1,234 research outputs found
The seismic properties of low-mass He-core white dwarf stars
We present here a detailed pulsational study applied to low-mass He-core
white dwarfs, based on full evolutionary models representative of these
objects. The background stellar models on which our pulsational analysis was
carried out were derived by taking into account the complete evolutionary
history of the progenitor stars, with special emphasis on the diffusion
processes acting during the white dwarf cooling phase. We computed nonradial
-modes to assess the dependence of the pulsational properties of these
objects with stellar parameters such as the stellar mass and the effective
temperature, and also with element diffusion processes. We also performed a g-
and p-mode pulsational stability analysis on our models and found well-defined
blue edges of the instability domain, where these stars should start to exhibit
pulsations. We found substantial differences in the seismic properties of white
dwarfs with and the extremely low-mass (ELM) white
dwarfs (). Specifically, -mode pulsation modes
in ELM white dwarfs mainly probe the core regions and are not dramatically
affected by mode-trapping effects by the He/H interface, whereas the opposite
is true for more massive He-core white dwarfs. We found that element diffusion
processes substantially affects the shape of the He/H chemical transition
region, leading to non-negligible changes in the period spectrum of low-mass
white dwarfs. Our stability analysis successfully predicts the pulsations of
the only known variable low-mass white dwarf (SDSS J184037.78+642312.3), and
also predicts both - and -mode pulsational instabilities in a significant
number of known low-mass and ELM white dwarfs.Comment: 14 pages, 15 figures, 2 tables. To be published in Astronomy &
Astrophysic
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A Status Report On A Planet Search Around White Dwarf Stars
We have continued monitoring a pilot sample of 15 isolated, pulsating DA white dwarfs for center-of-mass motion caused by a planetary companion. Roughly 7 years into our survey, we have preliminary evidence for periodic variations in pulse arrival times for at least two white dwarfs in our sample. The variations in these systems are unlikely to be caused by secular evolution and are possibly the result of motion of the white dwarf around a center of mass. We have yet to claim confirmation of a planet. GD66 is a previously published candidate system, with a modulation in pulse arrival times that could be caused by a 2.0 M-J sin i planetary companion with an 8.3 year orbital period. Another candidate system, WD1354+0108, has a phase modulation consistent with a 0.7 M-J sin i planet at 2.3 AU (a 4.5 year orbit). We see similar behavior in two independent frequencies within this star, and while a sinusoid is currently a marginally better fit to the data than a straight line (as we might expect from cooling alone in a DAV), we are hesitant to over-interpret our results. Finally, we have a third system, WD0018+0031, that shows a change in pulse arrival times inconsistent with cooling alone; a 2.7 M-J planet at an orbit of about 5 AU could cause the observed trend. Observations of these candidate systems are ongoing in order to constrain any planetary companions that may be present.Astronom
Crystallization in suspensions of hard spheres: A Monte Carlo and Molecular Dynamics simulation study
The crystallization of a metastable melt is one of the most important non
equilibrium phenomena in condensed matter physics, and hard sphere colloidal
model systems have been used for several decades to investigate this process by
experimental observation and computer simulation. Nevertheless, there is still
an unexplained discrepancy between simulation data and experimental nucleation
rate densities. In this paper we examine the nucleation process in hard spheres
using molecular dynamics and Monte Carlo simulation. We show that the
crystallization process is mediated by precursors of low orientational
bond-order and that our simulation data fairly match the experimental data
sets
Caracterização isotópica em água de poço e tanques de piscicultura no município de Petrolina, PE- Semiárido brasileiro.
An Application of Simple and Compact Genetic Algorithms for Estimating Harmonic Components
This work presents an approach for the harmonic components estimation problem present in electrical power systems by making use of evolutionary algorithms. The referential data were obtained by the ATP (Alternative Transients Program) software. Compact and simple genetic algorithms were applied to estimate the parameters of non-linear function to generate a waveform as similar as possible to the one provided by the ATP software. The results yielded by the aforementioned evolutionary algorithms were then compared with one another in a number of scenarios, using the values obtained by the waveform of reference generated by the ATP software. The comparisons were used to seek evidence of which algorithm solved the problem in a setting with limited availability of computational resources. Based on the generated results, it has been found that compact genetic algorithm satisfactorily solves the proposed problem and it is the most indicated method, when less computational effort is required
High-speed Photometric Observations of ZZ Ceti White Dwarf Candidates
We present high-speed photometric observations of ZZ Ceti white dwarf
candidates drawn from the spectroscopic survey of bright DA stars from the
Villanova White Dwarf Catalog by Gianninas et al., and from the recent
spectroscopic survey of white dwarfs within 40 parsecs of the Sun by Limoges et
al. We report the discovery of six new ZZ Ceti pulsators from these surveys,
and several photometrically constant DA white dwarfs, which we then use to
refine the location of the ZZ Ceti instability strip.Comment: 4 pages, 1 table, 2 figures, to appear in "19th European White Dwarf
Workshop" in the ASP Conference Serie
Controllability on infinite-dimensional manifolds
Following the unified approach of A. Kriegl and P.W. Michor (1997) for a
treatment of global analysis on a class of locally convex spaces known as
convenient, we give a generalization of Rashevsky-Chow's theorem for control
systems in regular connected manifolds modelled on convenient
(infinite-dimensional) locally convex spaces which are not necessarily
normable.Comment: 19 pages, 1 figur
Random close packing of polydisperse hard spheres
We study jammed configurations of hard spheres as a function of compression
speed using an event-driven molecular dynamics algorithm. We find that during
the compression, the pressure follows closely the metastable liquid branch
until the system gets arrested into a glass state as the relaxation time
exceeds the compression speed. Further compression yields a jammed
configuration that can be regarded as the infinite pressure configuration of
that glass state. Consequently, we find that the density of jammed packings
varies from 0.638 to 0.658 for polydisperse hard spheres and from 0.635 to
0.645 for pure hard spheres upon decreasing the compression rate. This
demonstrates that the density at which the systems falls out of equilibrium
determines the density at which the system jams at infinite pressure. In
addition, we give accurate data for the jamming density as a function of
compression rate and size polydispersity.Comment: Four pages, three figure
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