986 research outputs found
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
The Luminous and Carbon-Rich Supernova 2006gz: A Double Degenerate Merger?
Spectra and light curves of SN 2006gz show the strongest signature of
unburned carbon and one of the slowest fading light curves ever seen in a type
Ia event (Delta m_15 = 0.69 +/- 0.04). The early-time Si II velocity is low,
implying it was slowed by an envelope of unburned material. Our best estimate
of the luminosity implies M_V = -19.74 and the production of ~ 1.2 M_sun of
56Ni. This suggests a super-Chandrasekhar mass progenitor. A double degenerate
merger is consistent with these observations.Comment: Accepted for publication in ApJL (5 pages, 4 figures). UBVr'i' light
curves, UVOIR light curves, and spectra available at
http://www.cfa.harvard.edu/supernova/SN2006g
Instabilities and Clumping in Type Ia Supernova Remnants
We present two-dimensional high-resolution hydrodynamical simulations in
spherical polar coordinates of a Type Ia supernova interacting with a constant
density interstellar medium. The ejecta are assumed to be freely expanding with
an exponential density profile. The interaction gives rise to a double-shocked
structure susceptible to hydrodynamic instabilities. The Rayleigh-Taylor
instability initially grows, but the Kelvin-Helmholtz instability takes over,
producing vortex rings. The nonlinear instability initially evolves toward
longer wavelengths and eventually fades away when the reverse shock front is in
the flatter part of the supernova density distribution. Based on observations
of X-ray knots and the protrusion in the southeast outlin of Tycho's supernova
remnant, we include clumping in the ejecta. The clump interaction with the
reverse shock induces Rayleigh-Taylor and Kelvin-Helmholtz instabilities on the
clump surface that facilitate fragmentation. In order to survive crushing and
to have a bulging effect on the forward shock, the clump's initial density
ratio to the surrounding ejecta must be at least 100 for the conditions in
Tycho's remnant. The 56Ni bubble effect may be important for the development of
clumpiness in the ejecta. The observed presence of an Fe clump would then
require a non-radioactive origin for this Fe, possibly 54Fe. The large radial
distance of the X-ray emitting Si and S ejecta from the remnant center
indicates that they were initially in clumps.Comment: 27 pages, 4 postscript figures, 5 GIF figures submitted to
Astrophysical Journa
Analysis of the Flux and Polarization Spectra of the Type Ia Supernova SN 2001el: Exploring the Geometry of the High-velocity Ejecta
SN 2001el is the first normal Type Ia supernova to show a strong, intrinsic
polarization signal. In addition, during the epochs prior to maximum light, the
CaII IR triplet absorption is seen distinctly and separately at both normal
photospheric velocities and at very high velocities. The high-velocity triplet
absorption is highly polarized, with a different polarization angle than the
rest of the spectrum. The unique observation allows us to construct a
relatively detailed picture of the layered geometrical structure of the
supernova ejecta: in our interpretation, the ejecta layers near the photosphere
(v \approx 10,000 km/s) obey a near axial symmetry, while a detached,
high-velocity structure (v \approx 18,000-25,000 km/s) with high CaII line
opacity deviates from the photospheric axisymmetry. By partially obscuring the
underlying photosphere, the high-velocity structure causes a more incomplete
cancellation of the polarization of the photospheric light, and so gives rise
to the polarization peak and rotated polarization angle of the high-velocity IR
triplet feature. In an effort to constrain the ejecta geometry, we develop a
technique for calculating 3-D synthetic polarization spectra and use it to
generate polarization profiles for several parameterized configurations. In
particular, we examine the case where the inner ejecta layers are ellipsoidal
and the outer, high-velocity structure is one of four possibilities: a
spherical shell, an ellipsoidal shell, a clumped shell, or a toroid. The
synthetic spectra rule out the spherical shell model, disfavor a toroid, and
find a best fit with the clumped shell. We show further that different
geometries can be more clearly discriminated if observations are obtained from
several different lines of sight.Comment: 14 pages (emulateapj5) plus 18 figures, accepted by The Astrophysical
Journa
Reconstruction of the optical potential from scattering data
We propose a method for reconstruction of the optical potential from
scattering data. The algorithm is a two-step procedure. In the first step the
real part of the potential is determined analytically via solution of the
Marchenko equation. At this point we use a diagonal Pad\'{e} approximant of the
corresponding unitary -matrix. In the second step the imaginary part of the
potential is determined via the phase equation of the variable phase approach.
