576 research outputs found
Atmospheric dynamics of red supergiant stars and applications to Interferometry
We have written a 3D radiative transfer code that computes emerging spectra
and intensity maps. We derive from radiative hydrodynamic (RHD) simulations of
RSG stars carried out with CO5BOLD (Freytag et al. 2002) observables expected
for red supergiant stars (RSG) especially for interferometric observations,
with emphasis on small scale structures. We show that the convection-related
surface structures are detectable in the H band with today's interferometers
and that the diameter measurement should not be too dependent on the adopted
model. The simulations are a great improvement over parametric models for the
interpretation of interferometric observations.Comment: 6 pages, SF2A 2007 conferenc
Ultrasensitive interferometric on-chip microscopy of transparent objects
Light microscopes can detect objects through several physical processes, such as scattering, absorption, and reflection. In transparent objects, these mechanisms are often too weak, and interference effects are more suitable to observe the tiny refractive index variations that produce phase shifts. We propose an on-chip microscope design that exploits birefringence in an unconventional geometry. It makes use of two sheared and quasi-overlapped illuminating beams experiencing relative phase shifts when going through the object, and a complementary metal-oxide-semiconductor image sensor array to record the resulting interference pattern. Unlike conventional microscopes, the beams are unfocused, leading to a very large field of view (20 mm(2)) and detection volume (more than 0.5 cm(3)), at the expense of lateral resolution. The high axial sensitivity (<1 nm) achieved using a novel phase-shifting interferometric operation makes the proposed device ideal for examining transparent substrates and reading microarrays of biomarkers. This is demonstrated by detecting nanometer-thick surface modulations on glass and single and double protein layers.Peer ReviewedPostprint (published version
Some propositions to find optimal conditions to simulate a flexible transport using an Agent-Based Model
This paper presents a method to assess the sensitivity of a flexible transport model based on agents and simulated using NetLogo. We simulate and we analyse a set of 124 transportation scenarios on several virtual networks and we assess their performance. Our main objective is to detect thresholds in the system scalability and efficiency. The research leads to three main results: (i) using Agent-Based Model, it is possible to significantly improve the global transport efficiency without any general objective function, (ii) there exists an optimal balance between the demand frequency and the number of simulated agents to simulate and perform a good flexible transport, (ii) to some extent, the network topological structure plays a non-negligible role in transport efficiency
A search for CO+ in planetary nebulae
We have carried out a systematic search for the molecular ion CO+ in a sample
of 8 protoplanetary and planetary nebulae in order to determine the origin of
the unexpectedly strong HCO+ emission previously detected in these sources. An
understanding of the HCO+ chemistry may provide direct clues to the physical
and chemical evolution of planetary nebulae. We find that the integrated
intensity of the CO+ line may be correlated with that of HCO+, suggesting that
the reaction of CO+ with molecular hydrogen may be an important formation route
for HCO+ in these planetary nebulae.Comment: 6 pages, 4 figures, accepted for publication in MNRA
Transmission of matter wave solitons through nonlinear traps and barriers
The transmissions of matter wave solitons through linear and nonlinear
inhomogeneities induced by the spatial variations of the trap and the
scattering length in Bose-Einstein condensates are investigated. New phenomena,
such as the enhanced transmission of a soliton through a linear trap by a
modulation of the scattering length, are exhibited. The theory is based on the
perturbed Inverse Scattering Transform for solitons, and we show that radiation
effects are important. Numerical simulations of the Gross-Pitaevskii equation
confirm the theoretical predictions.Comment: 6 pages, 4 figure
The low wind expansion velocity of metal-poor carbon stars in the Halo and the Sagittarius stream
We report the detection, from observations using the James Clerk Maxwell
Telescope, of CO J 3 2 transition lines in six carbon stars, selected
as members of the Galactic Halo and having similar infrared colors. Just one
Halo star had been detected in CO before this work. Infrared observations show
that these stars are red (J-K 3), due to the presence of large dusty
circumstellar envelopes. Radiative transfer models indicates that these stars
are losing mass with rather large dust mass-loss rates in the range 1--3.3
Myr, similar to what can be observed in the
Galactic disc. We show that two of these stars are effectively in the Halo, one
is likely linked to the stream of the Sagittarius Dwarf Spheroidal galaxy (Sgr
dSph), and the other three stars certainly belong to the thick disc. The wind
expansion velocities of the observed stars are low compared to carbon stars in
the thin disc and are lower for the stars in the Halo and the Sgr dSph stream
than in the thick disc. We discuss the possibility that the low expansion
velocities result from the low metallicity of the Halo carbon stars. This
implies that metal-poor carbon stars lose mass at a rate similar to metal-rich
carbon stars, but with lower expansion velocities, as predicted by recent
theoretical models. This result implies that the current estimates of mass-loss
rates from carbon stars in Local Group galaxies will have to be reconsidered.Comment: 10 pages, 7 figures, accepted for publication in MNRA
Modeling active electrolocation in weakly electric fish
In this paper, we provide a mathematical model for the electrolocation in
weakly electric fishes. We first investigate the forward complex conductivity
problem and derive the approximate boundary conditions on the skin of the fish.
