1,656 research outputs found
Experimental evidence of chaotic advection in a convective flow
Lagrangian chaos is experimentally investigated in a convective flow by means
of Particle Tracking Velocimetry. The Fnite Size Lyapunov Exponent analysis is
applied to quantify dispersion properties at different scales. In the range of
parameters of the experiment, Lagrangian motion is found to be chaotic.
Moreover, the Lyapunov depends on the Rayleigh number as . A
simple dimensional argument for explaining the observed power law scaling is
proposed.Comment: 7 pages, 3 figur
Cosmological extrapolation of MOND
Regime of MOND, which is used in astronomy to describe the gravitating
systems of island type without the need to postulate the existence of a
hypothetical dark matter, is generalized to the case of homogeneous
distribution of usual matter by introducing a linear dependence of the critical
acceleration on the size of region under consideration. We show that such the
extrapolation of MOND in cosmology is consistent with both the observed
dependence of brightness on the redshift for type Ia supernovae and the
parameters of large-scale structure of Universe in the evolution, that is
determined by the presence of a cosmological constant, the ordinary matter of
baryons and electrons as well as the photon and neutrino radiation without any
dark matter.Comment: 20 pages, 5 figures, comments adde
X-ray emission from the double-binary OB-star system QZ Car (HD 93206)
X-ray observations of the double-binary OB-star system QZ Car (HD 93206)
obtained with the Chandra X-ray Observatory over a period of roughly 2 years
are presented. The orbit of systems A (O9.7 I+b2 v, PA = 21 d) and B (O8 III+o9
v, PB = 6 d) are reasonably well sampled by the observations, allowing the
origin of the X-ray emission to be examined in detail. The X-ray spectra can be
well fitted by an attenuated three temperature thermal plasma model,
characterised by cool, moderate, and hot plasma components at kT ~ 0.2, 0.7,
and 2 keV, respectively, and a circumstellar absorption of ~ 0.2 x 10^22 cm-2.
Although the hot plasma component could be indicating the presence of wind-wind
collision shocks in the system, the model fluxes calculated from spectral fits,
with an average value of ~ 7 x 10^-13 erg s-1 cm-2, do not show a clear
correlation with the orbits of the two constituent binaries. A semi-analytical
model of QZ Car reveals that a stable momentum balance may not be established
in either system A or B. Yet, despite this, system B is expected to produce an
observed X-ray flux well in excess of the observations. If one considers the
wind of the O8 III star to be disrupted by mass transfer the model and
observations are in far better agreement, which lends support to the previous
suggestion of mass-transfer in the O8 III + o9 v binary. We conclude that the
X-ray emission from QZ Car can be reasonably well accounted for by a
combination of contributions mainly from the single stars and the mutual
wind-wind collision between systems A and B.Comment: 11 pages, 7 figures. Accepted for the ApJS Special Issue on the
Chandra Carina Complex Project (CCCP), scheduled for publication in May 2011.
All 16 CCCP Special Issue papers are available at
http://cochise.astro.psu.edu/Carina_public/special_issue.html through 2011 at
leas
Lorentz Violation of Quantum Gravity
A quantum gravity theory which becomes renormalizable at short distances due
to a spontaneous symmetry breaking of Lorentz invariance and diffeomorphism
invariance is studied. A breaking of Lorentz invariance with the breaking
patterns and , describing 3+1 and 2+1
quantum gravity, respectively, is proposed. A complex time dependent
Schr\"odinger equation (generalized Wheeler-DeWitt equation) for the wave
function of the universe exists in the spontaneously broken symmetry phase at
Planck energy and in the early universe, uniting quantum mechanics and general
relativity. An explanation of the second law of thermodynamics and the
spontaneous creation of matter in the early universe can be obtained in the
symmetry broken phase of gravity.Comment: 10 pages, minor change and reference added. Typos corrected. To be
published in Class. Quant. Grav
Spitzer observations of the Massive star forming complex S254-S258: structure and evolution
We present Spitzer-IRAC, NOAO 2.1meter-Flamingos, Keck-NIRC, and
FCRAO-SEQUOIA observations of the massive star forming complex S254-S258,
covering an area of 25x20 arc-minutes. Using a combination of the IRAC and NIR
data, we identify and classify the young stellar objects (YSO) in the complex.
