1,791 research outputs found
Carbon Ignition in Type Ia Supernovae: An Analytic Model
The observable properties of a Type Ia supernova are sensitive to how the
nuclear runaway ignites in a Chandrasekhar mass white dwarf - at a single point
at its center, off-center, or at multiple points and times. We present a simple
analytic model for the runaway based upon a combination of stellar
mixing-length theory and recent advances in understanding Rayleigh-Benard
convection. The convective flow just prior to runaway is likely to have a
strong dipolar component, though higher multipoles may contribute appreciably
at the very high Rayleigh number (10) appropriate to the white dwarf
core. A likely outcome is multi-point ignition with an exponentially increasing
number of ignition points during the few tenths of a second that it takes the
runaway to develop. The first sparks ignite approximately 150 - 200 km off
center, followed by ignition at smaller radii. Rotation may be important to
break the dipole asymmetry of the ignition and give a healthy explosion.Comment: 14 pages, 0 figures, submitted to ApJ, corrected typo in first
author's nam
A new model for deflagration fronts in reactive fluids
We present a new way of modeling deflagration fronts in reactive fluids, the
main emphasis being on turbulent thermonuclear deflagration fronts in white
dwarfs undergoing a Type Ia supernova explosion. Our approach is based on a
level set method which treats the front as a mathematical discontinuity and
allows full coupling between the front geometry and the flow field. With only
minor modifications, this method can also be applied to describe contact
discontinuities. Two different implementations are described and their
physically correct behaviour for simple testcases is shown. First results of
the method applied to the concrete problems of Type Ia supernovae and chemical
hydrogen combustion are briefly discussed; a more extensive analysis of our
astrophysical simulations is given in (Reinecke et al. 1998, MPA Green Report
1122b).Comment: 11 pages, 13 figures, accepted by A&A, corrected and extended
according to referee's comment
Direct Numerical Simulations of Type Ia Supernovae Flames II: The Rayleigh-Taylor Instability
A Type Ia supernova explosion likely begins as a nuclear runaway near the
center of a carbon-oxygen white dwarf. The outward propagating flame is
unstable to the Landau-Darrieus, Rayleigh-Taylor, and Kelvin-Helmholtz
instabilities, which serve to accelerate it to a large fraction of the speed of
sound. We investigate the Rayleigh-Taylor unstable flame at the transition from
the flamelet regime to the distributed-burning regime, around densities of
g/cc, through detailed, fully resolved simulations. A low Mach number,
adaptive mesh hydrodynamics code is used to achieve the necessary resolution
and long time scales. As the density is varied, we see a fundamental change in
the character of the burning--at the low end of the density range the
Rayleigh-Taylor instability dominates the burning, whereas at the high end the
burning suppresses the instability. In all cases, significant acceleration of
the flame is observed, limited only by the size of the domain we are able to
study. We discuss the implications of these results on the potential for a
deflagration to detonation transition.Comment: submitted to ApJ, some figures degraded due to size constraint
Can Deflagration-Detonation-Transitions occur in Type Ia Supernovae?
The mechanism for deflagration-detonation-transition (DDT) by turbulent
preconditioning, suggested to explain the possible occurrence of delayed
detonations in Type Ia supernova explosions, is argued to be conceptually
inconsistent. It relies crucially on diffusive heat losses of the burned
material on macroscopic scales. Regardless of the amplitude of turbulent
velocity fluctuations, the typical gradient scale for temperature fluctuations
is shown to be the laminar flame width or smaller, rather than the factor of
thousand more required for a DDT. Furthermore, thermonuclear flames cannot be
fully quenched in regions much larger than the laminar flame width as a
consequence of their simple ``chemistry''. Possible alternative explosion
scenarios are briefly discussed.Comment: 8 pages, uses aastex; added references. Accepted by ApJ Letter
Quasilinear Drift Of Cosmic Rays In Weak Turbulent Electromagnetic Fields
A general quasilinear transport parameter for particle drift in arbitrary
turbulence geometry is presented. The new drift coefficient is solely
characterized by a nonresonant term and is evaluated for slab and
two-dimensional turbulence geometry. The calculations presented here
demonstrate that fluctuating electric fields are a key quantity for
understanding quasilinear particle drift in slab geometry. It is shown that
particle drift does not exist in unpolarized and purely magnetic slab
fluctuations. This is in stark contrast to previous models, which are
restricted to slab geometry and the field line random walk limit. The
evaluation of the general transport parameter for two-dimensional turbulence
geometry, presented here for the first time for dynamical magnetic turbulence,
results in a drift coefficient valid for a magnetic power spectrum and
turbulence decay rate varying arbitrarily in wavenumber. For a two-component,
slab/two-dimensional turbulence model, numerical calculations are presented.
