41,844 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&
An LU implicity scheme for high speed inlet analysis
A numerical method is developed to analyze the inviscid flowfield of a high speed inlet by the solution of the Euler equations. The lower-upper implicit scheme in conjunction with adaptive dissipation proves to be an efficient and robust nonoscillatory shock capturing technique for high Mach number flows as well as for transonic flows
A practical approach for the design of nonuniform lapped transforms
We propose a simple method for the design of lapped transforms with nonuniform frequency resolution and good time localization. The method is a generalization of an approach previously proposed by Princen, where the nonuniform filter bank is obtained by joining uniform cosine-modulated filter banks (CMFBs) using a transition filter. We use several transition filters to obtain a near perfect-reconstruction (PR) nonuniform lapped transform with significantly reduced overall distortion. The main advantage of the proposed method is in reducing the length of the transition filters, which leads to a reduction in processing delay that can be useful for applications such as real-time audio coding
Detecting multiple authorship of United States Supreme Court legal decisions using function words
This paper uses statistical analysis of function words used in legal
judgments written by United States Supreme Court justices, to determine which
justices have the most variable writing style (which may indicated greater
reliance on their law clerks when writing opinions), and also the extent to
which different justices' writing styles are distinguishable from each other.Comment: Published in at http://dx.doi.org/10.1214/10-AOAS378 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Binaries at Low Metallicity: ranges for case A, B and C mass transfer
The evolution of single stars at low metallicity has attracted a large
interest, while the effect of metallicity on binary evolution remains still
relatively unexplored. We study the effect of metallicity on the number of
binary systems that undergo different cases of mass transfer. We find that
binaries at low metallicity are more likely to start transferring mass after
the onset of central helium burning, often referred to as case C mass transfer.
In other words, the donor star in a metal poor binary is more likely to have
formed a massive CO core before the onset of mass transfer.
At solar metallicity the range of initial binary separations that result in
case C evolution is very small for massive stars, because they do not expand
much after the ignition of helium and because mass loss from the system by
stellar winds causes the orbit to widen, preventing the primary star to fill
its Roche lobe. This effect is likely to have important consequences for the
metallicity dependence of the formation rate of various objects through binary
evolution channels, such as long GRBs, double neutron stars and double white
dwarfs.Comment: To appear in the proceedings of "First Stars III", Santa Fe, New
Mexico, July 16-20, 2007, 3 pages, 3 figure
Coding overcomplete representations of audio using the MCLT
We propose a system for audio coding using the modulated complex
lapped transform (MCLT). In general, it is difficult to encode signals using
overcomplete representations without avoiding a penalty in rate-distortion
performance. We show that the penalty can be significantly reduced for
MCLT-based representations, without the need for iterative methods of
sparsity reduction. We achieve that via a magnitude-phase polar quantization
and the use of magnitude and phase prediction. Compared to systems based
on quantization of orthogonal representations such as the modulated lapped
transform (MLT), the new system allows for reduced warbling artifacts and
more precise computation of frequency-domain auditory masking functions
Quasilinear approach of the cumulative whistler instability in fast solar winds: Constraints of electron temperature anisotropy
Context. Solar outflows are a considerable source of free energy which
accumulates in multiple forms like beaming (or drifting) components and/or
temperature anisotropies. However, kinetic anisotropies of plasma particles do
not grow indefinitely and particle-particle collisions are not efficient enough
to explain the observed limits of these anisotropies. Instead, the
self-generated wave instabilities can efficiently act to constrain kinetic
anisotropies, but the existing approaches are simplified and do not provide
satisfactory explanations. Thus, small deviations from isotropy shown by the
electron temperature () in fast solar winds are not explained yet.
Aims. This paper provides an advanced quasilinear description of the whistler
instability driven by the anisotropic electrons in conditions typical for the
fast solar winds. The enhanced whistler-like fluctuations may constrain the
upper limits of temperature anisotropy ,
where are defined with respect to the magnetic field
direction.
Methods. Studied are the self-generated whistler instabilities, cumulatively
driven by the temperature anisotropy and the relative (counter)drift of the
electron populations, e.g., core and halo electrons. Recent studies have shown
that quasi-stable states are not bounded by the linear instability thresholds
but an extended quasilinear approach is necessary to describe them in this
case.
Results. Marginal conditions of stability are obtained from a quasilinear
theory of the cumulative whistler instability, and approach the quasi-stable
states of electron populations reported by the observations.The instability
saturation is determined by the relaxation of both the temperature anisotropy
and the relative drift of electron populations.Comment: Accepted for publication in A&
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