116 research outputs found
Development of a density inversion in driven granular gases
Granular materials fluidized by a rapidly vibrating bottom plate often
develop a fascinating density inversion: a heavier layer of granulate supported
by a lower-density region. We employ the Navier-Stokes granular hydrodynamics
to follow a density inversion as it develops in time. Assuming a dilute
low-Mach-number flow, we derive a reduced time-dependent model of the late
stage of the dynamics. The model looks especially simple in the Lagrangian
coordinates. The time-dependent solution describes the growth of a density peak
at an intermediate height. A transient temperature minimum is predicted to
develop in the region of the density peak. The temperature minimum disappears
at later times, as the system approaches the steady state. At late times, the
predictions of the low-Mach-number model are in good agreement with a numerical
solution of the full hydrodynamic equations. At an early stage of the dynamics,
pressure oscillations are predicted.Comment: 13 pages, 6 figures. To appear in "Granular Gas Dynamics", ed. by T.
Poeschel and N. Brilliantov, vol. 624 of "Lecture Notes in Physics", Springe
Imploding ignition waves: I. one dimensional analysis
We show that converging spherical and cylindrical shock waves may ignite a
detonation wave in a combustible medium, provided the radius at which the
shocks become strong exceeds a critical radius, R_c. An approximate analytic
expression for R_c is derived for an ideal gas equation of state and a simple
(power-law-Arrhenius) reaction law, and shown to reproduce the results of
numerical solutions. For typical acetylene-air experiments we find R_c~0.1 mm
(spherical) and R_c~1 mm (cylindrical). We suggest that the deflagration to
detonation transition (DDT) observed in these systems may be due to converging
shocks produced by the turbulent deflagration flow, which reaches sub (but
near) sonic velocities on scales >>R_c. Our suggested mechanism differs from
that proposed by Zel'dovich et al., in which a fine-tuned spatial gradient in
the chemical induction time is required to be maintained within the turbulent
deflagration flow. Our analysis may be readily extended to more complicated
equations of state and reaction laws. An order of magnitude estimate of R_c
within a white dwarf at the pre-detonation conditions believed to lead to Type
Ia supernova explosions is 0.1 km, suggesting that our proposed mechanism may
be relevant for DDT initiation in these systems. The relevance of our proposed
ignition mechanism to DDT initiation may be tested by both experiments and
numerical simulations.Comment: 12 pages, 10 figures. Somewhat modified, published in Ap
Early emission from type Ia supernovae
A unique feature of deflagration-to-detonation (DDT) white dwarf explosion
models of SNe of type Ia is the presence of a strong shock wave propagating
through the outer envelope. We consider the early emission expected in such
models, which is produced by the expanding shock-heated outer part of the
ejecta and precedes the emission driven by radioactive decay. We expand on
earlier analyses by considering the modification of the pre-detonation density
profile by the weak-shocks generated during the deflagration phase, the time
evolution of the opacity, and the deviation of the post-shock equation of state
from that obtained for radiation pressure domination. A simple analytic model
is presented and shown to provide an acceptable approximation to the results of
1D numerical DDT simulations. Our analysis predicts a thousand second long
UV/optical flash with a luminosity of ~1 to 3*1e39 erg/s. Lower luminosity
corresponds to faster (turbulent) deflagration velocity. The predicted
luminosity of the UV flash is an order of magnitude lower than that of earlier
estimates, and is expected to be strongly suppressed at times longer than an
hour due to the deviation from pure radiation domination.Comment: 10 pages, 4 figure
Multi-Dimensional Explorations in Supernova Theory
In this paper, we bring together various of our published and unpublished
findings from our recent 2D multi-group, flux-limited radiation hydrodynamic
simulations of the collapse and explosion of the cores of massive stars. Aided
by 2D and 3D graphical renditions, we motivate the acoustic mechanism of
core-collapse supernova explosions and explain, as best we currently can, the
phases and phenomena that attend this mechanism. Two major foci of our
presentation are the outer shock instability and the inner core g-mode
oscillations. The former sets the stage for the latter, which damp by the
generation of sound. This sound propagates outward to energize the explosion
and is relevant only if the core has not exploded earlier by some other means.
