey, corresponding to a surface
density of 0.013+/-.006 deg^{-2}. If the Edinburgh QSO Survey is included, an
overdensity of a factor 2.5 is observed, corresponding to a density of
0.016+/-0.005 deg^{-2}. In order to derive the QSO optical luminosity function
(LF) we used Monte Carlo simulations that take into account of the selection
criteria, photometric errors and QSO spectral slope distribution. The LF can be
represented with a Pure Luminosity Evolution (L(z)\propto(1+z)^k) of a two
power law both for q_0=0.5 and q_0=0.1. For q_0=0.5 k=3.26, slower than the
previous Boyle's (1992) estimations of k=3.45. A flatter slope beta=-3.72 of
the bright part of the LF is also required. The observed overdensity of bright
QSOs is concentrated at z<0.6. It results that in the range 0.3<z<0.6 the
luminosity function is flatter than observed at higher redshifts. In this
redshift range, for Mb<-25, 32 QSOs are observed instead of 19 expected from
our best-fit PLE model. This feature requires a luminosity dependent luminosity
evolution in order to satisfactorily represent the data in the whole 0.3<z<2.2
interval.Comment: Invited talk in "Wide Field Spectroscopy" (20-24 May 1996, Athens),
eds. M. Kontizas et al. 6 pages and 3 eps figures, LaTex file, uses epfs.sty
and crckapb.sty (included
Migration in der europäischen Geschichte seit dem späten Mittelalter [2002]
In Germany, migration research is still a relatively young line of research. Several obstacles complicated a critical recovery of research concepts on the history of population and migration that had been shaped as early as in the 1920s. This was the result of the multilayered disavowal of academic demography - because of its role in Nazi Germany, because of the long-lasting primate of history of politics in post-WW ll Germany, and finally because of the late emergence of the history of society. This situation has profoundly changed during the last decades of the twentieth century. Reasons were the increasing historical distance to the ‘fall of man’ of demography in Nazi Germany, the reorientation of historiography in the context of critical social and cultural sciences; the inclusion of labor-market research into migration research, and the shaping of interdisciplinary and integral research concepts
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