518 research outputs found
Carbon-enhanced metal-poor stars: a window on AGB nucleosynthesis and binary evolution. II. Statistical analysis of a sample of 67 CEMP- stars
Many observed CEMP stars are found in binary systems and show enhanced
abundances of -elements. The origin of the chemical abundances of these
CEMP- stars is believed to be accretion in the past of enriched material
from a primary star in the AGB phase. We investigate the mechanism of mass
transfer and the process of nucleosynthesis in low-metallicity AGB stars by
modelling the binary systems in which the observed CEMP- stars were formed.
For this purpose we compare a sample of CEMP- stars with a grid of
binary stars generated by our binary evolution and nucleosynthesis model. We
classify our sample CEMP- stars in three groups based on the observed
abundance of europium. In CEMP stars the europium-to-iron ratio is more
than ten times higher than in the Sun, whereas it is lower than this threshold
in CEMP stars. No measurement of europium is currently available for
CEMP- stars. On average our models reproduce well the abundances observed
in CEMP- stars, whereas in CEMP- stars and CEMP- stars the
abundances of the light- elements are systematically overpredicted by our
models and in CEMP- stars the abundances of the heavy- elements are
underestimated. In all stars our modelled abundances of sodium overestimate the
observations. This discrepancy is reduced only in models that underestimate the
abundances of most of the -elements. Furthermore, the abundance of lead is
underpredicted in most of our model stars. These results point to the
limitations of our AGB nucleosynthesis model, particularly in the predictions
of the element-to-element ratios. Finally, in our models CEMP- stars are
typically formed in wide systems with periods above 10000 days, while most of
the observed CEMP- stars are found in relatively close orbits with periods
below 5000 days.Comment: 23 pages, 8 figures, accepted for publication on Astronomy &
Astrophysic
Carbon-enhanced metal-poor stars: a window on AGB nucleosynthesis and binary evolution. I. Detailed analysis of 15 binary stars with known orbital periods
AGB stars are responsible for producing a variety of elements, including
carbon, nitrogen, and the heavy elements produced in the slow neutron-capture
process (-elements). There are many uncertainties involved in modelling the
evolution and nucleosynthesis of AGB stars, and this is especially the case at
low metallicity, where most of the stars with high enough masses to enter the
AGB have evolved to become white dwarfs and can no longer be observed. The
stellar population in the Galactic halo is of low mass () and only a few observed stars have evolved beyond the first
giant branch. However, we have evidence that low-metallicity AGB stars in
binary systems have interacted with their low-mass secondary companions in the
past. The aim of this work is to investigate AGB nucleosynthesis at low
metallicity by studying the surface abundances of chemically peculiar very
metal-poor stars of the halo observed in binary systems. To this end we select
a sample of 15 carbon- and -element-enhanced metal-poor (CEMP-) halo
stars that are found in binary systems with measured orbital periods. With our
model of binary evolution and AGB nucleosynthesis, we determine the binary
configuration that best reproduces, at the same time, the observed orbital
period and surface abundances of each star of the sample. The observed periods
provide tight constraints on our model of wind mass transfer in binary stars,
while the comparison with the observed abundances tests our model of AGB
nucleosynthesis.Comment: 18 pages, 20 figures, accepted for publication on A&
An investigation of artificial neural network structure and its effects on the estimation of the low-cycle fatigue parameters of various steels
Artificial neural networks (ANNs) are a widely used machine learning approach for estimating low-cycle fatigue parameters. ANN structure has its parameters such as hidden layers, hidden neurons, activation functions, training functions, and so forth, and these parameters have a significant influence over the results. Three hidden layer combinations, the hidden neurons ranging from 1 to 25, and different activation functions like hyperbolic tangent sigmoid (tansig), logistic sigmoid (logsig), and linear (purelin) were used, and their effects on the low-cycle fatigue parameter estimation were investigated to determine optimal ANN structure. Based on the results, suggestions regarding ANN structure for the estimation of the low-cycle fatigue parameters and transition fatigue life were presented. For the output layer and hidden layers, the most suitable activation function was tansig. The optimal hidden neuron range has been found between 4 and 9. The neural network structure with one hidden layer was determined to be most suitable in terms of less knowledge, structural complexity, and computational time and power
Rubidium and zirconium abundances in massive Galactic asymptotic giant branch stars revisited
Luminous Galactic OH/IR stars have been identified as massive (>4-5 M_s) AGB
stars experiencing HBB and Li production. Their Rb abundances and [Rb/Zr]
ratios derived from hydrostatic model atmospheres, are significantly higher
than predictions from AGB nucleosynthesis models, posing a problem to our
understanding of AGB evolution and nucleosynthesis. We report new Rb and Zr
abundances in the full sample of massive Galactic AGB stars, previously studied
with hydrostatic models, by using more realistic extended model atmospheres. We
use a modified version of the spectral synthesis code Turbospectrum and
consider the presence of a circumstellar envelope and radial wind. The Rb and
Zr abundances are determined from the 7800 A Rb I resonant line and the 6474 A
ZrO bandhead, respectively, and we explore the sensitivity of the derived
abundances to variations of the stellar (Teff) and wind (M_loss, beta and vexp)
parameters in the extended models. The Rb and Zr abundances derived from the
best spectral fits are compared with the most recent AGB nucleosynthesis
theoretical models. The new Rb abundances are much lower (even 1-2 dex) than
those derived with the hydrostatic models, while the Zr abundances are similar.