We detect 510 sources with near or mid IR-excess, and we classify 87 Class I,
and 165 Class II sources. The YSO are found in clusters surrounded by isolated
YSO in a low-density distributed population. The ratio of clustered to total
YSO is 0.8. We identify six new clusters in the complex. One of them,
G192.63-00, is located around the ionizing star of the HII region S255. We
hypothesize that the ionizing star of S255 was formed in this cluster. We also
detect a southern component of the cluster in HII region S256. The cluster
G192.54-0.15, located inside HII region S254 has a VLSR of 17 km/s with respect
to the main cloud, and we conclude that it is located in the background of the
complex. The structure of the molecular cloud is examined using 12CO and 13CO,
as well as a near-IR extinction map. The main body of the molecular cloud has
VLSR between 5 and 9 km/s. The arc-shaped structure of the molecular cloud,
following the border of the HII regions, and the high column density in the
border of the HII regions support the idea that the material has been swept up
by the expansion of the HII regions.Comment: Accepted for publication in The Astrophysical Journa
Complete solutions to the metric of spherically collapsing dust in an expanding spacetime with a cosmological constant
We present semi-analytical solutions to the background equations describing
the Lema\^itre-Tolman-Bondi (LTB) metric as well as the homogeneous Friedmann
equations, in the presence of dust, curvature and a cosmological constant
Lambda. For none of the presented solutions any numerical integration has to be
performed. All presented solutions are given for expanding and collapsing
phases, preserving continuity in time and radius. Hence, these solutions
describe the complete space time of a collapsing spherical object in an
expanding universe. In the appendix we present for completeness a solution of
the Friedmann equations in the additional presence of radiation, only valid for
the Robertson-Walker metric.Comment: 23 pages, one figure. Numerical module for evaluation of the
solutions released at
http://web.physik.rwth-aachen.de/download/valkenburg/ColLambda/ Matches
published version, published under Open Access. Note change of titl
Nielsen-Olesen vortex in varying-alpha theories
We consider soliton solutions to Bekenstein's theory, for which the fine
structure constant is allowed to vary due to the
presence of a dielectric field pervading the vacuum. More specifically we
investigate the effects of a varying upon a complex scalar field with
a U(1) electromagnetic gauge symmetry subject to spontaneous symmetry breaking.
We find vortex solutions to this theory, similar to the Nielsen-Olesen vortex.
Near the vortex core the electric charge is typically much larger than far away
from the string, lending these strings a superconducting flavour. In general
the dielectric field coats the usual local string with a global string
envelope. We discuss the cosmological implications of networks of such strings,
with particular emphasis on their ability to generate inhomogeneous
recombination scenarios. We also consider the possibility of the dielectric
being a charged free field. Even though the vacuum of such a field is trivial,
we find that the dielectric arranges itself in the shape of a local string,
with a quantized magnetic flux at the core -- presumably borrowing these
topological features from the underlying Nielsen-Olesen vortex.Comment: 7 pages, 1 figur
"Swiss-Cheese" Inhomogeneous Cosmology & the Dark Energy Problem
We study an exact swiss-cheese model of the Universe, where inhomogeneous LTB
patches are embedded in a flat FLRW background, in order to see how
observations of distant sources are affected. We find negligible integrated
effect, suppressed by (L/R_{H})^3 (where L is the size of one patch, and R_{H}
is the Hubble radius), both perturbatively and non-perturbatively. We
disentangle this effect from the Doppler term (which is much larger and has
been used recently \cite{BMN} to try to fit the SN curve without dark energy)
by making contact with cosmological perturbation theory.Comment: 35 pages, 6 figure
The Possibility of Thermal Instability in Early-Type Stars Due to Alfven Waves
It was shown by dos Santos et al. the importance of Alfv\'en waves to explain
the winds of Wolf-Rayet stars. We investigate here the possible importance of
Alfv\'en waves in the creation of inhomogeneities in the winds of early-type
stars. The observed infrared emission (at the base of the wind) of early-type
stars is often larger than expected. The clumping explains this characteristic
in the wind, increasing the mean density and hence the emission measure, making
possible to understand the observed infrared, as well as the observed
enhancement in the blue wing of the line. In this study, we
investigate the formation of these clumps a via thermal instability. The
heat-loss function used, , includes physical processes such as:
emission of (continuous and line) recombination radiation; resonance line
emission excited by electron collisions; thermal bremsstrahlung; Compton
heating and cooling; and damping of Alfv\'en waves. As a result of this
heat-loss function we show the existence of two stable equilibrium regions. The
stable equilibrium region at high temperature is the diffuse medium and at low
temperature the clumps. Using this reasonable heat-loss function, we show that
the two stable equilibrium regions can coexist over a narrow range of pressures
describing the diffuse medium and the clumps.Comment: 21 pages (psfig.sty), 5 figures (included), ApJ accepted. Also
available at http://www.iagusp.usp.br/preprints/preprint.htm
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