1,579 research outputs found
Influence of antisymmetric exchange interaction on quantum tunneling of magnetization in a dimeric molecular magnet Mn6
We present magnetization measurements on the single molecule magnet Mn6,
revealing various tunnel transitions inconsistent with a giant-spin
description. We propose a dimeric model of the molecule with two coupled spins
S=6, which involves crystal-field anisotropy, symmetric Heisenberg exchange
interaction, and antisymmetric Dzyaloshinskii-Moriya exchange interaction. We
show that this simplified model of the molecule explains the experimentally
observed tunnel transitions and that the antisymmetric exchange interaction
between the spins gives rise to tunneling processes between spin states
belonging to different spin multiplets.Comment: 5 pages, 4 figure
Solar-like oscillations in red giants observed with Kepler: comparison of global oscillation parameters from different methods
The large number of stars for which uninterrupted high-precision photometric
timeseries data are being collected with \textit{Kepler} and CoRoT initiated
the development of automated methods to analyse the stochastically excited
oscillations in main-sequence, subgiant and red-giant stars. Aims: We
investigate the differences in results for global oscillation parameters of G
and K red-giant stars due to different methods and definitions. We also
investigate uncertainties originating from the stochastic nature of the
oscillations. Methods: For this investigation we use Kepler data obtained
during the first four months of operation. These data have been analysed by
different groups using already published methods and the results are compared.
We also performed simulations to investigate the uncertainty on the resulting
parameters due to different realizations of the stochastic signal. Results: We
obtain results for the frequency of maximum oscillation power (nu_max) and the
mean large separation () from different methods for over one thousand
red-giant stars. The results for these parameters agree within a few percent
and seem therefore robust to the different analysis methods and definitions
used here. The uncertainties for nu_max and due to differences in
realization noise are not negligible and should be taken into account when
using these results for stellar modelling.Comment: 11 pages, 9 Figures and 7 tables, accepted for publication in
Astronomy and Astrophysic
Modelling a high-mass red giant observed by CoRoT
The G6 giant HR\,2582 (HD\,50890) was observed by CoRoT for approximately 55
days. Mode frequencies are extracted from the observed Fourier spectrum of the
light curve. Numerical stellar models are then computed to determine the
characteristics of the star (mass, age, etc...) from the comparison with
observational constraints. We provide evidence for the presence of solar-like
oscillations at low frequency, between 10 and 20\,Hz, with a regular
spacing of Hz between consecutive radial orders. Only radial
modes are clearly visible. From the models compatible with the observational
constraints used here, We find that HR\,2582 (HD\,50890) is a massive star with
a mass in the range (3--\,5\,), clearly above the red clump. It
oscillates with rather low radial order ( = 5\,--\,12) modes. Its
evolutionary stage cannot be determined with precision: the star could be on
the ascending red giant branch (hydrogen shell burning) with an age of
approximately 155 Myr or in a later phase (helium burning). In order to obtain
a reasonable helium amount, the metallicity of the star must be quite subsolar.
Our best models are obtained with a mixing length significantly smaller than
that obtained for the Sun with the same physical description (except
overshoot). The amount of core overshoot during the main-sequence phase is
found to be mild, of the order of 0.1\,.Comment: Accepted in A&
Galactic Archaeology with CoRoT and APOGEE: Creating mock observations from a chemodynamical model
In a companion paper, we have presented the combined
asteroseismic-spectroscopic dataset obtained from CoRoT lightcurves and APOGEE
infra-red spectra for 678 solar-like oscillating red giants in two fields of
the Galactic disc (CoRoGEE). We have measured chemical abundance patterns,
distances, and ages of these field stars which are spread over a large radial
range of the Milky Way's disc. Here we show how to simulate this dataset using
a chemodynamical Galaxy model. We also demonstrate how the observation
procedure influences the accuracy of our estimated ages.Comment: 5 pages, 6 figures. To appear in Astronomische Nachrichten, special
issue "Reconstruction the Milky Way's History: Spectroscopic surveys,
Asteroseismology and Chemo-dynamical models", Guest Editors C. Chiappini, J.
