12,244 research outputs found
Non-axisymmetric instability of shear-banded Taylor-Couette flow
Recent experiments show that shear-banded flows of semi-dilute worm-like
micelles in Taylor-Couette geometry exhibit a flow instability in the form of
Taylor-like vortices. Here we perform the non-axisymmetric linear stability
analysis of the diffusive Johnson-Segalman model of shear banding and show that
the nature of this instability depends on the applied shear rate. For the
experimentally relevant parameters, we find that at the beginning of the stress
plateau the instability is driven by the interface between the bands, while
most of the stress plateau is occupied by the bulk instability of the
high-shear-rate band. Our work significantly alters the recently proposed
stability diagram of shear-banded flows based on axisymmetric analysis.Comment: 6 pages, 5 figures, main text and supplementary material; accepted to
Phys. Rev. Let
The Electrostatic Ion Beam Trap : a mass spectrometer of infinite mass range
We study the ions dynamics inside an Electrostatic Ion Beam Trap (EIBT) and
show that the stability of the trapping is ruled by a Hill's equation. This
unexpectedly demonstrates that an EIBT, in the reference frame of the ions
works very similar to a quadrupole trap. The parallelism between these two
kinds of traps is illustrated by comparing experimental and theoretical
stability diagrams of the EIBT. The main difference with quadrupole traps is
that the stability depends only on the ratio of the acceleration and trapping
electrostatic potentials, not on the mass nor the charge of the ions. All kinds
of ions can be trapped simultaneously and since parametric resonances are
proportional to the square root of the charge/mass ratio the EIBT can be used
as a mass spectrometer of infinite mass range
Hydrodynamic limit for a boundary driven stochastic lattice gas model with many conserved quantities
We prove the hydrodynamic limit for a particle system in which particles may
have different velocities. We assume that we have two infinite reservoirs of
particles at the boundary: this is the so-called boundary driven process. The
dynamics we considered consists of a weakly asymmetric simple exclusion process
with collision among particles having different velocities
A Search for Dense Molecular Gas in High Redshift Infrared-Luminous Galaxies
We present a search for HCN emission from four high redshift far infrared
(IR) luminous galaxies. Current data and models suggest that these high IR
luminous galaxies represent a major starburst phase in the formation of
spheroidal galaxies, although many of the sources also host luminous active
galactic nuclei (AGN), such that a contribution to the dust heating by the AGN
cannot be precluded. HCN emission is a star formation indicator, tracing dense
molecular hydrogen gas within star-forming molecular clouds (n(H) cm). HCN luminosity is linearly correlated with IR luminosity for
low redshift galaxies, unlike CO emission which can also trace gas at much
lower density. We report a marginal detection of HCN (1-0) emission from the
QSO J1409+5628, with a velocity integrated line luminosity of
K km s pc, while we obtain
3 upper limits to the HCN luminosity of the QSO J0751+2716 of
K km s pc, K km s pc for the starburst galaxy
J1401+0252, and K km s pc for the QSO J1148+5251. We compare the HCN data on these sources, plus three
other high- IR luminous galaxies, to observations of lower redshift
star-forming galaxies. The values of the HCN/far-IR luminosity ratios (or
limits) for all the high sources are within the scatter of the relationship
between HCN and far-IR emission for low star-forming galaxies (truncated).Comment: aastex format, 4 figures. to appear in the Astrophysical Journal;
Revised lens magnification estimate for 1401+025
Association of radio polar cap brightening with bright patches and coronal holes
Radio-bright regions near the solar poles are frequently observed in Nobeyama
Radioheliograph (NoRH) maps at 17 GHz, and often in association with coronal
holes. However, the origin of these polar brightening has not been established
yet. We propose that small magnetic loops are the source of these bright
patches, and present modeling results that reproduce the main observational
characteristics of the polar brightening within coronal holes at 17 GHz. The
simulations were carried out by calculating the radio emission of the small
loops, with several temperature and density profiles, within a 2D coronal hole
atmospheric model. If located at high latitudes, the size of the simulated
bright patches are much smaller than the beam size and they present the
instrument beam size when observed. The larger bright patches can be generated
by a great number of small magnetic loops unresolved by the NoRH beam. Loop
models that reproduce bright patches contain denser and hotter plasma near the
upper chromosphere and lower corona. On the other hand, loops with increased
plasma density and temperature only in the corona do not contribute to the
emission at 17 GHz. This could explain the absence of a one-to-one association
between the 17 GHz bright patches and those observed in extreme ultraviolet.
Moreover, the emission arising from small magnetic loops located close to the
limb may merge with the usual limb brightening profile, increasing its
brightness temperature and width.Comment: 8 pages, 6 figures, 1 table. Accepted for publication in The
Astrophysical Journa
Model-based diagnosis of acute pulmonary embolism - results from a porcine model
peer reviewe
Gravitational Lensing of the X-Ray Background by Clusters of Galaxies
Gravitational lensing by clusters of galaxies affects the cosmic X-ray
background (XRB) by altering the observed density and flux distribution of
background X-ray sources. At faint detection flux thresholds, the resolved
X-ray sources appear brighter and diluted, while the unresolved component of
the XRB appears dimmer and more anisotropic, due to lensing. The diffuse X-ray
intensity in the outer halos of clusters might be lower than the sky-averaged
XRB, after the subtraction of resolved sources. Detection of the lensing signal
with a wide-field X-ray telescope could probe the mass distribution of a
cluster out to its virialization boundary. In particular, we show that the
lensing signature imprinted on the resolved component of the XRB by the cluster
A1689, should be difficult but possible to detect out to 8' at the 2-4 sigma
level, after 10^6 seconds of observation with the forthcoming AXAF satellite.
The lensing signal is fairly insensitive to the lens redshift in the range
0.1<z<0.6. The amplitude of the lensing signal is however sensitive to the
faint end slope of the number-flux relation for unresolved X-ray sources, and
can thus help constrain models of the XRB. A search for X-ray arcs or arclets
could identify the fraction of all faint sources which originate from extended
emission of distant galaxies. The probability for a 3 sigma detection of an
arclet which is stretched by a factor of about 3 after a 10^6 seconds
observation of A1689 with AXAF, is roughly comparable to the fraction of all
background X-ray sources that have an intrinsic size of order 1''.Comment: 41 LaTeX pages, 11 postscript figures, 1 table, in AASTeX v4.0
format. To appear in ApJ, April 1, 1997, Vol. 47
Habitat association predicts genetic diversity and population divergence in amazonian birds
© 2017 by The University of Chicago. The ecological traits of organisms may predict their genetic diversity and population genetic structure and mediate the action of evolutionary processes important for speciation and adaptation. Making these ecological-evolutionary links is difficult because it requires comparable genetic estimates from many species with differing ecologies. In Amazonian birds, habitat association is an important component of ecological diversity. Here, we examine the link between habitat association and genetic parameters using 20 pairs of closely related Amazonian bird species in which one member of the pair occurs primarily in forest edge and floodplains and the other occurs in upland forest interior. We use standardized geographic sampling and data from 2,416 genomic markers to estimate genetic diversity, population genetic structure, and statistics reflecting demographic and evolutionary processes. We find that species of upland forest have greater genetic diversity and divergence across the landscape as well as signatures of older histories and less gene flow than floodplain species. Our results reveal that species ecology in the form of habitat association is an important predictor of genetic diversity and population divergence and suggest that differences in diversity between floodplain and upland avifaunas in the Amazon may be driven by differences in the demographic and evolutionary processes at work in the two habitats
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