Hence, it is a more delayed mechanism than the traditional neutrino mechanism
that has been studied for the last twenty years since it was championed by
Bethe and Wilson. We discuss protoneutron star convection,
accretion-induced-collapse, gravitational wave emissions, pulsar kicks, the
angular anisotropy of the neutrino emissions, a subset of numerical issues, and
a new code we are designing that should supercede our current supernova code
VULCAN/2D. Whatever ideas last from this current generation of numerical
results, and whatever the eventual mechanism(s), we conclude that the breaking
of spherical symmetry will survive as one of the crucial keys to the supernova
puzzle.Comment: To be published in the "Centennial Festschrift for Hans Bethe,"
Physics Reports (Elsevier: Holland), ed. G.E. Brown, E. van den Heuvel, and
V. Kalogera, 200
Type II-Plateau supernova radiation: dependencies on progenitor and explosion properties
We explore the properties of Type II-Plateau (II-P) supernovae (SNe) together
with their red-supergiant (RSG) star progenitors. Using MESA STAR, we modulate
the parameters (e.g., mixing length, overshoot, rotation, metallicity) that
control the evolution of a 15Msun main-sequence star to produce a variety of
physical pre-SN models and SN II-P ejecta. We extend previous modeling of SN
II-P radiation to include photospheric and nebular phases, as well as
multi-band light curves and spectra. Our treatment does not assume local
thermodynamic equilibrium, is time dependent, treats explicitly the effects of
line blanketing, and incorporates non-thermal processes. We find that the color
properties of SNe II-P require large model atoms for FeI and FeII, much larger
than adopted in Dessart & Hillier (2011). The color properties also imply RSG
progenitors of limited extent (~500Rsun) --- larger progenitor stars produce a
SN II-P radiation that remains too blue for too long. This finding calls for a
reduction of RSG radii, perhaps through a strengthening of convective energy
transport in RSG envelopes. Increased overshoot and rotation reduce the ratio
of ejecta to helium-core mass, similarly to an increase in main-sequence mass,
and thus complicate the inference ofprogenitor masses. In contrast to the great
sensitivity on progenitor radius, SN II-P color evolution appears insensitive
to variations in explosion energy. Finally, we document the numerous SN II-P
signatures that vary with progenitor metallicity, revealing their potential for
metallicity determinations in the nearby and distant Universe.Comment: Paper accepted to MNRA
Results From Core-Collapse Simulations with Multi-Dimensional, Multi-Angle Neutrino Transport
We present new results from the only 2D multi-group, multi-angle calculations
of core-collapse supernova evolution. The first set of results from these
calculations was published in Ott et al. (2008). We have followed a nonrotating
and a rapidly rotating 20 solar mass model for ~400 ms after bounce. We show
that the radiation fields vary much less with angle than the matter quantities
in the region of net neutrino heating. This obtains because most neutrinos are
emitted from inner radiative regions and because the specific intensity is an
integral over sources from many angles at depth. The latter effect can only be
captured by multi-angle transport. We then compute the phase relationship
between dipolar oscillations in the shock radius and in matter and radiation
quantities throughout the postshock region. We demonstrate a connection between
variations in neutrino flux and the hydrodynamical shock oscillations, and use
a variant of the Rayleigh test to estimate the detectability of these neutrino
fluctuations in IceCube and Super-K. Neglecting flavor oscillations,
fluctuations in our nonrotating model would be detectable to ~10 kpc in
IceCube, and a detailed power spectrum could be measured out to ~5 kpc. These
distances are considerably lower in our rapidly rotating model or with
significant flavor oscillations. Finally, we measure the impact of rapid
rotation on detectable neutrino signals. Our rapidly rotating model has strong,
species-dependent asymmetries in both its peak neutrino flux and its light
curves. The peak flux and decline rate show pole-equator ratios of up to ~3 and
~2, respectively.Comment: 13 pages, 9 figures, ApJ accepted. Replaced with accepted versio
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