676 research outputs found
Interplay between pulsations and mass loss in the blue supergiant 55 Cygnus = HD 198478
Blue supergiant stars are known to display photometric and spectroscopic
variability that is suggested to be linked to stellar pulsations. Pulsational
activity in massive stars strongly depends on the star's evolutionary stage and
is assumed to be connected with mass-loss episodes, the appearance of
macroturbulent line broadening, and the formation of clumps in the wind. To
investigate a possible interplay between pulsations and mass-loss, we carried
out an observational campaign of the supergiant 55 Cyg over a period of five
years to search for photospheric activity and cyclic mass-loss variability in
the stellar wind. We modeled the H, He I, Si II and Si III lines using the
nonlocal thermal equilibrium atmosphere code FASTWIND and derived the
photospheric and wind parameters. In addition, we searched for variability in
the intensity and radial velocity of photospheric lines and performed a moment
analysis of the line profiles to derive frequencies and amplitudes of the
variations. The Halpha line varies with time in both intensity and shape,
displaying various types of profiles: P Cygni, pure emission, almost complete
absence, and double or multiple peaked. The star undergoes episodes of variable
mass-loss rates that change by a factor of 1.7-2 on different timescales. We
also observe changes in the ionization rate of Si II and determine a
multiperiodic oscillation in the He I absorption lines, with periods ranging
from a few hours to 22.5 days. We interpret the photospheric line variations in
terms of oscillations in p-, g-, and strange modes. We suggest that these
pulsations can lead to phases of enhanced mass loss. Furthermore, they can
mislead the determination of the stellar rotation. We classify the star as a
post-red supergiant, belonging to the group of alpha Cyg variables.Comment: 20 pages, 18 figures, 3 tables, accepted to Astronomy & Astrophysic
A Recognition-Based Alternative to Discrimination-Based Multi-layer Perceptrons
Though impressive classification accuracy is often obtained via discrimination-based learning techniques such as Multi-Layer Perceptrons (DMLP), these techniques often assume that the underlying training sets are optimally balanced (in terms of the number of positive and negative examples). Unfortunately, this is not always the case. In this paper, we look at a recognitionbased approach whose accuracy in such environments is superior to that obtained via more conventional mechanisms. At the heart of the new technique is a modified auto-encoder that allows for the incorporation of a recognition component into the conventional MLP mechanism. In short, rather than being associated with an output value of "1", positive examples are fully reconstructed at the network output layer while negative examples, rather than being associated with an output value of "0", have their inverse derived at the output layer. The result is an auto-encoder able to recognize positive examples while discriminating against negative ones by virtue of the fact that negative cases generate larger reconstruction errors. A simple technique is employed to exaggerate the impact of training with these negative examples so that reconstruction errors can be more reliably established. Preliminary testing on both seismic and sonar data sets has demonstrated that the new method produces lower error rates than standard connectionist systems in imbalanced settings. Our approach thus suggests a simple and more robust alternative to commonly used classification mechanisms
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Mid-infrared supercontinuum generation using dispersion-engineered Ge11.5As24Se64.5 chalcogenide channel waveguide
We numerically investigate mid-infrared supercontinuum (SC) generation in dispersion-engineered, air-clad, Ge11.5As24Se64.5 chalcogenide-glass channel waveguides employing two different materials, Ge11.5As24S64.5 or MgF2 glass for their lower cladding. We study the effect of waveguide parameters on the bandwidth of the SC at the output of 1-cm-long waveguide. Our results show that output can vary over a wide range depending on its design and the pump wavelength employed. At the pump wavelength of 2 µm the SC never extended beyond 4.5 µm for any of our designs. However, supercontinuum could be extended to beyond 5 µm for a pump wavelength of 3.1 µm. A broadband SC spanning from 2 µm to 6 µm and extending over 1.5 octave could be generated with a moderate peak power of 500 W at a pump wavelength of 3.1 µm using an air-clad, all-chalcogenide, channel waveguide. We show that SC can be extended even further when MgF2 glass is used for the lower cladding of chalcogenide waveguide. Our numerical simulations produced SC spectra covering the wavelength range 1.8-7.7 µm (> two octaves) by using this geometry. Both ranges exceed the broadest SC bandwidths reported so far. Moreover, we realize it using 3.1 µm pump source and relatively low peak power pulses. By employing the same pump source, we show that SC spectra can cover a wavelength range of 1.8-11 µm (> 2.5 octaves) in a channel waveguide employing MgF2 glass for its lower cladding with a moderate peak power of 3000 W
Hot subdwarf wind models with accurate abundances I. Hydrogen dominated stars HD 49798 and BD+182647
Hot subdwarfs are helium burning objects in late stages of their evolution.
These stars can develop winds driven by light absorption in the lines of
heavier elements. The wind strength depends on chemical composition which can
significantly vary from star to star. We aim to understand the influence of
metallicity on the strength of the winds of the hot hydrogen-rich subdwarfs HD
49798 and BD+182647. We used UV and optical spectra to derive stellar
parameters and abundances. For derived stellar parameters, we predicted wind
structure (including mass-loss rates and terminal velocities) with our METUJE
code. We derived effective temperature K and mass
for HD 49798 and K and
for BD+182647. The abundances can be interpreted as
a result of interplay between stellar evolution and diffusion. HD 49798 has a
strong wind that does not allow for chemical separation and consequently it
shows solar chemical composition modified by hydrogen burning. On the other
hand, we did not find any wind in BD+182647 and its abundances are
therefore most likely affected by radiative diffusion. Accurate abundances do
not lead to a significant modification of wind mass-loss rate for HD 49798,
because the increase of the contribution of Fe and Ni to the radiative force is
compensated by the decrease of the force due to other elements. The resulting
wind mass-loss rate predicts
an X-ray light curve during the eclipse which closely agrees with observations.
On the other hand, the absence of the wind in BD+182647 for accurate
abundances is a result of its peculiar chemical composition. Wind models with
accurate abundances provide more reliable wind parameters, but the influence of
abundances on the wind parameters is limited in many cases.Comment: 11 pages, accepted for publication in Astronomy & Astrophysic
Learning object relationships which determine the outcome of actions
Peer reviewedPublisher PD
Structure and Evolution of Nearby Stars with Planets. I. Short-Period Systems
Using the Yale stellar evolution code, we have calculated theoretical models
for nearby stars with planetary-mass companions in short-period nearly circular
orbits: 51 Pegasi, Tau Bootis, Upsilon Andromedae, Rho Cancri, and Rho Coronae
Borealis. We present tables listing key stellar parameters such as mass,
radius, age, and size of the convective envelope as a function of the
observable parameters (luminosity, effective temperature, and metallicity), as
well as the unknown helium fraction. For each star we construct best models
based on recently published spectroscopic data and the present understanding of
galactic chemical evolution. We discuss our results in the context of planet
formation theory, and, in particular, tidal dissipation effects and stellar
metallicity enhancements.Comment: 48 pages including 13 tables and 5 figures, to appear in Ap
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