3,603 research outputs found
Thermoelectric Outer Planets Spacecraft (TOPS) electronic packaging and cabling development summary report
Electronic packaging and cabling activities performed in support of the Thermoelectric Outer Planets Spacecraft (TOPS) Advanced Systems Technology (AST) project are detailed. It describes new electronic compartment, electronic assembly, and module concepts, and a new high-density, planar interconnection technique called discrete multilayer (DML). Development and qualification of high density cabling techniques, using small gage wire and microminiature connectors, are also reported
Chemical equilibrium and stable stratification of a multi-component fluid: thermodynamics and application to neutron stars
A general thermodynamic argument shows that multi-component matter in full
chemical equilibrium, with uniform entropy per baryon, is generally stably
stratified. This is particularly relevant for neutron stars, in which the
effects of entropy are negligible compared to those of the equilibrium
composition gradient established by weak interactions. It can therefore be
asserted that, regardless of the uncertainties in the equation of state of
dense matter, neutron stars are stably stratified. This has important,
previously discussed consequences for their oscillation modes, magnetic field
evolution, and internal angular momentum transport.Comment: AASTeX, 8 pages, including 1 PS figure. Accepted for publication in
The Astrophysical Journa
Theoretical Studies of Accretion of Matter onto White Dwarfs and the Single Degenerate Scenario for Supernovae of Type Ia
We present a brief summary of the Single Degenerate Scenario for the
progenitors of Type Ia Supernovae in which it is assumed that a low mass
carbon-oxygen white dwarf is growing in mass as a result of accretion from a
secondary star in a close binary system. Recent hydrodynamic simulations of
accretion of solar material onto white dwarfs without mixing always produce a
thermonuclear runaway and steady burning does not occur. For a broad range in
WD mass (0.4 Solar masses to 1.35 Solar Masses), the maximum ejected material
occurs for the 1.25 Solar Mass sequences and then decreases as the white dwarf
mass decreases. Therefore, the white dwarfs are growing in mass as a
consequence of the accretion of solar material and as long as there is no
mixing of accreted material with core material. In contrast, a thermonuclear
runaway in the accreted hydrogen-rich layers on the low luminosity WDs in close
binary systems where mixing of core matter with accreted material has occurred
is the outburst mechanism for Classical, Recurrent, and Symbiotic novae. The
differences in characteristics of these systems is likely the WD mass and mass
accretion rate. The high levels of enrichment of CN ejecta in elements ranging
from carbon to sulfur confirm that there is dredge-up of matter from the core
of the WD and enable them to contribute to the chemical enrichment of the
interstellar medium. Therefore, studies of CNe can lead to an improved
understanding of Galactic nucleosynthesis, some sources of pre-solar grains,
and the Extragalactic distance scale. The characteristics of the outburst
depend on the white dwarf mass, luminosity, mass accretion rate, and the
chemical composition of both the accreting material and WD material. The
properties of the outburst also depends on when, how, and if the accreted
layers are mixed with the WD core and the mixing mechanism is still unknown.Comment: 25 Pages, Bulletin of the Astronomical Society of India (BASI) in
pres
Modeling core collapse supernovae in 2 and 3 dimensions with spectral neutrino transport
The overwhelming evidence that the core collapse supernova mechanism is
inherently multidimensional, the complexity of the physical processes involved,
and the increasing evidence from simulations that the explosion is marginal
presents great computational challenges for the realistic modeling of this
event, particularly in 3 spatial dimensions. We have developed a code which is
scalable to computations in 3 dimensions which couples PPM Lagrangian with
remap hydrodynamics [1], multigroup, flux-limited diffusion neutrino transport
