35,245 research outputs found
Presupernova evolution of accreting white dwarfs with rotation
We discuss the effects of rotation on the evolution of accreting
carbon-oxygen white dwarfs, with the emphasis on possible consequences in Type
Ia supernova (SN Ia) progenitors. Starting with a slowly rotating white dwarf,
we simulate the accretion of matter and angular momentum from a quasi-Keplerian
accretion disk. The role of the various rotationally induced hydrodynamic
instabilities for the transport of angular momentum inside the white dwarf is
investigated. We find that the dynamical shear instability is the most
important one in the highly degenerate core. Our results imply that accreting
white dwarfs rotate differentially throughout,with a shear rate close to the
threshold value for the onset of the dynamical shear instability. As the latter
depends on the temperature of the white dwarf, the thermal evolution of the
white dwarf core is found to be relevant for the angular momentum
redistribution. As found previously, significant rotation is shown to lead to
carbon ignition masses well above 1.4 Msun. Our models suggest a wide range of
white dwarf explosion masses, which could be responsible for some aspects of
the diversity observed in SNe Ia. We analyze the potential role of the bar-mode
and the r-mode instability in rapidly rotating white dwarfs, which may impose
angular momentum loss by gravitational wave radiation. We discuss the
consequences of the resulting spin-down for the fate of the white dwarf, and
the possibility to detect the emitted gravitational waves at frequencies of 0.1
>... 1.0 Hz in nearby galaxies with LISA. Possible implications of fast and
differentially rotating white dwarf cores for the flame propagation in
exploding white dwarfs are also briefly discussed.Comment: 22 pages, 16 figures, Accepted to A&
Object oriented development of engineering software using CLIPS
Engineering applications involve numeric complexity and manipulations of a large amount of data. Traditionally, numeric computation has been the concern in developing an engineering software. As engineering application software became larger and more complex, management of resources such as data, rather than the numeric complexity, has become the major software design problem. Object oriented design and implementation methodologies can improve the reliability, flexibility, and maintainability of the resulting software; however, some tasks are better solved with the traditional procedural paradigm. The C Language Integrated Production System (CLIPS), with deffunction and defgeneric constructs, supports the procedural paradigm. The natural blending of object oriented and procedural paradigms has been cited as the reason for the popularity of the C++ language. The CLIPS Object Oriented Language's (COOL) object oriented features are more versatile than C++'s. A software design methodology based on object oriented and procedural approaches appropriate for engineering software, and to be implemented in CLIPS was outlined. A method for sensor placement for Space Station Freedom is being implemented in COOL as a sample problem
Lessons from digital switchover in South Korea
In this article, I discuss the lessons from the South Korea’s digital switchover, considering its development in the process and the remaining challenges. South Korea completed the digital switchover on 31st December 2012 but the analogue cable conversion to digital cable has not yet been completed for some 10 million households. Furthermore, mere 2.6% of all the households eligible for the government support benefitted from the support scheme. Although Korea embarked on its journey towards digital television relatively early, its progress has been slow and the outcome was only a partial success. I examine the national politics leading up to pilot programs for eliminating analogue television signals in two phases: Wuljin, Kangjin and Danyang during September and November 2010, followed by Jeju in June 2011. Based on these experiences and the diagnosis of the current situation after the switchover, I conclude that the South Korea’s digital switchover is a partial success and there is continued need for a centralized hub collaborating between the government, the broadcasters, and the television manufacturers to communicate more effectively and to increase public awareness to overcome the remaining challenges
The Evolution of Helium Star Plus Carbon-Oxygen White Dwarf Binary Systems and Implications for Diverse Stellar Transients and Hypervelocity Stars
Helium accretion induced explosions in CO white dwarfs (WDs) are considered
promising candidates for a number of observed types of stellar transients,
including supernovae (SNe) of Type Ia and Type Iax. However, a clear favorite
outcome has not yet emerged. We explore the conditions of helium ignition in
the white dwarf and the final fates of helium star-WD binaries as function of
their initial orbital periods and component masses. We compute 274 model binary
systems with the Binary Evolution Code (BEC), where both components are fully
resolved. Stellar and orbital evolution is computed simultaneously, including
mass and angular momentum transfer, tides, and gravitational wave emission, as
well as differential rotation and internal hydrodynamic and magnetic angular
momentum transport. We find that helium detonations are expected only in
systems with the shortest initial orbital periods, and for initially massive
white dwarfs (MWD > 1.0 MSun ) and lower mass donors (Mdonor < 0.8 MSun), with
accumulated helium layers mostly exceeding 0.1 MSun. Upon detonation, these
systems would release the donor as a hypervelocity pre-WD runaway star, for
which we predict the expected range of kinematic and stellar properties.
Systems with more massive donors or initial periods exceeding 1.5 h will likely
undergo helium deflagrations after accumulating 0.1 - 0.001 MSun of helium.
Helium ignition in the white dwarf is avoided in systems with helium donor
stars below - 0.6 MSun, and lead to three distinctly different groups of double
white dwarf systems. The size of the parameter space open to helium detonation
corresponds to only about 3 % of the galactic SN Ia rate, and to 10 % of the SN
Iax rate, while the predicted large amounts of helium (>0.1 MSun) in
progenitors cannot easily be reconciled with observations of archetypical SN
Ia. ...Comment: Accepted for publication in A&A, 28 pages, 16 figures, 6 table
Theoretical correction to the neutral meson asymmetry
Certain types of asymmetries in neutral meson physics have not been treated
properly, ignoring the difference of normalization factors with an assumption
of the equality of total decay width. Since the corrected asymmetries in
meson are different from known asymmetries by a shift in the first order of CP-
and CPT-violation parameters, experimental data should be analyzed with the
consideration of this effect as in meson physics.Comment: 7 page
Generation of a composite grid for turbine flows and consideration of a numerical scheme
A composite grid was generated for flows in turbines. It consisted of the C-grid (or O-grid) in the immediate vicinity of the blade and the H-grid in the middle of the blade passage between the C-grids and in the upstream region. This new composite grid provides better smoothness, resolution, and orthogonality than any single grid for a typical turbine blade with a large camber and rounded leading and trailing edges. The C-H (or O-H) composite grid has an unusual grid point that is connected to more than four neighboring nodes in two dimensions (more than six neighboring nodes in three dimensions). A finite-volume lower-upper (LU) implicit scheme to be used on this grid poses no problem and requires no special treatment because each interior cell of this composite grid has only four neighboring cells in two dimensions (six cells in three dimensions). The LU implicit scheme was demonstrated to be efficient and robust for external flows in a broad flow regime and can be easily applied to internal flows and extended from two to three dimensions
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