1,218 research outputs found
Spectral Signatures of Gravitationally Confined Thermonuclear Supernova Explosions
We consider some of the spectral and polarimetric signatures of the
gravitational confined detonation scenario for Type Ia supernova explosions. In
this model, material produced by an off-center deflagration (which itself fails
to produce the explosion) forms a metal-rich atmosphere above the white dwarf
surface. Using hydrodynamical simulations, we show that this atmosphere is
compressed and accelerated during the subsequent interaction with the supernova
ejecta. This leads ultimately to the formation of a high-velocity pancake of
metal-rich material that is geometrically detached from the bulk of the ejecta.
When observed at the epochs near maximum light, this absorbing pancake produces
a highly blueshifted and polarized calcium IR triplet absorption feature
similar to that observed in several Type~Ia supernovae. We discuss the
orientation effects present in our model and contrast them to those expected in
other supernova explosion models. We propose that a large sample of
spectropolarimetric observations can be used to critically evaluate the
different theoretical scenarios.Comment: 4 pages, 3 figures. To appear in ApJ Letters. For higher resolution
images and movies see http://panisse.lbl.gov/~dnkasen/gcd.htm
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
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
Local Ignition in Carbon/Oxygen White Dwarfs -- I: One-zone Ignition and Spherical Shock Ignition of Detonations
The details of ignition of Type Ia supernovae remain fuzzy, despite the
importance of this input for any large-scale model of the final explosion.
Here, we begin a process of understanding the ignition of these hotspots by
examining the burning of one zone of material, and then investigate the
ignition of a detonation due to rapid heating at single point.
We numerically measure the ignition delay time for onset of burning in
mixtures of degenerate material and provide fitting formula for conditions of
relevance in the Type Ia problem. Using the neon abundance as a proxy for the
white dwarf metallicity, we then find that ignition times can decrease by ~20%
with addition of even 5% of neon by mass. When temperature fluctuations that
successfully kindle a region are very rare, such a reduction in ignition time
can increase the probability of ignition by orders of magnitude. If the neon
comes largely at the expense of carbon, a similar increase in the ignition time
can occur.
We then consider the ignition of a detonation by an explosive energy input in
one localized zone, eg a Sedov blast wave leading to a shock-ignited
detonation. Building on previous work on curved detonations, we find that
surprisingly large inputs of energy are required to successfully launch a
detonation, leading to required matchheads of ~4500 detonation thicknesses -
tens of centimeters to hundreds of meters - which is orders of magnitude larger
than naive considerations might suggest. This is a very difficult constraint to
meet for some pictures of a deflagration-to-detonation transition, such as a
Zel'dovich gradient mechanism ignition in the distributed burning regime.Comment: 29 pages; accepted to ApJ. Comments welcome at
http://www.cita.utoronto.ca/~ljdursi/thisweek/ . Updated version addressing
referee comment
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