313 research outputs found
Neural Systems for solving the inverse problem of recovering the Primary Signal Waveform in potential transformers
The inverse problem of recovering the potential transformer primary signal waveform using secondary signal waveform and information about the secondary load is solved here via two inverse neural network models. The first model uses two recurrent neural networks trained in an off-line mode. The second model is designed with the use a Dynamic Evolving Neural-Fuzzy Interface System (DENFIS) and suited for on-line application and integration into existing protection algorithms as a parallel module. It has the ability of learning and adjusting its structure in an on-line mode to reflect changes in the environment. The model is suited for real time applications and improvement of protection relay operation. The two models perform better than any existing and published models so far and are useful not only for the reconstruction of the primary signal, but for predicting the signal waveform for some time steps ahead and thus for estimating the drifts in the incoming signals and events
The origin and orbit of the old, metal-rich, open cluster NGC 6791: Insights from kinematics
NGC 6791 is a unique stellar system among Galactic open clusters being at the
same time one of the oldest open clusters and the most metal rich. Combination
of its properties is puzzling and poses question of its origin. One possible
scenario is that the cluster formed close to the Galactic Center and later
migrated outwards to its current location. In this work we study the cluster's
orbit and investigate the possible migration processes which might have
displaced NGC 6791 to its present-day position, under the assumption that it
actually formed in the inner disk. To this aim we performed integrations of NGC
6791's orbit in a potential consistent with the main Milky Way parameters. In
addition to analytical expressions for halo, bulge and disk, we also consider
the effect of bar and spiral arm perturbations, which are expected to be very
important for the disk dynamical evolution, especially inside the solar circle.
Starting from state-of-the art initial conditions for NGC 6791, we calculate
1000 orbits back in time for about 1 Gyr turning on and off different
non-axisymmetric components of the global potential. We then compare
statistical estimates of the cluster's recent orbital parameters with the
orbital parameters of 10^4 test-particles originating close to the Galactic
Center (having initial galocentric radii in the range of 3-5 kpc) and
undergoing radial migration during 8 Gyr of forward integration. We find that a
model which incorporates a strong bar and spiral arm perturbations can indeed
be responsible for the migration of NGC 6791 from the inner disk (galocentric
radii of 3-5 kpc) to its present-day location. Such a model can provide orbital
parameters which are close enough to the observed ones. However, the
probability of this scenario as it results from our investigations is very low.Comment: 11 pages, 9 figures, 7 tables, accepted for publication in A&A || v2:
minor changes to match the published versio
Abundance trend with condensation temperature for stars with different Galactic birth places
During the past decade, several studies reported a correlation between
chemical abundances of stars and condensation temperature (also known as Tc
trend). However, the real astrophysical nature of this correlation is still
debated. The main goal of this work is to explore the possible dependence of
the Tc trend on stellar Galactocentric distances, Rmean. We used high-quality
spectra of about 40 stars observed with the HARPS and UVES spectrographs to
derive precise stellar parameters, chemical abundances, and stellar ages. A
differential line-by-line analysis was applied to achieve the highest possible
precision in the chemical abundances. We confirm previous results that [X/Fe]
abundance ratios depend on stellar age and that for a given age, some elements
also show a dependence on Rmean. When using the whole sample of stars, we
observe a weak hint that the Tc trend depends on Rmean. The observed dependence
is very complex and disappears when only stars with similar ages are
considered. To conclude on the possible dependence of the Tc trend on the
formation place of stars, a larger sample of stars with very similar
atmospheric parameters and stellar ages observed at different Galactocentric
distances is neededComment: Accepted by A&
Constraints on the Galactic bar from the Hercules stream as traced with RAVE across the Galaxy