The new quasilinear drift, induced by the magnetic perturbations, is compared
with a standard drift expression related to the curvature and gradient of an
unperturbed heliospheric background magnetic field. The considerations
presented here offer a solid ground and natural explanation for the hitherto
puzzling observation that drift models often describe observations much better
when drift effects are reduced.Comment: 23 pages, 6 figures, accepted for publication in Ap
Technology Supports for Individuals with Disabilities in New York State: A Survey of Current Status
This exploratory study examines the current use of instructional technology, and assistive technologies for support of individuals with learning disabilities as well as other disabilities in New York State.
The researchers used SurveyMonkey and postings on social media websites for various professional organizations to solicit responses to a questionnaire from individuals working with or caring for persons with disabilities. A small sample of responses (N=122) revealed barriers to the use of technology, as well as the preferred type and most convenient for persons with disabilities. School districts may want to use this survey with students, parents and community members
Flame Evolution During Type Ia Supernovae and the Deflagration Phase in the Gravitationally Confined Detonation Scenario
We develop an improved method for tracking the nuclear flame during the
deflagration phase of a Type Ia supernova, and apply it to study the variation
in outcomes expected from the gravitationally confined detonation (GCD)
paradigm. A simplified 3-stage burning model and a non-static ash state are
integrated with an artificially thickened advection-diffusion-reaction (ADR)
flame front in order to provide an accurate but highly efficient representation
of the energy release and electron capture in and after the unresolvable flame.
We demonstrate that both our ADR and energy release methods do not generate
significant acoustic noise, as has been a problem with previous ADR-based
schemes. We proceed to model aspects of the deflagration, particularly the role
of buoyancy of the hot ash, and find that our methods are reasonably
well-behaved with respect to numerical resolution. We show that if a detonation
occurs in material swept up by the material ejected by the first rising bubble
but gravitationally confined to the white dwarf (WD) surface (the GCD
paradigm), the density structure of the WD at detonation is systematically
correlated with the distance of the deflagration ignition point from the center
of the star. Coupled to a suitably stochastic ignition process, this
correlation may provide a plausible explanation for the variety of nickel
masses seen in Type Ia Supernovae.Comment: 14 pages, 10 figures, accepted to the Astrophysical Journa
Similarity and contrasts between thermodynamic properties at the critical point of liquid alkali metals and of electron-hole droplets
The recent experimental study by means of time-resolved luminescence
measurements of an electron-hole liquid (EHL) in diamond by Shimano et al.
[Phys. Rev. Lett. 88 (2002) 057404] prompts us to compare and contrast critical
temperature T_c and critical density n_c relations in liquid alkali metals with
those in electron-hole liquids. The conclusion drawn is that these systems have
similarities with regard to critical properties. In both cases the critical
temperature is related to the cube root of the critical density. The existence
of this relation is traced to Coulomb interactions and to systematic trends in
the dielectric constant of the electron-hole systems. Finally a brief
comparison between the alkalis and EHLs of the critical values for the
compressibility ratio Z_c is also given
Pregnancy outcome in CharcotâMarieâTooth disease: results of the CMTâNET cohort study in Germany
Optical control of coherent interactions between quantum dot electron spins
Coherent interactions between spins in quantum dots are a key requirement for
quantum gates. We have performed pump-probe experiments in which pulsed lasers
emitting at different photon energies manipulate two distinct subsets of
electron spins within an inhomogeneous InGaAs quantum dot ensemble. The spin
dynamics are monitored through their precession about an external magnetic
field. These measurements demonstrate spin precession phase shifts and
modulations of the magnitude of one subset of oriented spins after optical
orientation of the second subset. The observations are consistent with results
from a model using a Heisenberg-like interaction with microeV-strength.Comment: 5 pages, 4 figure
- âŠ