555 research outputs found
Direct probing of band-structure Berry phase in diluted magnetic semiconductors
We report on experimental evidence of the Berry phase accumulated by the
charge carrier wave function in single-domain nanowires made from a
(Ga,Mn)(As,P) diluted ferromagnetic semiconductor layer. Its signature on the
mesoscopic transport measurements is revealed as unusual patterns in the
magnetoconductance, that are clearly distinguished from the universal
conductance fluctuations. We show that these patterns appear in a magnetic
field region where the magnetization rotates coherently and are related to a
change in the band-structure Berry phase as the magnetization direction
changes. They should be thus considered as a band structure Berry phase
fingerprint of the effective magnetic monopoles in the momentum space. We argue
that this is an efficient method to vary the band structure in a controlled way
and to probe it directly. Hence, (Ga,Mn)As appears to be a very interesting
test bench for new concepts based on this geometrical phase.Comment: 7 pages, 6 figure
Populations of rotating stars II. Rapid rotators and their link to Be-type stars
Even though it is broadly accepted that single Be stars are rapidly rotating
stars surrounded by a flat rotating circumstellar disk, there is still a debate
about how fast these stars rotate and also about the mechanisms involved in the
angular-momentum and mass input in the disk. We study the properties of stars
that rotate near their critical-rotation rate and investigate the properties of
the disks formed by equatorial mass ejections. We used the most recent Geneva
stellar evolutionary tracks for rapidly rotating stars that reach the critical
limit and used a simple model for the disk structure. We obtain that for a 9
Msun star at solar metallicity, the minimum average velocity during the Main
Sequence phase to reach the critical velocity is around 330 km/s, whereas it
would be 390 km/s at the metallicity of the Small Magellanic Cloud (SMC). Red
giants or supergiants originating from very rapid rotators rotate six times
faster and show N/C ratios three times higher than those originating from
slowly rotating stars. This difference becomes stronger at lower metallicity.
It might therefore be very interesting to study the red giants in clusters that
show a large number of Be stars on the MS band. On the basis of our single-star
models, we show that the observed Be-star fraction with cluster age is
compatible with the existence of a temperature-dependent lower limit in the
velocity rate required for a star to become a Be star. The mass, extension, and
diffusion time of the disks produced when the star is losing mass at the
critical velocity, obtained from simple parametrized expressions, are not too
far from those estimated for disks around Be-type stars. At a given
metallicity, the mass and the extension of the disk increase with the initial
mass and with age on the MS phase. Denser disks are expected in low-metallicity
regions.Comment: Accepted for publication in A&A, language edite
LSTM-Based Goal Recognition in Latent Space
Approaches to goal recognition have progressively relaxed the requirements
about the amount of domain knowledge and available observations, yielding
accurate and efficient algorithms capable of recognizing goals. However, to
recognize goals in raw data, recent approaches require either human engineered
domain knowledge, or samples of behavior that account for almost all actions
being observed to infer possible goals. This is clearly too strong a
requirement for real-world applications of goal recognition, and we develop an
approach that leverages advances in recurrent neural networks to perform goal
recognition as a classification task, using encoded plan traces for training.
We empirically evaluate our approach against the state-of-the-art in goal
recognition with image-based domains, and discuss under which conditions our
approach is superior to previous ones.Comment: Added/Fixed some reference
Localized magnetoplasmon modes arising from broken translational symmetry in semiconductor superlattices
The electromagnetic propagator associated with the localized collective
magnetoplasmon excitations in a semiconductor superlattice with broken
translational symmetry, is calculated analytically within linear response
theory. We discuss the properties of these collective excitations in both
radiative and non-radiative regimes of the electromagnetic spectra. We find
that low frequency retarded modes arise when the surface density of carriers at
the symmetry breaking layer is lower than the density at the remaining layers.
