455 research outputs found
Surface friction of rock in terrestrial and simulated lunar environments
The conventional probe-on-the rotating-disk concept was used to determine the surface friction in mineral probe/specimen interfaces. Nine rocks or minerals and two stainless steels were tested in both new (NT) and same track (ST) tests under three different pressure environments-atmospheric, UHV, and dry nitrogen. Each environment was further subdivided into two testing conditions, that is, ambient and elevated (135 C) temperatures. In NT tests, friction was the lowest in an atmospheric pressure condition for all rock types and increased to the largest in UHV ambient condition except for pyroxene and stainless steel. Friction values measured in dry nitrogen ambient condition lie between the two extremes. Heating tends to increase friction in atmospheric and dry nitrogen environment but decreases in UHV environment with the exception of stainless steel, basalt, and pyroxene. In ST tests, friction was the lowest in the first run and increased in subsequent runs except for stainless steel where the reverse was true. The increases leveled off after a few runs ranging from the second to the seventh depending on rock types
The Diversity of Type Ia Supernovae from Broken Symmetries
Type Ia supernovae result when carbon-oxygen white dwarfs in binary systems
accrete mass from companion stars, reach a critical mass, and explode. The near
uniformity of their light curves makes these supernovae good standard candles
for measuring cosmic expansion, but a correction must be applied to account for
the fact that the brighter supernovae have broader light curves.
One-dimensional modelling, with a certain choice of parameters, can reproduce
this general trend in the width-luminosity relation, but the processes of
ignition and detonation have recently been shown to be intrinsically
asymmetric. Here we report on multi-dimensional modelling of the explosion
physics and radiative transfer that reveals that the breaking of spherical
symmetry is a critical factor in determining both the width luminosity relation
and the observed scatter about it. The deviation from sphericity can also
explain the finite polarization detected in the light from some supernovae. The
slope and normalization of the width-luminosity relation has a weak dependence
on certain properties of the white dwarf progenitor, in particular the trace
abundances of elements other than carbon and oxygen. Failing to correct for
this effect could lead to systematic overestimates of up to 2% in the distance
to remote supernovae.Comment: Accepted to Natur
Off-center ignition in type Ia supernova: I. Initial evolution and implications for delayed detonation
The explosion of a carbon-oxygen white dwarf as a Type Ia supernova is known
to be sensitive to the manner in which the burning is ignited. Studies of the
pre-supernova evolution suggest asymmetric, off-center ignition, and here we
explore its consequences in two- and three-dimensional simulations. Compared
with centrally ignited models, one-sided ignitions initially burn less and
release less energy. For the distributions of ignition points studied, ignition
within two hemispheres typically leads to the unbinding of the white dwarf,
while ignition within a small fraction of one hemisphere does not. We also
examine the spreading of the blast over the surface of the white dwarf that
occurs as the first plumes of burning erupt from the star. In particular, our
studies test whether the collision of strong compressional waves can trigger a
detonation on the far side of the star as has been suggested by Plewa et al.
(2004). The maximum temperature reached in these collisions is sensitive to how
much burning and expansion has already gone on, and to the dimensionality of
the calculation. Though detonations are sometimes observed in 2D models, none
ever happens in the corresponding 3D calculations. Collisions between the
expansion fronts of multiple bubbles also seem, in the usual case, unable to
ignite a detonation. "Gravitationally confined detonation" is therefore not a
robust mechanism for the explosion. Detonation may still be possible in these
models however, either following a pulsation or by spontaneous detonation if
the turbulent energy is high enough.Comment: 13 pages, 10 figures (resolution of some figures reduced to comply
with astro-ph file size restriction); submitted to the Astrophysical Journal
on 8/3/200
Modeling the Diversity of Type Ia Supernova Explosions
Type Ia supernovae (SNe Ia) are a prime tool in observational cosmology. A
relation between their peak luminosities and the shapes of their light curves
allows to infer their intrinsic luminosities and to use them as distance
indicators. This relation has been established empirically. However, a
theoretical understanding is necessary in order to get a handle on the
systematics in SN Ia cosmology. Here, a model reproducing the observed
diversity of normal SNe Ia is presented. The challenge in the numerical