Montalb\'an, and M. Steffe
E´ chelle diagrams and period spacings of g modes in: Doradus stars from four years of Kepler observations
We use photometry from the Kepler Mission to study oscillations in Doradus stars. Some stars show remarkably clear sequences of g modes and we use period ´echelle diagrams to measure period spacings and identifyrotationally split multiplets with ` = 1 and ` = 2.We find small deviations from regular period spacings that arise from the gradient in the chemical composition just outside the convective core. We also find stars for which the period spacing shows a strong linear trend as a function of period, consistent with relatively rapid rotation. Overall, th
Preparation of Kepler lightcurves for asteroseismic analyses
The Kepler mission is providing photometric data of exquisite quality for the
asteroseismic study of different classes of pulsating stars. These analyses
place particular demands on the pre-processing of the data, over a range of
timescales from minutes to months. Here, we describe processing procedures
developed by the Kepler Asteroseismic Science Consortium (KASC) to prepare
light curves that are optimized for the asteroseismic study of solar-like
oscillating stars in which outliers, jumps and drifts are corrected.Comment: Accepted for publication in MNRAS. 5 pages, 2 figure
Atmospheric parameters and chemical properties of red giants in the CoRoT asteroseismology fields
A precise characterisation of the red giants in the seismology fields of the
CoRoT satellite is a prerequisite for further in-depth seismic modelling.
High-resolution FEROS and HARPS spectra were obtained as part of the
ground-based follow-up campaigns for 19 targets holding great asteroseismic
potential. These data are used to accurately estimate their fundamental
parameters and the abundances of 16 chemical species in a self-consistent
manner. Some powerful probes of mixing are investigated (the Li and CNO
abundances, as well as the carbon isotopic ratio in a few cases). The
information provided by the spectroscopic and seismic data is combined to
provide more accurate physical parameters and abundances. The stars in our
sample follow the general abundance trends as a function of the metallicity
observed in stars of the Galactic disk. After an allowance is made for the
chemical evolution of the interstellar medium, the observational signature of
internal mixing phenomena is revealed through the detection at the stellar
surface of the products of the CN cycle. A contamination by NeNa-cycled
material in the most massive stars is also discussed. With the asteroseismic
constraints, these data will pave the way for a detailed theoretical
investigation of the physical processes responsible for the transport of
chemical elements in evolved, low- and intermediate-mass stars.Comment: Accepted for publication in A&A, 25 pages, 13 colour figures (revised
version after language editing
Reconstruction of three-dimensional porous media using generative adversarial neural networks
To evaluate the variability of multi-phase flow properties of porous media at
the pore scale, it is necessary to acquire a number of representative samples
of the void-solid structure. While modern x-ray computer tomography has made it
possible to extract three-dimensional images of the pore space, assessment of
the variability in the inherent material properties is often experimentally not
feasible. We present a novel method to reconstruct the solid-void structure of
porous media by applying a generative neural network that allows an implicit
description of the probability distribution represented by three-dimensional
image datasets. We show, by using an adversarial learning approach for neural
networks, that this method of unsupervised learning is able to generate
representative samples of porous media that honor their statistics. We
successfully compare measures of pore morphology, such as the Euler
characteristic, two-point statistics and directional single-phase permeability
of synthetic realizations with the calculated properties of a bead pack, Berea
sandstone, and Ketton limestone. Results show that GANs can be used to
reconstruct high-resolution three-dimensional images of porous media at
different scales that are representative of the morphology of the images used
to train the neural network. The fully convolutional nature of the trained
neural network allows the generation of large samples while maintaining
computational efficiency. Compared to classical stochastic methods of image
reconstruction, the implicit representation of the learned data distribution
can be stored and reused to generate multiple realizations of the pore
structure very rapidly.Comment: 21 pages, 20 figure
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