We assume that the real and the imaginary parts of the optical potential are
proportional. We use the phase equation to calculate the proportionality
coefficient. A numerical algorithm is developed for a single and for coupled
partial waves. The developed procedure is applied to analysis of
, , and data.Comment: 26 pages, 8 figures, results of nucl-th/0410092 are refined, some new
results are presente
Nematic Ordering of Rigid Rods in a Gravitational Field
The isotropic-to-nematic transition in an athermal solution of long rigid
rods subject to a gravitational (or centrifugal) field is theoretically
considered in the Onsager approximation. The new feature emerging in the
presence of gravity is a concentration gradient which coupled with the nematic
ordering. For rodlike molecules this effect becomes noticeable at centrifugal
acceleration g ~ 10^3--10^4 m/s^2, while for biological rodlike objects, such
as tobacco mosaic virus, TMV, the effect is important even for normal
gravitational acceleration conditions. Rods are concentrated near the bottom of
the vessel which sometimes leads to gravity induced nematic ordering. The
concentration range corresponding to phase separation increases with increasing
g. In the region of phase separation the local rod concentration, as well as
the order parameter, follow a step function with height.Comment: Full article http://prola.aps.org/abstract/PRE/v60/i3/p2973_
The Role of Electron Captures in Chandrasekhar Mass Models for Type Ia Supernovae
The Chandrasekhar mass model for Type Ia Supernovae (SNe Ia) has received
increasing support from recent comparisons of observations with light curve
predictions and modeling of synthetic spectra. It explains SN Ia events via
thermonuclear explosions of accreting white dwarfs in binary stellar systems,
being caused by central carbon ignition when the white dwarf approaches the
Chandrasekhar mass. As the electron gas in white dwarfs is degenerate,
characterized by high Fermi energies for the high density regions in the
center, electron capture on intermediate mass and Fe-group nuclei plays an
important role in explosive burning. Electron capture affects the central
electron fraction Y_e, which determines the composition of the ejecta from such
explosions. Up to the present, astrophysical tabulations based on shell model
matrix elements were only available for light nuclei in the sd-shell. Recently
new Shell Model Monte Carlo (SMMC) and large-scale shell model diagonalization
calculations have also been performed for pf-shell nuclei. These lead in
general to a reduction of electron capture rates in comparison with previous,
more phenomenological, approaches. Making use of these new shell model based
rates, we present the first results for the composition of Fe-group nuclei
produced in the central regions of SNe Ia and possible changes in the
constraints on model parameters like ignition densities and burning front
speeds.Comment: 26 pages, 8 figures, submitted to Ap
Double-detonation sub-Chandrasekhar supernovae: synthetic observables for minimum helium shell mass models
Abridged. In the double detonation scenario for Type Ia supernovae (SNe Ia) a
detonation initiates in a shell of He-rich material accreted from a companion
star by a sub-Chandrasekhar-mass White Dwarf (WD). This shell detonation drives
a shock front into the carbon-oxygen (C/O) WD that triggers a secondary
detonation in the core. The core detonation results in a complete disruption of
the WD. Earlier studies concluded that this scenario has difficulties in
accounting for the observed properties of SNe Ia since the explosion ejecta are
surrounded by the products of explosive He burning in the shell. Recently, it
was proposed that detonations might be possible for much less massive He shells
than previously assumed. Moreover, it was shown that even detonations of these
minimum He shell masses robustly trigger detonations of the C/O core. Here we
present time-dependent multi-wavelength radiative transfer calculations for
models with minimum He shell mass and derive synthetic observables for both the
optical and {\gamma}-ray spectral regions. These differ strongly from those
found in earlier simulations of sub-Chandrasekhar-mass explosions in which more
massive He shells were considered. Our models predict light curves which cover
both the range of brightnesses and the rise and decline times of observed SNe
Ia. However, their colours and spectra do not match the observations. In
particular, their B-V colours are generally too red. We show that this
discrepancy is mainly due to the composition of the burning products of the He
shell of our models which contain significant amounts of Ti and Cr. Using a toy
model, we also show that the burning products of the He shell depend crucially
on its initial composition. This leads us to conclude that good agreement
between sub-Chandrasekhar-mass explosions and observed SNe Ia may still be
feasible but further study of the shell properties is required.Comment: 17 pages, 13 figures. Accepted for publication by Ap
Constraints on Type Ia Supernova Models from X-ray Spectra of Galaxy Clusters
We present constraints on theoretical models of Type Ia supernovae using
spatially resolved ASCA X-ray spectroscopy of three galaxy clusters: Abell 496,
Abell 2199 and Abell 3571. All three clusters have central iron abundance
enhancements; an ensemble of abundance ratios are used to show that most of the
iron in the central regions of the clusters comes from SN Ia. These
observations are consistent with the suppressed galactic wind scenario proposed
by Dupke and White (1999). At the center of each cluster, simultaneous analysis
of spectra from all ASCA instruments shows that the nickel to iron abundance
ratio (normalized by the solar ratio) is Ni/Fe ~ 4. We use the nickel to iron
ratio as a discriminator between SN Ia explosion models: the Ni/Fe ratio of
ejecta from the "Convective Deflagration" model W7 is consistent with the
observations, while those of "delayed detonation" models are not consistent at
the 90% confidence level.Comment: 20 pages, 2 figures, accepted by The Astrophysical Journa
Evolution of bias in different cosmological models
We study the evolution of the halo-halo correlation function and bias in four
cosmological models (LCDM, OCDM, tauCDM, and SCDM) using very high-resolution
N-body simulations. The high force and mass resolution allows dark matter (DM)
halos to survive in the tidal fields of high-density regions and thus prevents
the ambiguities related with the ``overmerging problem.'' This allows us to
estimate for the first time the evolution of the correlation function and bias
at small (down to ~100/h kpc) scales. We find that at all epochs the 2-point
correlation function of galaxy-size halos xi_hh is well approximated by a
power-law with slope ~1.6-1.8. The difference between the shape of xi_hh and
the shape of the correlation function of matter results in the scale-dependent
bias at scales <7/h Mpc, which we find to be a generic prediction of the
hierarchical models. The bias evolves rapidly from a high value of ~2-5 at
z~3-7 to the anti-bias of b~0.5-1 at small <5/h Mpc scales at z=0. We find that
our results agree well with existing clustering data at different redshifts.
Particularly, we find an excellent agreement in both slope and the amplitude
between xi_hh(z=0) in our LCDM simulation and the galaxy correlation function
measured using the APM galaxy survey. At high redshifts, the observed
clustering of the Lyman-break galaxies is also well reproduced by the models.
The agreement with the data at high and low z indicates the general success of
the hierarchical models of structure formation in which galaxies form inside
the host DM halos. (Abridged)Comment: submitted to the Astrophys.Journal; 21 pages, LaTeX (uses
emulateapj.sty); full resolution versions of figs.1 and 2 are available at
http://astro.nmsu.edu/~akravtso/GROUP/group_publications.html or at
ftp://charon.nmsu.edu/pub/kravtsov/PAPERS/Bias
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