78 research outputs found
Carbon Ignition in Type Ia Supernovae: An Analytic Model
The observable properties of a Type Ia supernova are sensitive to how the
nuclear runaway ignites in a Chandrasekhar mass white dwarf - at a single point
at its center, off-center, or at multiple points and times. We present a simple
analytic model for the runaway based upon a combination of stellar
mixing-length theory and recent advances in understanding Rayleigh-Benard
convection. The convective flow just prior to runaway is likely to have a
strong dipolar component, though higher multipoles may contribute appreciably
at the very high Rayleigh number (10) appropriate to the white dwarf
core. A likely outcome is multi-point ignition with an exponentially increasing
number of ignition points during the few tenths of a second that it takes the
runaway to develop. The first sparks ignite approximately 150 - 200 km off
center, followed by ignition at smaller radii. Rotation may be important to
break the dipole asymmetry of the ignition and give a healthy explosion.Comment: 14 pages, 0 figures, submitted to ApJ, corrected typo in first
author's nam
Small-scale Interaction of Turbulence with Thermonuclear Flames in Type Ia Supernovae
Microscopic turbulence-flame interactions of thermonuclear fusion flames
occuring in Type Ia Supernovae were studied by means of incompressible direct
numerical simulations with a highly simplified flame description. The flame is
treated as a single diffusive scalar field with a nonlinear source term. It is
characterized by its Prandtl number, Pr << 1, and laminar flame speed, S_L. We
find that if S_L ~ u', where u' is the rms amplitude of turbulent velocity
fluctuations, the local flame propagation speed does not significantly deviate
from S_L even in the presence of velocity fluctuations on scales below the
laminar flame thickness. This result is interpreted in the context of
subgrid-scale modeling of supernova explosions and the mechanism for
deflagration-detonation-transitions.Comment: 8 pages, 6 figures, accepted by Astrophys.
Tidal Interaction between a Fluid Star and a Kerr Black Hole in Circular Orbit
We present a semi-analytic study of the equilibrium models of close binary
systems containing a fluid star (mass and radius ) and a Kerr black
hole (mass ) in circular orbit. We consider the limit where
spacetime is described by the Kerr metric. The tidally deformed star is
approximated by an ellipsoid, and satisfies the polytropic equation of state.
The models also include fluid motion in the stellar interior, allowing binary
models with nonsynchronized stellar spin (as expected for coalescing neutron
star-black hole binaries) to be constructed. Tidal disruption occurs at orbital
radius , but the dimensionless ratio depends on the spin parameter of
the black hole as well as on the equation of state and the internal rotation of
the star. We find that the general relativistic tidal field disrupts the star
at a larger than the Newtonian tide; the difference is
particularly prominent if the disruption occurs in the vicinity of the black
hole's horizon. In general, is smaller for a (prograde
rotating) Kerr black hole than for a Schwarzschild black hole. We apply our
results to coalescing black hole-neutron star and black hole-white dwarf
binaries. The tidal disruption limit is important for characterizing the
expected gravitational wave signals and is relevant for determining the
energetics of gamma ray bursts which may result from such disruption.Comment: 29 pages including 8 figures. Minor changes and update. To appear in
ApJ, March 20, 2000 (Vol.532, #1
Stability of the r-modes in white dwarf stars
Stability of the r-modes in rapidly rotating white dwarf stars is
investigated. Improved estimates of the growth times of the
gravitational-radiation driven instability in the r-modes of the observed DQ
Her objects are found to be longer (probably considerably longer) than 6x10^9y.
This rules out the possibility that the r-modes in these objects are emitting
gravitational radiation at levels that could be detectable by LISA. More
generally it is shown that the r-mode instability can only be excited in a very
small subset of very hot (T>10^6K), rather massive (M>0.9M_sun) and very
rapidly rotating (P_min<P<1.2P_min) white dwarf stars. Further, the growth
times of this instability are so long that these conditions must persist for a
very long time (t>10^9y) to allow the amplitude to grow to a dynamically
significant level. This makes it extremely unlikely that the r-mode instability
plays a significant role in any real white dwarf stars.Comment: 5 Pages, 5 Figures, revte
Thermoluminescence, photoluminescence and optically stimulated luminescence characteristics of CaSO4:Eu phosphor: experimental and density functional theory (DFT) investigations
The CaSO4:Eu phosphor in nanocrystalline form was obtained by chemical method. The sample was annealed at various temperatures and quenched. The structural, electronic and optical properties are studied using various experimental techniques. As synthesized CaSO4:Eu particles have nanorod shapes with diameter of ~15 nm and length of ~250 nm. After annealing (at around 900 °C) a significant increase in their size (~2â4 ÎŒm) with phase transformation from hexagonal to orthorhombic was observed. Thermoluminescence (TL) and optically stimulated luminescence (OSL) intensities were found to increase with temperature up to 900 °C and decrease thereafter for 1 Gy of test dose of ÎČ-rays from 90Sr-90Yr source. However, the maximum OSL sensitivity was found to be more than that of CaSO4:Eu microcrystalline phosphor (prepared by acid recrystallization method) contrary to the usually found in the literature but much less than that of commercially available α-Al2O3:C phosphor. The activation energy for thermally assisted OSL process was found to be 0.0572 ± 0.0028 eV. The dose ranges of TL and OSL response was found from 0.04 Gy to 100 Gy and 0.02 Gyâ100 Gy, respectively. The experimental results are also correlated with computational calculations based on density functional theory (DFT). The crystal structures and electronic structures of both hexagonal and orthorhombic CaSO4 and CaSO4:Eu materials show that they are direct band gap (5.67â5.86 eV) insulators, with Ca2+ substitution by Eu2+ found to introduce donor states in the band gap near Fermi level and the valence band edge of CaSO4 on doping with Eu2+ impurity ions
Physics of Neutron Star Crusts
The physics of neutron star crusts is vast, involving many different research
fields, from nuclear and condensed matter physics to general relativity. This
review summarizes the progress, which has been achieved over the last few
years, in modeling neutron star crusts, both at the microscopic and macroscopic
levels. The confrontation of these theoretical models with observations is also
briefly discussed.Comment: 182 pages, published version available at
<http://www.livingreviews.org/lrr-2008-10
Shear Viscosity and Oscillations of Neutron Star Crusts
We calculate the electron shear viscosity (determined by Coulomb electron
collisions) for a dense matter in a wide range of parameters typical for white
dwarf cores and neutron star crusts. In the density range from ~10^3 g cm^-3 to
10^7-10^10 g cm^-3 we consider the matter composed of widely abundant
astrophysical elements, from H to Fe. For higher densities, 10^10-10^14 g
cm^-3, we employ the ground-state nuclear composition, taking into account
finite sizes of atomic nuclei and the distribution of proton charge over the
nucleus. Numerical values of the viscosity are approximated by an analytic
expression convenient for applications. Using the approximation of
plane-parallel layer we study eigenfrequencies, eigenmodes and viscous damping
times of oscillations of high multipolarity, l~500-1000, localized in the outer
crust of a neutron star. For instance, at l~500 oscillations have frequencies f
>= 40 kHz and are localized not deeper than ~300 m from the surface. When the
crust temperature decreases from 10^9 K to 10^7 K, the dissipation time of
these oscillations (with a few radial nodes) decreases from ~1 year to 10-15
days.Comment: 23 pages, 13 figure
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