The Rb I line profile and Rb abundance are very sensitive to the M_loss rate
but much less sensitive to variations of the wind velocity-law and the
vexp(OH). We confirm the earlier preliminary results based on a smaller sample
of massive O-rich AGB stars, that the use of extended atmosphere models can
solve the discrepancy between the AGB nucleosynthesis theoretical models and
the observations of Galactic massive AGB stars. The Rb abundances, however, are
still strongly dependent of the M_loss, which is unknown in these AGB stars.
Accurate M_loss rates in these massive Galactic AGB stars are needed in order
to break the models degeneracy and get reliable Rb abundances in these stars.Comment: Accepted for publication in A&A, 14 pages, 12 figures, 4 table
Modelling the observed properties of carbon-enhanced metal-poor stars using binary population synthesis
The stellar population in the Galactic halo is characterised by a large
fraction of CEMP stars. Most CEMP stars are enriched in -elements (CEMP-
stars), and some of these are also enriched in -elements (CEMP- stars).
One formation scenario proposed for CEMP stars invokes wind mass transfer in
the past from a TP-AGB primary star to a less massive companion star which is
presently observed. We generate low-metallicity populations of binary stars to
reproduce the observed CEMP-star fraction. In addition, we aim to constrain our
wind mass-transfer model and investigate under which conditions our synthetic
populations reproduce observed abundance distributions. We compare the CEMP
fractions and the abundance distributions determined from our synthetic
populations with observations. Several physical parameters of the binary
stellar population of the halo are uncertain, e.g. the initial mass function,
the mass-ratio and orbital-period distributions, and the binary fraction. We
vary the assumptions in our model about these parameters, as well as the wind
mass-transfer process, and study the consequent variations of our synthetic
CEMP population. The CEMP fractions calculated in our synthetic populations
vary between 7% and 17%, a range consistent with the CEMP fractions among very
metal-poor stars recently derived from the SDSS/SEGUE data sample. The results
of our comparison between the modelled and observed abundance distributions are
different for CEMP- stars and for CEMP- stars. For the latter, our
simulations qualitatively reproduce the observed distributions of C, Na, Sr,
Ba, Eu, and Pb. Contrarily, for CEMP- stars our model cannot reproduce the
large abundances of neutron-rich elements such as Ba, Eu, and Pb. This result
is consistent with previous studies, and suggests that CEMP- stars
experienced a different nucleosynthesis history to CEMP- stars.Comment: 17 pages, 11 figures, accepted for publication on Astronomy and
Astrophysic
Hot bottom burning and s-process nucleosynthesis in massive AGB stars at the beginning of the thermally-pulsing phase
We report the first spectroscopic identification of massive Galactic
asymptotic giant branch (AGB) stars at the beginning of the thermal pulse (TP)
phase. These stars are the most Li-rich massive AGBs found to date, super
Li-rich AGBs with logE(Li)~3-4. The high Li overabundances are accompanied by
weak or no s-process element (i.e. Rb and Zr) enhancements. A comparison of our
observations with the most recent hot bottom burning (HBB) and s-process
nucleosynthesis models confirms that HBB is strongly activated during the first
TPs but the 22Ne neutron source needs many more TP and third dredge-up episodes
to produce enough Rb at the stellar surface. We also show that the short-lived
element Tc, usually used as an indicator of AGB genuineness, is not detected in
massive AGBs which is in agreement with the theoretical predictions when the
22Ne neutron source dominates the s-process nucleosynthesis.Comment: Accepted for publication in Astronomy & Astrophysics Letters (7
pages, 5 figures and 1 table); final version (language corrected
HR4049: signature of nova nucleosynthesis ?
The post-Asymptotic Giant Branch (AGB) star HR4049 is in an eccentric binary
system with a relatively short period probably surrounded by a dusty
circumbinary disk. Extremely anomalous oxygen isotopic ratios, O16/O17 ~
O16/O18 ~ 7, have been measured from CO_2 molecules likely residing in the
disk. Such a composition cannot be explained in the framework of AGB and
post-AGB evolution while it can be qualitatively associated with the
nucleosynthesis occurring during nova outbursts. We discuss nova models, the
presence of a white dwarf companion to HR4049 and possible scenarios for the
dynamical evolution of this binary system. Circumbinary disks in which mixing
occurs between red-giant and nova material may also be invoked as the site of
formation of some rare types of meteoritic presolar grains.Comment: 4 pages, 2 figures, submitted for the proceedings of the 8th Nuclei
in the Cosmos symposium (Vancouver, Canada, 19-23 July 2004
ERP source reconstruction by using Particle Swarm Optimization
Localization of the sources of Event Related Potentials (ERP) is a challenging inverse problem, especially to resolve sources of neural activity occurring simultaneously. By using an effective dipole source model, we propose a new technique for accurate source localization of ERP signals. The parameters of the dipole ERP sources are optimally chosen by using Particle Swarm Optimization technique. Obtained results on synthetic data sets show that proposed method well localizes the dipoles on their actual locations. On real data sets, the fit error between the actual and reconstructed data is successfully reduced to noise level by localizing a few dipoles in the brain. ©2009 IEEE
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