324 research outputs found
On the seismic modelling of rotating B-type pulsators in the traditional approximation
The CoRoT and Kepler data revolutionised our view on stellar pulsation. For
massive stars, the space data revealed the simultaneous presence of
low-amplitude low-order modes and dominant high-order gravity modes in several
B-type pulsators. The interpretation of such a rich set of detected
oscillations requires new tools. We present computations of oscillations for
B-type pulsators taking into account the effects of the Coriolis force in the
so-called traditional approximation. We discuss the limitations of classical
frequency matching to tune these stars seismically and show that the predictive
power is limited in the case of high-order gravity mode pulsators, except if
numerous modes of consecutive radial order can be identified.Comment: 8 pages, 4 figures. Paper submitted for publication in the
Proceedings of the 61st Fujihara Seminar: Progress in solar/stellar physics
with helio- and asteroseismology to appear in ASP Conference Serie
Asteroseismic inversions in the Kepler era: application to the Kepler Legacy sample
In the past few years, the CoRoT and Kepler missions have carried out what is
now called the space photometry revolution. This revolution is still ongoing
thanks to K2 and will be continued by the Tess and Plato2.0 missions. However,
the photometry revolution must also be followed by progress in stellar
modelling, in order to lead to more precise and accurate determinations of
fundamental stellar parameters such as masses, radii and ages. In this context,
the long-lasting problems related to mixing processes in stellar interior is
the main obstacle to further improvements of stellar modelling. In this
contribution, we will apply structural asteroseismic inversion techniques to
targets from the Kepler Legacy sample and analyse how these can help us
constrain the fundamental parameters and mixing processes in these stars. Our
approach is based on previous studies using the SOLA inversion technique to
determine integrated quantities such as the mean density, the acoustic radius,
and core conditions indicators, and has already been successfully applied to
the 16Cyg binary system. We will show how this technique can be applied to the
Kepler Legacy sample and how new indicators can help us to further constrain
the chemical composition profiles of stars as well as provide stringent
constraints on stellar ages.Comment: To appear in the proceedings of the Kasc 9 Tasc 2 worksho
Revised instability domains of SPB and beta Cephei stars
The excitation of pulsation modes in beta Cephei and Slowly Pulsating B stars
is known to be very sensitive to opacity changes in the stellar interior where
T~2 10^5 K. In this region differences in opacity up to ~50% can be induced by
the choice between OPAL and OP opacity tables, and between two different metal
mixtures (Grevesse and Noels 1993 and Asplund et al. 2005). We have extended
the non-adiabatic computations presented in Miglio et al. (2007) towards models
of higher mass and pulsation modes of degree l=3, and we present here the
instability domains in the HR- and log(P)-log(Teff) diagrams resulting from
different choices of opacity tables, and for three different metallicities.Comment: 9 pages, 4 figures. Accepted for publication in Communications in
Asteroseismolog
Nonradial nonadiabatic stellar pulsations: A numerical method and its application to a beta Cephei model
A new general method for the computation of nonradial nonadiabatic oscillations of a given stellar model is presented for a linear approximation. A simple and useful modelling of the atmosphere is included, allowing to obtain credible values for the eigenfunctions in the atmosphere. Some of the results obtained for a 10 M[SUB]sun[/SUB] model are shown as an illustration. Our study opens the way to different applications. Better theoretical line-profile variations could be obtained from our method, allowing a more detailed comparison with observations. More generally, our study is relevant for asteroseismology, giving a way for a better knowledge of stellar interiors.Peer reviewe
Mode identification in rapidly rotating stars from BRITE data
Apart from recent progress in Gamma Dor stars, identifying modes in rapidly
rotating stars is a formidable challenge due to the lack of simple, easily
identifiable frequency patterns. As a result, it is necessary to look to
observational methods for identifying modes. Two popular techniques are
spectroscopic mode identification based on line profile variations (LPVs) and
photometric mode identification based on amplitude ratios and phase differences
between multiple photometric bands. In this respect, the BRITE constellation is
particularly interesting as it provides space-based multi-colour photometry.
The present contribution describes the latest developments in obtaining
theoretical predictions for amplitude ratios and phase differences for
pulsation modes in rapidly rotating stars. These developments are based on full
2D non-adiabatic pulsation calculations, using models from the ESTER code, the
only code to treat in a self-consistent way the thermal equilibrium of rapidly
rotating stars. These predictions are then specifically applied to the BRITE
photometric bands to explore the prospects of identifying modes based on BRITE
observations.Comment: 8 pages, 3 figures, proceedings of the 3rd BRITE Science Worksho
Non-adiabatic pulsations in ESTER models
One of the greatest challenges in interpreting the pulsations of rapidly
rotating stars is mode identification, i.e. correctly matching theoretical
modes to observed pulsation frequencies. Indeed, the latest observations as
well as current theoretical results show the complexity of pulsation spectra in
such stars, and the lack of easily recognisable patterns. In the present
contribution, the latest results on non-adiabatic effects in such pulsations
are described, and we show how these come into play when identifying modes.