implementation arises from the vast range of scales involved in the physical
mechanism. Simulating the supernova on scales of the exploding white dwarf
requires specific models of the microphysics involved in the thermonuclear
combustion process. Such techniques are discussed and results of simulations
are presented.Comment: 6 pages, ASTRONUM-2009 "Numerical Modeling of Space Plasma Flows",
Chamonix, France, July 2009, to appear in ASP Conf. Pro
A Common Explosion Mechanism for Type Ia Supernovae
Type Ia supernovae, the thermonuclear explosions of white dwarf stars
composed of carbon and oxygen, were instrumental as distance indicators in
establishing the acceleration of the universe's expansion. However, the physics
of the explosion are debated. Here we report a systematic spectral analysis of
a large sample of well observed type Ia supernovae. Mapping the velocity
distribution of the main products of nuclear burning, we constrain theoretical
scenarios. We find that all supernovae have low-velocity cores of stable
iron-group elements. Outside this core, nickel-56 dominates the supernova
ejecta. The outer extent of the iron-group material depends on the amount of
nickel-56 and coincides with the inner extent of silicon, the principal product
of incomplete burning. The outer extent of the bulk of silicon is similar in
all SNe, having an expansion velocity of ~11000 km/s and corresponding to a
mass of slightly over one solar mass. This indicates that all the supernovae
considered here burned similar masses, and suggests that their progenitors had
the same mass. Synthetic light curve parameters and three-dimensional explosion
simulations support this interpretation. A single explosion scenario, possibly
a delayed detonation, may thus explain most type Ia supernovae.Comment: 8 pages, 2 figure
Excess low energy photon pairs from pion annihilation at the chiral phase transition
The photon pair production by pion annihilation in a hot and dense medium at
the chiral phase transition is investigated within a chiral quark model. As a
direct consequence of this transition the meson appears as a bound
state in the domain of temperatures and chemical potentials where the condition
is fulfilled. This effect results in a
strong enhancement of the cross section for the pion annihilation process compared with the vacuum case. The calculation of the photon
pair production rate as function of the invariant mass shows a strong
enhancement and narrowing of the meson resonance at threshold due to
chiral symmetry restoration.Comment: 15 pages, LaTeX, 6 figures, Phys. Lett.
REMARKS ON THE MORPHOLOGY OF THE TEAK MOTH, HYBLAEA PUERA CR.
abstract not availabl
A New Model for Electron-Capture Supernovae in Galactic Chemical Evolution
We examine the contribution of electron-capture supernovae (ECSNe), low-mass
SNe from collapsing Fe cores (FeCCSNe), and rotating massive stars to the
chemical composition of the Galaxy. Our model includes contributions to
chemical evolution from both thermonuclear ECSNe (tECSNe) and gravitational
collapse ECSNe (cECSNe). We show that if ECSNe are predominantly gravitational
collapse SNe but about 15% are partial thermonuclear explosions, the model is
able to reproduce the solar abundances of several important and problematic
isotopes including Ca, Ti and Cr together with Fe,
Ni, Se and Kr and several of the Zn--Zr isotopes. A model
in which no cECSNe occur, only tECSNe with low-mass FeCCSNe or rotating massive
stars, proves also very successful at reproducing the solar abundances for
these isotopes. Despite the small mass range for the progenitors of ECSNe and
low-mass FeCCSNe, the large production factors suffice for the solar inventory
of the above isotopes. Our model is compelling because it introduces no new
tensions with the solar abundance distribution for a Milky Way model -- only
tending to improve the model predictions for several isotopes. The proposed
astrophysical production model thus provides a natural and elegant way to
explain one of the last uncharted territories on the periodic table of
astrophysical element production.Comment: 10 pages, 4 figures; accepted for publication in the Astrophysical
Journa
Nucleation and cluster formation in low-density nucleonic matter: A mechanism for ternary fission
Ternary fission yields in the reaction 241Pu(nth,f) are calculated using a
new model which assumes a nucleation-time moderated chemical equilibrium in the
low density matter which constitutes the neck region of the scissioning system.
The temperature, density, proton fraction and fission time required to fit the
experimental data are derived and discussed. A reasonably good fit to the
experimental data is obtained. This model provides a natural explanation for
the observed yields of heavier isotopes relative to those of the lighter
isotopes, the observation of low proton yields relative to 2H and 3H yields and
the non-observation of 3He, all features which are shared by similar thermal
neutron induced and spontaneous fissioning systems.Comment: 6 pages, 3 figure
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