3,986 research outputs found
VLTI/AMBER spectro-interferometric imaging of VX Sgr's inhomogenous outer atmosphere
Aims. We aim to explore the photosphere of the very cool late-type star VX
Sgr and in particular the existence and characterization of molecular layers
above the continuum forming photosphere. Methods. We obtained interferometric
observations with the VLTI/AMBER interferometer using the fringe tracker FINITO
in the spectral domain 1.45-2.50 micron with a spectral resolution of about 35
and baselines ranging from 15 to 88 meters.We perform independent image
reconstruction for different wavelength bins and fit the interferometric data
with a geometrical toy model.We also compare the data to 1D dynamical models of
Miras atmosphere and to 3D hydrodynamical simulations of red supergiant (RSG)
and asymptotic giant branch (AGB) stars. Results. Reconstructed images and
visibilities show a strong wavelength dependence. The H-band images display two
bright spots whose positions are confirmed by the geometrical toy model. The
inhomogeneities are qualitatively predicted by 3D simulations. At about 2,00
micron and in the region 2,35 - 2,50 micron, the photosphere appears extended
and the radius is larger than in the H band. In this spectral region, the
geometrical toy model locates a third bright spot outside the photosphere that
can be a feature of the molecular layers. The wavelength dependence of the
visibility can be qualitatively explained by 1D dynamical models of Mira
atmospheres. The best-fitting photospheric models show a good match with the
observed visibilities and give a photospheric diameter of theta = 8,82+-0,50
mas. The H2O molecule seems to be the dominant absorber in the molecular
layers. Conclusions. We show that the atmosphere of VX Sgr rather resembles
Mira/AGB star model atmospheres than RSG model atmospheres. In particular, we
see molecular (water) layers that are typical for Mira stars.Comment: 9 Pages, Accepted for publication on Astronomy & Astrophysics, two
references update
Characterization of integrated optics components for the second generation of VLTI instruments
Two of the three instruments proposed to ESO for the second generation
instrumentation of the VLTI would use integrated optics for beam combination.
Several design are studied, including co-axial and multi-axial recombination.
An extensive quantity of combiners are therefore under test in our
laboratories. We will present the various components, and the method used to
validate and compare the different combiners. Finally, we will discuss the
performances and their implication for both VSI and Gravity VLTI instruments.Comment: SPIE Astronomical Instrumentation 2008 in Marseille, France --
Equation (7) update
Magnetic Behavior of Co/Pt and TbCo Nanocaps Assembly for Bit Pattern Media
Large area patterning of self-assembled alumina nanobumps, with hexagonally close-packed order, has
been used to create ordered array of bit pattern magnetic media. We have studied the magnetic properties
of perpendicular magnetic TbCo alloy and Co/Pt multilayers deposited on self assembled alumina
nanobumps. Measurement of reversal field as a function of field intensity, as well as magnetic force
microscopy images confirm the weakness of exchange coupling between bits in the case of Co/Pt multilayer
while stronger coupling is observed in the case of TbCo alloys.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3535
Magnetic Behavior of Co/Pt and TbCo Nanocaps Assembly for Bit Pattern Media
Large area patterning of self-assembled alumina nanobumps, with hexagonally close-packed order, has
been used to create ordered array of bit pattern magnetic media. We have studied the magnetic properties
of perpendicular magnetic TbCo alloy and Co/Pt multilayers deposited on self assembled alumina
nanobumps. Measurement of reversal field as a function of field intensity, as well as magnetic force
microscopy images confirm the weakness of exchange coupling between bits in the case of Co/Pt multilayer
while stronger coupling is observed in the case of TbCo alloys.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3535
Planet Formation Imager (PFI): Introduction and Technical Considerations
Complex non-linear and dynamic processes lie at the heart of the planet
formation process. Through numerical simulation and basic observational
constraints, the basics of planet formation are now coming into focus. High
resolution imaging at a range of wavelengths will give us a glimpse into the
past of our own solar system and enable a robust theoretical framework for
predicting planetary system architectures around a range of stars surrounded by
disks with a diversity of initial conditions. Only long-baseline interferometry
can provide the needed angular resolution and wavelength coverage to reach
these goals and from here we launch our planning efforts. The aim of the
"Planet Formation Imager" (PFI) project is to develop the roadmap for the
construction of a new near-/mid-infrared interferometric facility that will be
optimized to unmask all the major stages of planet formation, from initial dust
coagulation, gap formation, evolution of transition disks, mass accretion onto
planetary embryos, and eventual disk dispersal. PFI will be able to detect the
emission of the cooling, newly-formed planets themselves over the first 100
Myrs, opening up both spectral investigations and also providing a vibrant look
into the early dynamical histories of planetary architectures. Here we
introduce the Planet Formation Imager (PFI) Project
(www.planetformationimager.org) and give initial thoughts on possible facility
architectures and technical advances that will be needed to meet the
challenging top-level science requirements.Comment: SPIE Astronomical Telescopes and Instrumentation conference, June
2014, Paper ID 9146-35, 10 pages, 2 Figure
On the structure of the transition disk around TW Hya
For over a decade, the structure of the inner cavity in the transition disk
of TW Hydrae has been a subject of debate. Modeling the disk with data obtained
at different wavelengths has led to a variety of proposed disk structures.