[2], with many improvements), and a nuclear network [3]. The neutrino transport
is performed in a ray-by-ray plus approximation wherein all the lateral effects
of neutrinos are included (e.g., pressure, velocity corrections, advection)
except the transport. A moving radial grid option permits the evolution to be
carried out from initial core collapse with only modest demands on the number
of radial zones. The inner part of the core is evolved after collapse along
with the rest of the core and mantle by subcycling the lateral evolution near
the center as demanded by the small Courant times. We present results of 2-D
simulations of a symmetric and an asymmetric collapse of both a 15 and an 11 M
progenitor. In each of these simulations we have discovered that once the
oxygen rich material reaches the shock there is a synergistic interplay between
the reduced ram pressure, the energy released by the burning of the shock
heated oxygen rich material, and the neutrino energy deposition which leads to
a revival of the shock and an explosion.Comment: 10 pages, 3 figure
3D hydrodynamic simulations of carbon burning in massive stars
We present the first detailed 3D hydrodynamic implicit large eddy simulations of turbulent convection of carbon burning in massive stars. Simulations begin with radial profiles mapped from a carbon-burning shell within a 15âMâ 1D stellar evolution model. We consider models with 1283, 2563, 5123, and 10243 zones. The turbulent flow properties of these carbon-burning simulations are very similar to the oxygen-burning case. We performed a mean field analysis of the kinetic energy budgets within the Reynolds-averaged NavierâStokes framework. For the upper convective boundary region, we find that the numerical dissipation is insensitive to resolution for linear mesh resolutions above 512 grid points. For the stiffer, more stratified lower boundary, our highest resolution model still shows signs of decreasing sub-grid dissipation suggesting it is not yet numerically converged. We find that the widths of the upper and lower boundaries are roughly 30 per cent and 10 per cent of the local pressure scaleheights, respectively. The shape of the boundaries is significantly different from those used in stellar evolution models. As in past oxygen-shell-burning simulations, we observe entrainment at both boundaries in our carbon-shell-burning simulations. In the large PĂ©clet number regime found in the advanced phases, the entrainment rate is roughly inversely proportional to the bulk Richardson number, RiB (âRiBâα, 0.5 âČ Î± âČ 1.0). We thus suggest the use of RiB as a means to take into account the results of 3D hydrodynamics simulations in new 1D prescriptions of convective boundary mixing
Observational Tests and Predictive Stellar Evolution II: Non-standard Models
We examine contributions of second order physical processes to results of
stellar evolution calculations amenable to direct observational testing. In the
first paper in the series (Young et al. 2001) we established baseline results
using only physics which are common to modern stellar evolution codes. In the
current paper we establish how much of the discrepancy between observations and
baseline models is due to particular elements of new physics. We then consider
the impact of the observational uncertainties on the maximum predictive
accuracy achievable by a stellar evolution code. The sun is an optimal case
because of the precise and abundant observations and the relative simplicity of
the underlying stellar physics. The Standard Model is capable of matching the
structure of the sun as determined by helioseismology and gross surface
observables to better than a percent. Given an initial mass and surface
composition within the observational errors, and no additional constraints for
which the models can be optimized, it is not possible to predict the sun's
current state to better than ~7%. Convectively induced mixing in radiative
regions, seen in multidimensional hydrodynamic simulations, dramatically
improves the predictions for radii, luminosity, and apsidal motions of
eclipsing binaries while simultaneously maintaining consistency with observed
light element depletion and turnoff ages in young clusters (Young et al. 2003).