Otherwise a doublet of localized, high-frequency magnetoplasmon-like modes
occurs.Comment: Revtex file + separate pdf figure
Angular Momentum Loss Rates in Be Stars Determined by the Viscous Decretion Disc Model
Circumstellar discs around Be stars are formed by the material ejected by the
central star. This process removes excess angular momentum from the star as
viscosity facilitates the mass and angular momentum transfer within the disc
and its growth. The angular momentum loss rates (AMLR) of Be stars is a subject
of debate in the literature. Through the modelling of the disc formation and
dissipation phases observed from Be stars, their average AMLR can be determined
and this is the goal of this work. We use the viscous decretion disc (VDD)
model to provide a range of the average AMLR for Be stars and compare these
rates with predicted values from the literature. We explore the reasons for
discrepancies between the predicted values of average AMLR using the VDD and
Geneva stellar evolution (GSE) models that were previously reported in
literature and find that the largest differences occur when Be stars are
rotating below their critical speeds. We show that the time over which the mass
reservoir builds up is inversely proportional to the average AMLR. Also, we
determine a revised value of the average AMLR for the Galactic Be star omega
CMa of 4.7x10^36 g cm^2/s^2, which is in better agreement with the values
expected for a typical B2 type star. Finally, the effect of disc truncation due
to the presence of a companion star is investigated and we find that this has a
minimal effect on the average AMLR
The impact of mass-loss on the evolution and pre-supernova properties of red supergiants
The post main-sequence evolution of massive stars is very sensitive to many
parameters of the stellar models. Key parameters are the mixing processes, the
metallicity, the mass-loss rate and the effect of a close companion. We study
how the red supergiant lifetimes, the tracks in the Hertzsprung-Russel diagram
(HRD), the positions in this diagram of the pre-supernova progenitor as well as
the structure of the stars at that time change for various mass-loss rates
during the red supergiant phase (RSG), and for two different initial rotation
velocities. The surface abundances of RSGs are much more sensitive to rotation
than to the mass-loss rates during that phase. A change of the RSG mass-loss
rate has a strong impact on the RSG lifetimes and therefore on the luminosity
function of RSGs. At solar metallicity, the enhanced mass-loss rate models do
produce significant changes on the populations of blue, yellow and red
supergiants. When extended blue loops or blue ward excursions are produced by
enhanced mass-loss, the models predict that a majority of blue (yellow)
supergiants are post RSG objects. These post RSG stars are predicted to show
much smaller surface rotational velocities than similar blue supergiants on
their first crossing of the HR gap. The position in the HRD of the end point of
the evolution depends on the mass of the hydrogen envelope. More precisely,
whenever, at the pre-supernova stage, the H-rich envelope contains more than
about 5\% of the initial mass, the star is a red supergiant, and whenever the
H-rich envelope contains less than 1\% of the total mass the star is a blue
supergiant. For intermediate situations, intermediate colors/effective
temperatures are obtained. Yellow progenitors for core collapse supernovae can
be explained by the enhanced mass-loss rate models, while the red progenitors
are better fitted by the standard mass-loss rate models.Comment: 19 pages, 11 figures, 6 tables, accepted for publication in Astronomy
and Astrophysic
Quiescent and active phases in Be stars : A WISE snapshot of Young Galactic Open Clusters
Through the modeling of near-infrared photometry of star-plus disk systems with the codes bedisk/beray, we successfully describe the Wide-Field Infrared Survey Explorer (WISE) photometric characteristics of Be stars in five young open clusters, NGC 663, NGC 869, NGC 884, NGC 3766, and NGC 4755, broadly studied in the literature. WISE photometry allows previously known Be stars to be detected and to find new Be candidates which could be confirmed spectroscopically. The location of Be stars in the WISE color-magnitude diagram, separates them in two groups; active (Be stars hosting a developed circumstellar disk) and quiescent objects (Be stars in a diskless phase), and this way, we can explore how often stars are observed in these different stages. The variability observed in most active variable Be stars is compatible with a disk dissipation phase. We find that 50% of Be stars in the studied open clusters are in an active phase. We can interpret this as Be stars having a developed circumstellar disk one-half of the time. The location of Be stars with a developed disk in the color-magnitude diagram require mass loss rates in agreement with values recently reported in the literature. For these objects, we expect to have a tight relation between the equivalent width of the Hα line and the mass of the disk, if the inclination is known. Also, near-infrared photometry of Be stars in stellar clusters has the potential of being useful to test whether there is a preferential viewing angle.Instituto de Astrofísica de La Plat
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