Rather than being inconsistent, the individual models might point to the
different faces of physical processes going on in disks, such as dust growth
and planet formation. Our aim is to investigate the structure of the transition
disk again and to find to what extent we can reconcile apparent model
differences. A large set of high-angular-resolution data was collected from
near-infrared to centimeter wavelengths. We investigated the existing disk
models and established a new self-consistent radiative-transfer model. A
genetic fitting algorithm was used to automatize the parameter fitting. Simple
disk models with a vertical inner rim and a radially homogeneous dust
composition from small to large grains cannot reproduce the combined data set.
Two modifications are applied to this simple disk model: (1) the inner rim is
smoothed by exponentially decreasing the surface density in the inner ~3 AU,
and (2) the largest grains (>100 um) are concentrated towards the inner disk
region. Both properties can be linked to fundamental processes that determine
the evolution of protoplanetary disks: the shaping by a possible companion and
the different regimes of dust-grain growth, respectively. The full
interferometric data set from near-infrared to centimeter wavelengths requires
a revision of existing models for the TW Hya disk. We present a new model that
incorporates the characteristic structures of previous models but deviates in
two key aspects: it does not have a sharp edge at 4 AU, and the surface density
of large grains differs from that of smaller grains. This is the first
successful radiative-transfer-based model for a full set of interferometric
data.Comment: 22 pages, 12 figures, accepted for publication in Astronomy &
Astrophysic
Critical behavior of a fluid in a disordered porous matrix: An Ornstein-Zernike approach
Using a liquid-state approach based on Ornstein-Zernike equations, we study
the behavior of a fluid inside a porous disordered matrix near the liquid-gas
critical point.The results obtained within various standard approximation
schemes such as lowest-order -ordering and the mean-spherical
approximation suggest that the critical behavior is closely related to that of
the random-field Ising model (RFIM).Comment: 10 pages, revtex, to appear in Physical Review Letter
The Subaru Coronagraphic Extreme Adaptive Optics system: enabling high-contrast imaging on solar-system scales
The Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument is a
multipurpose high-contrast imaging platform designed for the discovery and
detailed characterization of exoplanetary systems and serves as a testbed for
high-contrast imaging technologies for ELTs. It is a multi-band instrument
which makes use of light from 600 to 2500nm allowing for coronagraphic direct
exoplanet imaging of the inner 3 lambda/D from the stellar host. Wavefront
sensing and control are key to the operation of SCExAO. A partial correction of
low-order modes is provided by Subaru's facility adaptive optics system with
the final correction, including high-order modes, implemented downstream by a
combination of a visible pyramid wavefront sensor and a 2000-element deformable
mirror. The well corrected NIR (y-K bands) wavefronts can then be injected into
any of the available coronagraphs, including but not limited to the phase
induced amplitude apodization and the vector vortex coronagraphs, both of which
offer an inner working angle as low as 1 lambda/D. Non-common path, low-order
aberrations are sensed with a coronagraphic low-order wavefront sensor in the
infrared (IR). Low noise, high frame rate, NIR detectors allow for active
speckle nulling and coherent differential imaging, while the HAWAII 2RG
detector in the HiCIAO imager and/or the CHARIS integral field spectrograph
(from mid 2016) can take deeper exposures and/or perform angular, spectral and
polarimetric differential imaging. Science in the visible is provided by two
interferometric modules: VAMPIRES and FIRST, which enable sub-diffraction
limited imaging in the visible region with polarimetric and spectroscopic
capabilities respectively. We describe the instrument in detail and present
preliminary results both on-sky and in the laboratory.Comment: Accepted for publication, 20 pages, 10 figure
- …