Systematic errors in core size for models of massive binaries disappear with
more complete mixing physics, and acceptable fits are achieved for all of the
binaries without calibration of free parameters. The lack of accurate abundance
determinations for binaries is now the main obstacle to improving stellar
models using this type of test.Comment: 33 pages, 8 figures, accepted for publication in the Astrophysical
Journa
Relative Importance of Convective Uncertainties in Massive Stars
In this work, we investigate the impact of uncertainties due to convective boundary mixing (CBM), commonly called âovershootâ, namely the boundary location and the amount of mixing at the convective boundary, on stellar structure and evolution. For this we calculated two grids of stellar evolution models with the MESA code, each with the Ledoux and the Schwarzschild boundary criterion, and vary the amount of CBM. We calculate each grid with the initial masses 15, 20 and . We present the stellar structure of the models during the hydrogen and helium burning phases. In the latter, we examine the impact on the nucleosynthesis. We find a broadening of the main-sequence with more CBM, which is more in agreement with observations. Furthermore during the core hydrogen burning phase there is a convergence of the convective boundary location due to CBM. The uncertainties of the intermediate convective zone remove this convergence. The behaviour of this convective zone strongly affects the surface evolution of the model, i.e. how fast it evolves red-wards. The amount of CBM impacts the size of the convective cores and the nucleosynthesis, e.g. the 12C to 16O ratio and the weak s-process. Lastly, we determine the uncertainty that the range of parameter values investigated introduce and we find differences of up to for the core masses and the total mass of the star
Stellar Hydrodynamics in Radiative Regions
We present an analysis of the response of a radiative region to waves
generated by a convective region of the star; this wave treatment of the
classical problem of ``overshooting'' gives extra mixing relative to the
treatment traditionally used in stellar evolutionary codes. The interface
between convectively stable and unstable regions is dynamic and nonspherical,
so that the nonturbulent material is driven into motion, even in the absence of
``penetrative overshoot.'' These motions may be described by the theory of
nonspherical stellar pulsations, and are related to motion measured by
helioseismology. Multi-dimensional numerical simulations of convective flow
show puzzling features which we explain by this simplified physical model.
Gravity waves generated at the interface are dissipated, resulting in slow
circulation and mixing seen outside the formal convection zone. The approach
may be extended to deal with rotation and composition gradients. Tests of this
description in the stellar evolution code TYCHO produce carbon stars on the
asymptotic giant branch (AGB), an isochrone age for the Hyades and three young
clusters with lithium depletion ages from brown dwarfs, and lithium and
beryllium depletion consistent with observations of the Hyades and Pleiades,
all without tuning parameters. The insight into the different contributions of
rotational and hydrodynamic mixing processes could have important implications
for realistic simulation of supernovae and other questions in stellar
evolution.Comment: 27 pages, 5 figures, accepted to the Astrophysical Journa
A dynamical model of surrogate reactions
A new dynamical model is developed to describe the whole process of surrogate
reactions; transfer of several nucleons at an initial stage, thermal
equilibration of residues leading to washing out of shell effects and decay of
populated compound nuclei are treated in a unified framework. Multi-dimensional
Langevin equations are employed to describe time-evolution of collective
coordinates with a time-dependent potential energy surface corresponding to
different stages of surrogate reactions. The new model is capable of
calculating spin distributions of the compound nuclei, one of the most
important quantity in the surrogate technique. Furthermore, various observables
of surrogate reactions can be calculated, e.g., energy and angular distribution
of ejectile, and mass distributions of fission fragments. These features are
important to assess validity of the proposed model itself, to understand
mechanisms of the surrogate reactions and to determine unknown parameters of
the model. It is found that spin distributions of compound nuclei produced in
O+U O+U and O+U
O+U reactions are equivalent and much less than
10, therefore satisfy conditions proposed by Chiba and Iwamoto (PRC 81,
044604(2010)) if they are used as a pair in the surrogate ratio method.Comment: 17 pages, 5 figure
Observing Supernova 1987A with the Refurbished Hubble Space Telescope
Observations with the Hubble Space Telescope (HST), conducted since 1990, now
offer an unprecedented glimpse into fast astrophysical shocks in the young
remnant of supernova 1987A. Comparing observations taken in 2010 using the
refurbished instruments on HST with data taken in 2004, just before the Space
Telescope Imaging Spectrograph failed, we find that the Ly-a and H-a lines from
shock emission continue to brighten, while their maximum velocities continue to
decrease. We observe broad blueshifted Ly-a, which we attribute to resonant
scattering of photons emitted from hotspots on the equatorial ring. We also
detect NV~\lambda\lambda 1239,1243 A line emission, but only to the red of
Ly-A. The profiles of the NV lines differ markedly from that of H-a, suggesting
that the N^{4+} ions are scattered and accelerated by turbulent electromagnetic
fields that isotropize the ions in the collisionless shock.Comment: Science, accepted. Science Express, 02 Sept 2010. 5 figures.
Supporting online material can be found at
http://www.sciencemag.org/cgi/content/full/sci;science.1192134/DC
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