1,124 research outputs found
The Thermonuclear Explosion Of Chandrasekhar Mass White Dwarfs
The flame born in the deep interior of a white dwarf that becomes a Type Ia
supernova is subject to several instabilities. We briefly review these
instabilities and the corresponding flame acceleration. We discuss the
conditions necessary for each of the currently proposed explosion mechanisms
and the attendant uncertainties. A grid of critical masses for detonation in
the range - g cm is calculated and its
sensitivity to composition explored. Prompt detonations are physically
improbable and appear unlikely on observational grounds. Simple deflagrations
require some means of boosting the flame speed beyond what currently exists in
the literature. ``Active turbulent combustion'' and multi-point ignition are
presented as two plausible ways of doing this. A deflagration that moves at the
``Sharp-Wheeler'' speed, , is calculated in one dimension
and shows that a healthy explosion is possible in a simple deflagration if the
front moves with the speed of the fastest floating bubbles. The relevance of
the transition to the ``distributed burning regime'' is discussed for delayed
detonations. No model emerges without difficulties, but detonation in the
distributed regime is plausible, will produce intermediate mass elements, and
warrants further study.Comment: 28 pages, 4 figures included, uses aaspp4.sty. Submitted to Ap
Small-scale Interaction of Turbulence with Thermonuclear Flames in Type Ia Supernovae
Microscopic turbulence-flame interactions of thermonuclear fusion flames
occuring in Type Ia Supernovae were studied by means of incompressible direct
numerical simulations with a highly simplified flame description. The flame is
treated as a single diffusive scalar field with a nonlinear source term. It is
characterized by its Prandtl number, Pr << 1, and laminar flame speed, S_L. We
find that if S_L ~ u', where u' is the rms amplitude of turbulent velocity
fluctuations, the local flame propagation speed does not significantly deviate
from S_L even in the presence of velocity fluctuations on scales below the
laminar flame thickness. This result is interpreted in the context of
subgrid-scale modeling of supernova explosions and the mechanism for
deflagration-detonation-transitions.Comment: 8 pages, 6 figures, accepted by Astrophys.
Magnetization control of the nematicity direction and nodal points in a superconducting doped topological insulator
We study the effects of magnetization on the properties of the doped
topological insulator with nematic superconductivity. We found that the
direction of the in-plane magnetization fixes the direction of the nematicity
in the system. The chiral state is more favorable than the nematic state for
large values of out-of-plane magnetization. Overall, the critical temperature
of the nematic state is resilient against magnetization. We explore the
spectrum of the system with the pinned direction of the nematic order parameter
in details. Without magnetization, there is a full gap in the
spectrum. At strong enough out-of-plane or orthogonal in-plane
magnetization, the spectrum is closed at the nodal points that are split by the
magnetization. Flat Majorana surface states connect such split bulk nodal
points. Parallel magnetization lifts nodal points and opens a full gap in
the spectrum. We discuss relevant experiments and propose experimental
verifications of our theory
Primary Sequences of Protein-Like Copolymers: Levy Flight Type Long Range Correlations
We consider the statistical properties of primary sequences of two-letter HP
copolymers (H for hydrophobic and P for polar) designed to have water soluble
globular conformations with H monomers shielded from water inside the shell of
P monomers. We show, both by computer simulations and by exact analytical
calculation, that for large globules and flexible polymers such sequences
exhibit long-range correlations which can be described by Levy-flight
statistics.Comment: 4 pages, including 2 figures; several references added, some
formulations improve
Nucleon-nucleon wave function with short-range nodes and high-energy deuteron photodisintegration
We review a concept of the Moscow potential (MP) of the interaction. On
the basis of this concept we derive by quantum inversion optical partial
potentials from the modern partial-wave analysis (PWA) data and deuteron
properties. Point-form (PF) relativistic quantum mechanics (RQM) is applied to
the two-body deuteron photodisintegration. Calculations of the cross-section
angular distributions cover photon energies between 1.1 and 2.5 GeV. Good
agreement between our theory and recent experimental data confirms the concept
of deep attractive Moscow potential with forbidden - and -states.Comment: 31 pages, 9 figures. typos, extended formalism, review of the Moscow
potential model adde
Reconstruction of the optical potential from scattering data
We propose a method for reconstruction of the optical potential from
scattering data. The algorithm is a two-step procedure. In the first step the
real part of the potential is determined analytically via solution of the
Marchenko equation. At this point we use a diagonal Pad\'{e} approximant of the
corresponding unitary -matrix. In the second step the imaginary part of the
potential is determined via the phase equation of the variable phase approach.
We assume that the real and the imaginary parts of the optical potential are
proportional. We use the phase equation to calculate the proportionality
coefficient. A numerical algorithm is developed for a single and for coupled
partial waves. The developed procedure is applied to analysis of
, , and data.Comment: 26 pages, 8 figures, results of nucl-th/0410092 are refined, some new
results are presente
Thermonuclear Supernovae: Simulations of the Deflagration Stage and Their Implications
Large-scale three-dimensional numerical simulations of the deflagration stage
of a thermonuclear supernova explosion show the formation and evolution of a
highly convoluted turbulent flame in a gravitational field of an expanding
carbon-oxygen white dwarf. The flame dynamics is dominated by the
gravity-induced Rayleigh-Taylor instability that controls the burning rate. The
thermonuclear deflagration releases enough energy to produce a healthy
explosion. The turbulent flame, however, leaves large amounts of unburnt and
partially burnt material near the star center, whereas observations imply these
materials only in outer layers. This disagreement could be resolved if the
deflagration triggers a detonation.Comment: 17 pages, 5 figures. To appear in Science, January 200
On the small-scale stability of thermonuclear flames in Type Ia supernovae
We present a numerical model which allows us to investigate thermonuclear
flames in Type Ia supernova explosions. The model is based on a finite-volume
explicit hydrodynamics solver employing PPM. Using the level-set technique
combined with in-cell reconstruction and flux-splitting schemes we are able to
describe the flame in the discontinuity approximation. We apply our
implementation to flame propagation in Chandrasekhar-mass Type Ia supernova
models. In particular we concentrate on intermediate scales between the flame
width and the Gibson-scale, where the burning front is subject to the
Landau-Darrieus instability. We are able to reproduce the theoretical
prediction on the growth rates of perturbations in the linear regime and
observe the stabilization of the flame in a cellular shape. The increase of the
mean burning velocity due to the enlarged flame surface is measured. Results of
our simulation are in agreement with semianalytical studies.Comment: 9 pages, 7 figures, Uses AASTEX, emulateapj5.sty, onecolfloat.sty.
Replaced with accepted version (ApJ), Figures 1 and 3 are ne
Dynamics of Collapse of flexible Polyelectrolytes and Polyampholytes
We provide a theory for the dynamics of collapse of strongly charged
polyelectrolytes (PEs) and flexible polyampholytes (PAs) using Langevin
equation. After the initial stage, in which counterions condense onto PE, the
mechanism of approach to the globular state is similar for PE and PA. In both
instances, metastable pearl-necklace structures form in characteristic time
scale that is proportional to N^{4/5} where N is the number of monomers. The
late stage of collapse occurs by merger of clusters with the largest one
growing at the expense of smaller ones (Lifshitz- Slyozov mechanism). The time
scale for this process T_{COLL} N. Simulations are used to support the proposed
collapse mechanism for PA and PE.Comment: 14 pages, 2 figure
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