4,646 research outputs found
Resolving asymmetries along the pulsation cycle of the Mira star X Hya
The mass-loss process in Mira stars probably occurs in an asymmetric way
where dust can form in inhomogeneous circumstellar molecular clumps. Following
asymmetries along the pulsation cycle can give us clues about these mass-loss
processes. We imaged the Mira star X Hya and its environnement at different
epochs to follow the evolution of the morphology in the continuum and in the
molecular bands. We observed X Hya with AMBER in J-H-K at low resolution at two
epochs. We modelled squared visibilities with geometrical and physical models.
We also present imaging reconstruction results obtained with MiRA and based on
the physical a priori images. We report on the angular scale change of X Hya
between the two epochs. 1D CODEX profiles allowed us to understand and model
the spectral variation of squared visibilities and constrain the stellar
parameters. Reconstructed model-dependent images enabled us to reproduce
closure phase signals and the azimuthal dependence of squared visibilities.
They show evidence for material inhomogeneities located in the immediate
environment of the star.Comment: Accepted for publication in A&A, 17 pages, 16 figure
Cross-Product Extensions of the Gene Ontology
The Gene Ontology is being normalized and extended to include computable logical definitions. These definitions are partitioned into mutually exclusive cross-product sets, many of which reference other OBO Foundry ontologies. The results can be used to reason over the ontology, and to make cross-ontology queries
A close halo of large transparent grains around extreme red giant stars
Intermediate-mass stars end their lives by ejecting the bulk of their
envelope via a slow dense wind back into the interstellar medium, to form the
next generation of stars and planets. Stellar pulsations are thought to elevate
gas to an altitude cool enough for the condensation of dust, which is then
accelerated by radiation pressure from starlight, entraining the gas and
driving the wind. However accounting for the mass loss has been a problem due
to the difficulty in observing tenuous gas and dust tens of milliarcseconds
from the star, and there is accordingly no consensus on the way sufficient
momentum is transferred from the starlight to the outflow. Here, we present
spatially-resolved, multi-wavelength observations of circumstellar dust shells
of three stars on the asymptotic giant branch of the HR diagram. When imaged in
scattered light, dust shells were found at remarkably small radii (<~ 2 stellar
radii) and with unexpectedly large grains (~300 nm radius). This proximity to
the photosphere argues for dust species that are transparent to starlight and
therefore resistant to sublimation by the intense radiation field. While
transparency usually implies insufficient radiative pressure to drive a wind,
the radiation field can accelerate these large grains via photon scattering
rather than absorption - a plausible mass-loss mechanism for lower-amplitude
pulsating stars.Comment: 13 pages, 1 table, 6 figure
Inhomogeneities in molecular layers of Mira atmospheres
We obtained K-band spectro-interferometric observations of the Miras R Cnc, X
Hya, W Vel, and RW Vel with a spectral resolution of 1500 using the VLTI/AMBER
instrument. We obtained concurrent JHKL photometry using the the Mk II
instrument at the SAAO. Our sources have wavelength-dependent visibility values
that are consistent with earlier low-resolution AMBER observations of S Ori and
with the predictions of dynamic model atmosphere series based on self-excited
pulsation models. The wavelength-dependent UD diameters show a minimum near the
near-continuum bandpass at 2.25 um. They increase by up to 30% toward the H2O
band at 2.0 um and by up to 70% at the CO bandheads. The dynamic model
atmosphere series show a consistent wavelength-dependence, and their parameters
such as the visual phase, effective temperature, and distances are consistent
with independent estimates. The closure phases have significantly
wavelength-dependent non-zero values indicating deviations from point symmetry.
For example, the R Cnc closure phase is 110 degr in the 2.0 um H2O band,
corresponding for instance to an additional unresolved spot contributing 3% of
the total flux at a separation of ~4 mas. Our observations are consistent with
the predictions of the latest dynamic model atmosphere series based on
self-excited pulsation models. The wavelength-dependent radius variations are
interpreted as the effect of molecular layers. The wavelength-dependent closure
phase values are indicative of deviations from point symmetry at all
wavelengths, thus a complex non-spherical stratification of the extended
atmosphere. In particular, the significant deviation from point symmetry in the
H2O band is interpreted as a signature on large scales of inhomogeneities or
clumps in the water vapor layer. The observed inhomogeneities might be caused
by pulsation- and shock-induced chaotic motion in the extended atmosphere.Comment: 5 pages, 2 figures, accepted for publication as a Letter in Astronomy
and Astrophysic
The radius and mass of the subgiant star bet Hyi from interferometry and asteroseismology
We have used the Sydney University Stellar Interferometer (SUSI) to measure
the angular diameter of beta Hydri. This star is a nearby G2 subgiant whose
mean density was recently measured with high precision using asteroseismology.
We determine the radius and effective temperature of the star to be
1.814+/-0.017 R_sun (0.9%) and 5872+/-44 K (0.7%) respectively. By combining
this value with the mean density, as estimated from asteroseismology, we make a
direct estimate of the stellar mass. We find a value of 1.07+/-0.03 M_sun
(2.8%), which agrees with published estimates based on fitting in the H-R
diagram, but has much higher precision. These results place valuable
constraints on theoretical models of beta Hyi and its oscillation frequencies.Comment: 3 figures, 3 tables, to appear in MNRAS Letter
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Blue and Red Cathodoluminescent Emission of Y2O3:Eu Phosphor Studied as a Function of Temperature in a Transmission Electron Microscope
Combined EELS, CL and STEM imaging are used to characterize and study Y2O3:Eu to confirm the variation in CL properties and photo-luminescent properties observed. The synthetic procedures used have facilitated this material to manifest a strong blue emission at low temperature with the expected red emission dominating at room temperature
Quantum Theory and Galois Fields
We discuss the motivation and main results of a quantum theory over a Galois
field (GFQT). The goal of the paper is to describe main ideas of GFQT in a
simplest possible way and to give clear and simple arguments that GFQT is a
more natural quantum theory than the standard one. The paper has been prepared
as a presentation to the ICSSUR' 2005 conference (Besancon, France, May 2-6,
2005).Comment: Latex, 24 pages, 1 figur
Investigating the emission characteristics of single crystal YAG when activated by high power laser beams
© The Author(s) 2016. Limitations associated with light emitting diodes (LEDs) operating under high current densities due to the efficiency droop has created a need to look for alternative light sources; here we report investigations on the potential of laser diodes (LDs) for high brightness lighting solutions. High power laser diodes require phosphor targets with certain performance criteria such as high thermal conductivity, efficiency and structural geometry. Here we examine the possibility of using single crystal YAG:Ce phosphor materials as potential targets for generation of light via laser diodes. We report on the emission properties of the crystals with different sizes and examine the effect of laser beam incident angle incident on crystal target emission
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