Then we provide a dipole approximation for small targets away from the fish.
Based on this approximation, we obtain a non-iterative location search
algorithm using multi-frequency measurements. We present numerical experiments
to illustrate the performance and the stability of the proposed multi-frequency
location search algorithm. Finally, in the case of disk- and ellipse-shaped
targets, we provide a method to reconstruct separately the conductivity, the
permittivity, and the size of the targets from multi-frequency measurements.Comment: 37 pages, 11 figure
It's a wonderful tail: the mass loss history of Mira
Recent observations of the Mira AB binary system have revealed a surrounding
arc-like structure and a stream of material stretching 2 degrees away in
opposition to the arc. The alignment of the proper motion vector and the
arc-like structure shows the structures to be a bow shock and accompanying
tail. We have successfully hydrodynamically modelled the bow shock and tail as
the interaction between the asymptotic giant branch (AGB) wind launched from
Mira A and the surrounding interstellar medium. Our simulations show that the
wake behind the bow shock is turbulent: this forms periodic density variations
in the tail similar to those observed. We investigate the possiblity of
mass-loss variations, but find that these have limited effect on the tail
structure. The tail is estimated to be approximately 450,000 years old, and is
moving with a velocity close to that of Mira itself. We suggest that the
duration of the high mass-loss phase on the AGB may have been underestimated.
Finally, both the tail curvature and the rebrightening at large distance can be
qualitatively understood if Mira recently entered the Local Bubble. This is
estimated to have occured 17 pc downstream from its current location.Comment: 12 pages, 3 colour figures, accepted by ApJ Part II (Letters
Molecular Hydrogen Emission Lines in Far Ultraviolet Spectroscopic Explorer Observations of Mira B
We present new Far Ultraviolet Spectroscopic Explorer (FUSE) observations of
Mira A's wind-accreting companion star, Mira B. We find that the strongest
lines in the FUSE spectrum are H2 lines fluoresced by H I Lyman-alpha. A
previously analyzed Hubble Space Telescope (HST) spectrum also shows numerous
Lyman-alpha fluoresced H2 lines. The HST lines are all Lyman band lines, while
the FUSE H2 lines are mostly Werner band lines, many of them never before
identified in an astrophysical spectrum. We combine the FUSE and HST data to
refine estimates of the physical properties of the emitting H2 gas. We find
that the emission can be reproduced by an H2 layer with a temperature and
column density of T=3900 K and log N(H2)=17.1, respectively. Another similarity
between the HST and FUSE data, besides the prevalence of H2 emission, is the
surprising weakness of the continuum and high temperature emission lines,
suggesting that accretion onto Mira B has weakened dramatically. The UV fluxes
observed by HST on 1999 August 2 were previously reported to be over an order
of magnitude lower than those observed by HST and the International Ultraviolet
Explorer (IUE) from 1979--1995. Analysis of the FUSE data reveals that Mira B
was still in a similarly low state on 2001 November 22.Comment: 23 pages, 6 figures; AASTEX v5.0 plus EPSF extensions in mkfig.sty;
accepted by Ap
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