These calculations fully take into account the effects of rapid rotation,
including centrifugal distortion, and are based on models from the ESTER
project, currently the only rapidly rotating models in which the energy
conservation equation is satisfied, a prerequisite for calculating
non-adiabatic effects. Non-adiabatic effects determine which modes are excited
and play a key role in the near-surface pulsation-induced temperature
variations which intervene in multi-colour amplitude ratios and phase
differences, as well as line profile variations.Comment: Proceedings for the Joint TASC2 & KASC9 Workshop, Terceira, Azores,
201
Impact of helium diffusion and helium-flash-induced carbon production on gravity-mode pulsations in subdwarf B stars
Realistic stellar models are essential to the forward modelling approach in
asteroseismology. For practicality however, certain model assumptions are also
required. For example, in the case of subdwarf B stars, one usually starts with
zero-age horizontal branch structures without following the progenitor
evolution. We analyse the effects of common assumptions in subdwarf B models on
the g-mode pulsational properties. We investigate if and how the pulsation
periods are affected by the H-profile in the core-envelope transition zone.
Furthermore, the effects of C-production and convective mixing during the core
helium flash are evaluated. Finally, we reanalyse the effects of stellar
opacities on the mode excitation in subdwarf B stars. We find that helium
settling causes a shift in the theoretical blue edge of the g-mode instability
domain to higher effective temperatures. This results in a closer match to the
observed instability strip of long-period sdB pulsators, particularly for l<=3
modes. We show further that the g-mode spectrum is extremely sensitive to the
H-profile in the core-envelope transition zone. If atomic diffusion is
efficient, details of the initial shape of the profile become less important in
the course of evolution. Diffusion broadens the chemical gradients, and results
in less effective mode trapping and different pulsation periods. Furthermore,
we report on the possible consequences of the He-flash for the g-modes. The
outer edge of a flash-induced convective region introduces an additional
chemical transition in the stellar models, and the corresponding spike in the
Brunt-Vaisala frequency produces a complicated mode trapping signature in the
period spacings.Comment: 9 pages, 6 figures, 1 table, accepted for publication in A&
Ledoux's convection criterion in evolution and asteroseismology of massive stars
Saio et al. (2006) have shown that the presence of an intermediate convective
zone (ICZ) in post-main sequence models could prevent the propagation of
g-modes in the radiative interior and hence avoid the corresponding radiative
damping. The development of such a convective region highly depends on the
structure of the star in the mu-gradient region surrounding the convective core
during the main sequence phase. In particular,the development of this ICZ
depends on physical processes such as mass loss, overshooting (Chiosi & Maeder
1986, Chiosi et al. 1992, see also Godart et al., these proceedings) and
convective instability criterion (Schwarzschild's or Ledoux's criteria). In
this paper we study the consequences of adopting the Ledoux's criterion on the
evolution of the convective regions in massive stars (15 and 20 Msun), and on
the pulsation spectrum of these new B-type variables (also called SPBsg).Comment: Contribution to the Proceedings of the 38th LIAC/HELAS-ESTA/BAG, 2008
Accepted for publication in CoAs
SpikeGrad: An ANN-equivalent Computation Model for Implementing Backpropagation with Spikes
Event-based neuromorphic systems promise to reduce the energy consumption of
deep learning tasks by replacing expensive floating point operations on dense
matrices by low power sparse and asynchronous operations on spike events. While
these systems can be trained increasingly well using approximations of the
back-propagation algorithm, these implementations usually require high
precision errors for training and are therefore incompatible with the typical
communication infrastructure of neuromorphic circuits. In this work, we analyze
how the gradient can be discretized into spike events when training a spiking
neural network. To accelerate our simulation, we show that using a special
implementation of the integrate-and-fire neuron allows us to describe the
accumulated activations and errors of the spiking neural network in terms of an
equivalent artificial neural network, allowing us to largely speed up training
compared to an explicit simulation of all spike events. This way we are able to
demonstrate that even for deep networks, the gradients can be discretized
sufficiently well with spikes if the gradient is properly rescaled. This form
of spike-based backpropagation enables us to achieve equivalent or better
accuracies on the MNIST and CIFAR10 dataset than comparable state-of-the-art
spiking neural networks trained with full precision gradients. The algorithm,
which we call SpikeGrad, is based on accumulation and comparison operations and
can naturally exploit sparsity in the gradient computation, which makes it an
interesting choice for a spiking neuromorphic systems with on-chip learning
capacities
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