Non-axisymmetries in the Galactic potential (spiral arms and bar) induce kinematic groups such as the Hercules stream. Assuming that Hercules is caused by the effects of the outer Lindblad resonance of the Galactic bar, we model analytically its properties as a function of position in the Galaxy and its dependence on the bar's pattern speed and orientation. Using data from the RAVE survey we find that the azimuthal velocity of the Hercules structure decreases as a function of Galactocentric radius, in a manner consistent with our analytical model. This allows us to obtain new estimates of the parameters of the Milky Way's bar. The combined likelihood function of the bar's pattern speed and angle has its maximum for a pattern speed of Omega(b) = (1.89 +/- 0.08) x Omega(0), where Omega(0) is the local circular frequency. Assuming a solar radius of 8.05 kpc and a local circular velocity of 238 km s(-1), this corresponds to Omega(b) = 56 +/- 2km s(-1) kpc(-1). On the other hand, the bar's orientation phi(b) cannot be constrained with the available data. In fact, the likelihood function shows that a tight correlation exists between the pattern speed and the orientation, implying that a better description of our best fit results is given by the linear relation Omega(b)/Omega(0) = 1.91+0.0044 (phi(b)(deg) - 48), with standard deviation of 0.02. For example, for an angle of phi(b) = 30 deg the pattern speed is 54.0 +/- 0.5 km s(-1) kpc(-1). These results are not very sensitive to the other Galactic parameters such as the circular velocity curve or the peculiar motion of the Sun, and are robust to biases in distance
The Relationship Between Mono-Abundance and Mono-Age Stellar Populations in the Milky Way Disk
Studying the Milky Way disk structure using stars in narrow bins of [Fe/H] and [α/Fe] has recently been proposed as a powerful method to understand the Galactic thick and thin disk formation. It has been assumed so far that these mono-abundance populations (MAPs) are also coeval, or mono-age, populations. Here we study this relationship for a Milky Way chemodynamical model and show that equivalence between MAPs and mono-age populations exists only for the high-[α/Fe] tail, where the chemical evolution curves of different Galactic radii are far apart. At lower [α/Fe]-values an MAP is composed of stars with a range in ages, even for small observational uncertainties and a small MAP bin size. Due to the disk inside-out formation, for these MAPs younger stars are typically located at larger radii, which results in negative radial age gradients that can be as large as 2 Gyr kpc−1. Positive radial age gradients can result for MAPs at the lowest [α/Fe] and highest [Fe/H] end. Such variations with age prevent the simple interpretation of observations for which accurate ages are not available. Studying the variation with radius of the stellar surface density and scale height in our model, we find good agreement to recent analyses of the APOGEE red-clump (RC) sample when 1–4 Gyr old stars dominate (as expected for the RC). Our results suggest that the APOGEE data are consistent with a Milky Way model for which mono-age populations flare for all ages. We propose observational tests for the validity of our predictions and argue that using accurate age measurements, such as from asteroseismology, is crucial for putting constraints on Galactic formation and evolution
The Effect of Spiral Structure on the Stellar Velocity Distribution in the Solar Neighborhood
Clumps in the solar neighborhood's stellar velocity distribution could be
caused by spiral density waves. In the solar neighborhood, stellar velocities
corresponding to orbits that are nearly closed in the frame rotating with a
spiral pattern represent likely regions for stellar concentrations. Via
particle integration, we show that orbits can intersect the solar neighborhood
when they are excited by Lindblad resonances with a spiral pattern. We find
that a two-armed spiral density wave with pattern speed placing the Sun near
the 4:1 Inner Lindblad Resonance (ILR) can cause two families of nearly closed
orbits in the solar neighborhood. One family corresponds to square shaped
orbits aligned so their peaks lie on top of, and support, the two dominant
stellar arms. The second family correspond to orbits 45 degrees out of phase
with the other family. Such a spiral density pattern could account for two
major clumps in the solar neighborhood's velocity distribution. The
Pleiades/Hyades moving group corresponds to the first family of orbits and the
Coma Berenices moving group corresponds to the second family. This model
requires a spiral pattern speed of approximately 0.66 +- 0.03 times the angular
rotation rate of the Sun or 18.1 +- 0.8 km/s/kpc.Comment: Accepted for publication in A
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