1,398 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
Maximum Brightness and Post-Maximum Decline of Light Curves of SN~Ia: A Comparison of Theory and Observations
We compare the observed correlations between the maximum brightness,
postmaximum decline rate and color at maximum light of Type Ia supernovae (SN
Ia) with model predictions.
The observations are based on a total of 40 SN Ia with 29 SN of the Calan
Tololo Supernova Search and 11 local SN which cover a range of 2 mag in the
absolute visual brightness.
The observed correlations are not tight, one dimensional relations.
Supernovae with the same postmaximum decline or the same color have a spread in
visual magnitude of about 0.7 mag. The dispersion in the color-magnitude
relation may result from uncertainties in the distance determinations or the
interstellar reddening within the host galaxy. The dispersion in the decline
rate-magnitude relation suggests that an intrinsic spread in the supernova
properties exists that cannot be accounted for by any single relation between
visual brightness and postmaximum decline.
Theoretical correlations are derived from a grid of models which encompasses
delayed detonations, pulsating delayed detonations, the merging scenario and
helium detonations.
We find that the observed correlations can be understood in terms of
explosions of Chandrasekhar mass white dwarfs.
Our models show an intrinsic spread in the relations of about 0.5 mag in the
maximum brightness and about 0.1 mag in the B-V color.
Our study provides strong evidence against the mechanism of helium detonation
for subluminous, red SN Ia.Comment: 7 pages, 3 figures, macros ''aaspp.sty'. LaTeX Style. Astrophysical
Journal Letters, submitted Jul. 1995, revised Aug. 1995, resubmitted Sep.
199
A scalar hyperbolic equation with GR-type non-linearity
We study a scalar hyperbolic partial differential equation with non-linear
terms similar to those of the equations of general relativity. The equation has
a number of non-trivial analytical solutions whose existence rely on a delicate
balance between linear and non-linear terms. We formulate two classes of
second-order accurate central-difference schemes, CFLN and MOL, for numerical
integration of this equation. Solutions produced by the schemes converge to
exact solutions at any fixed time when numerical resolution is increased.
However, in certain cases integration becomes asymptotically unstable when
is increased and resolution is kept fixed. This behavior is caused by subtle
changes in the balance between linear and non-linear terms when the equation is
discretized. Changes in the balance occur without violating second-order
accuracy of discretization. We thus demonstrate that a second-order accuracy
and convergence at finite do not guarantee a correct asymptotic behavior
and long-term numerical stability.
Accuracy and stability of integration are greatly improved by an exponential
transformation of the unknown variable.Comment: submitted to Class. Quantum Gra
Isotropic-nematic phase equilibria of polydisperse hard rods: The effect of fat tails in the length distribution
We study the phase behaviour of hard rods with length polydispersity, treated
within a simplified version of the Onsager model. We give a detailed
description of the unusual phase behaviour of the system when the rod length
distribution has a "fat" (e.g. log-normal) tail up to some finite cutoff. The
relatively large number of long rods in the system strongly influences the
phase behaviour: the isotropic cloud curve, which defines the where a nematic
phase first occurs as density is increased, exhibits a kink; at this point the
properties of the coexisting nematic shadow phase change discontinuously. A
narrow three-phase isotropic-nematic-nematic coexistence region exists near the
kink in the cloud curve, even though the length distribution is unimodal. A
theoretical derivation of the isotropic cloud curve and nematic shadow curve,
in the limit of large cutoff, is also given. The two curves are shown to
collapse onto each other in the limit. The coexisting isotropic and nematic
phases are essentially identical, the only difference being that the nematic
contains a larger number of the longest rods; the longer rods are also the only
ones that show any significant nematic ordering. Numerical results for finite
but large cutoff support the theoretical predictions for the asymptotic scaling
of all quantities with the cutoff length.Comment: 21 pages, 13 figure
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
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
Bolometric light curves of supernovae and post-explosion magnetic fields
The various effects leading to diversity in the bolometric light curves of
supernovae are examined: nucleosynthesis, kinematic differences, ejected mass,
degree of mixing, and configuration and intensity of the magnetic field are
discussed. In Type Ia supernovae, a departure in the bolometric light curve
from the full-trapping decline of Co can occur within the two and a half
years after the explosion, depending on the evolutionary path followed by the
WD during the accretion phase. If convection has developed in the WD core
during the presupernova evolution, starting several thousand years before the
explosion, a tangled magnetic field close to the equipartition value should
have grown in the WD. Such an intense magnetic field would confine positrons
where they originate from the Co decays, and preclude a strong departure
from the full-trapping decline, as the supernova expands. This situation is
expected to occur in C+O Chandrasekhar WDs as opposed to edge-lit detonated
sub-Chandrasekhar WDs. If the pre-explosion magnetic field of the WD is less
intense than 10G, a lack of confinement of the positrons emitted in the
Co decay and a departure from full-trapping decline would occur. The
time at which it takes place can provide estimates of the original magnetic
field of the WD, its configuration, and also of the mass of the supernova
ejecta. In SN 1991bg, the bolometric light curve suggests absence of a
significant tangled magnetic field (intensity lower than G).
Chandrasekhar-mass models do not reproduce the bolometric light curve of this
supernova. For SN 1972E, on the contrary, there is evidence for a tangled
configuration of the magnetic field and its light curve is well reproduced by a
Chandrasekhar WD explosion.Comment: 54 pages, including 8 figures. To appear in Ap
Hydrodynamical simulations of galaxy formation: effects of supernova feedback
We numerically simulate some of the most critical physical processes in
galaxy formation: The supernova feedback, in conjunction with gasdynamics and
gravity, plays a crucial role in determining how galaxies arise within the
context of a model for large-scale structure. Our treatment incorporates a
multi-phase model of the interstellar medium and includes the effects of
cooling, heating and metal enrichment by supernovae, and evaporation of cold
clouds. The star formation happens inside the clouds of cold gas, which are
produced via thermal instability. We simulate the galaxy formation in standard
biased CDM model for a variety of parameters and for several resolutions in the
range 2--20kpc. In our picture, supernova feedback regulates the
evolution of the gas components and star formation. The efficiency of cloud
evaporation by supernova strongly influences star formation rates. This
feedback results in a steady rate of star formation in large galaxies (mass
larger than 2-3x10^{11}\Msun) at a level of (1-10)\Msun\yr for .
Supernova feedback has an even stronger effect on the evolution of dwarf
galaxies, most of which have a small fraction of stars and extremely low
luminosities: . In the case of both large and small galaxies, the
distribution of luminous matter (stars) is strongly BIASED with respect to the
dark matter. We find an approximate biasing measure of the form for z=0 and overdensities exceeding 1000. Deviations
from this relation (a factor 2-3) depend on the environment. For halo masses
exceeding 2x10^{10}\Msun, the dependence of the absolute magnitude on the
total mass can be approximated as M_V=-18.5-4\log(M_{tot}/10^{11}\Msun), with
a scatter of less than 0.5mag.Comment: 17 pages LATEX (uses mn.sty). 12 PostScript figures and the text are
available at ftp://charon.nmsu.edu/pub/aklypin/HYDRO Corrected ftp adderes
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