74 research outputs found
Asymmetry and the Nucleosynthetic Signature of Nearly Edge-Lit Detonation in White Dwarf Cores
Most of the leading explosion scenarios for Type Ia supernovae involve the
nuclear incineration of a white dwarf star through a detonation wave. Several
scenarios have been proposed as to how this detonation may actually occur, but
the exact mechanism and environment in which it takes place remain unknown. We
explore the effects of an off-center initiated detonation on the spatial
distribution of the nucleosynthetic yield products in a toy model -- a
pre-expanded near Chandrasekhar-mass white dwarf. We find that a single-point
near edge-lit detonation results in asymmetries in the density and thermal
profiles, notably the expansion timescale, throughout the supernova ejecta. We
demonstrate that this asymmetry of the thermodynamic trajectories should be
common to off-center detonations where a small amount of the star is burned
prior to detonation. The sensitivity of the yields on the expansion timescale
results in an asymmetric distribution of the elements synthesized as reaction
products. We tabulate the shift in the center of mass of the various elements
produced in our model supernova and find an odd-even pattern for elements past
silicon. Our calculations show that off-center single-point detonations in
carbon-oxygen white dwarfs are marked by significant composition asymmetries in
their remnants which bear potentially observable signatures in both velocity
and coordinate space, including an elemental nickel mass fraction which varies
by a factor of two to three from one side of the remnant to the other.Comment: 7 pages, 7 figures, accepted for publication in the Astrophysical
Journa
Effects of magnetic fields on the propagation of nuclear flames in magnetic white dwarfs
We investigate effects of the magnetic field on the propagation of laminar
flames of nuclear reactions taking place in white dwarfs (WDs) with the mass
close to the Chandrasekhar limit. We calculate the velocities of laminar flames
parallel and perpendicular to uniform magnetic fields as eigenvalues of steady
solutions for magnetic hydrodynamical equations. As a result, we find that even
when the magnetic pressure does not dominate the entire pressure it is possible
for the magnetic field to suppress the flame propagation through the thermal
conduction. Above the critical magnetic field, the flame velocity decreases
with increasing magnetic field strength as . In media with
densities of 10^{7}, 10^{8}, \,\mathrm{and}\,10^{9} \unit{g\,cm^{-3}}, the
critical magnetic fields are orders of \sim 10^{10}, 10^{11},
\,\mathrm{and}\,10^{12} \unit{G}, respectively.Comment: 9 pages, 8 figures, 1 tables, to appear in Ap
Evaluating Systematic Dependencies of Type Ia Supernovae: The Influence of Deflagration to Detonation Density
We explore the effects of the deflagration to detonation transition (DDT)
density on the production of Ni-56 in thermonuclear supernova explosions (type
Ia supernovae). Within the DDT paradigm, the transition density sets the amount
of expansion during the deflagration phase of the explosion and therefore the
amount of nuclear statistical equilibrium (NSE) material produced. We employ a
theoretical framework for a well-controlled statistical study of
two-dimensional simulations of thermonuclear supernovae with randomized initial
conditions that can, with a particular choice of transition density, produce a
similar average and range of Ni-56 masses to those inferred from observations.
Within this framework, we utilize a more realistic "simmered" white dwarf
progenitor model with a flame model and energetics scheme to calculate the
amount of Ni-56 and NSE material synthesized for a suite of simulated
explosions in which the transition density is varied in the range 1-3x10^7
g/cc. We find a quadratic dependence of the NSE yield on the log of the
transition density, which is determined by the competition between plume rise
and stellar expansion. By considering the effect of metallicity on the
transition density, we find the NSE yield decreases by 0.055 +/- 0.004 solar
masses for a 1 solar metallicity increase evaluated about solar metallicity.
For the same change in metallicity, this result translates to a 0.067 +/- 0.004
solar mass decrease in the Ni-56 yield, slightly stronger than that due to the
variation in electron fraction from the initial composition. Observations
testing the dependence of the yield on metallicity remain somewhat ambiguous,
but the dependence we find is comparable to that inferred from some studies.Comment: 15 pages, 13 figures, accepted to ApJ on July 6, 201
On Silicon Group Elements Ejected by Supernovae Type Ia
There is compelling evidence that the peak brightness of a Type Ia supernova
is affected by the electron fraction Ye at the time of the explosion. The
electron fraction is set by the aboriginal composition of the white dwarf and
the reactions that occur during the pre explosive convective burning. To date,
determining the makeup of the white dwarf progenitor has relied on indirect
proxies, such as the average metallicity of the host stellar population. In
this paper, we present analytical calculations supporting the idea that the
electron fraction of the progenitor systematically influences the
nucleosynthesis of silicon group ejecta in Type Ia supernovae. In particular,
we suggest the abundances generated in quasi nuclear statistical equilibrium
are preserved during the subsequent freezeout. This allows one to potential
recovery of Ye at explosion from the abundances recovered from an observed
spectra. We show that measurement of 28Si, 32S, 40Ca, and 54Fe abundances can
be used to construct Ye in the silicon rich regions of the supernovae. If these
four abundances are determined exactly, they are sufficient to recover Ye to 6
percent. This is because these isotopes dominate the composition of
silicon-rich material and iron rich material in quasi nuclear statistical
equilibrium. Analytical analysis shows that the 28Si abundance is insensitive
to Ye, the 32S abundance has a nearly linear trend with Ye, and the 40Ca
abundance has a nearly quadratic trend with Ye. We verify these trends with
post-processing of 1D models and show that these trends are reflected in model
synthetic spectra.Comment: Submitted to the Ap
The Laminar Flame Speedup by Neon-22 Enrichment in White Dwarf Supernovae
Carbon-oxygen white dwarfs contain neon-22 formed from alpha-captures onto
nitrogen during core He burning in the progenitor star. In a white dwarf (type
Ia) supernova, the neon-22 abundance determines, in part, the neutron-to-proton
ratio and hence the abundance of radioactive nickel-56 that powers the
lightcurve. The neon-22 abundance also changes the burning rate and hence the
laminar flame speed. We tabulate the flame speedup for different initial carbon
and neon-22 abundances and for a range of densities. This increase in the
laminar flame speed--about 30% for a neon-22 mass fraction of 6%--affects the
deflagration just after ignition near the center of the white dwarf, where the
laminar speed of the flame dominates over the buoyant rise, and in regions of
lower density ~ 10^7 g/cm3 where a transition to distributed burning is
conjectured to occur. The increase in flame speed will decrease the density of
any transition to distributed burning.Comment: 5 pages, 2 figures, to be published in ApJ Letters. Table 2 is
avalible from http://www.pa.msu.edu/~ebrown/Research/typeIa
MAESTRO: An Adaptive Low Mach Number Hydrodynamics Algorithm for Stellar Flows
Many astrophysical phenomena are highly subsonic, requiring specialized
numerical methods suitable for long-time integration. In a series of earlier
papers we described the development of MAESTRO, a low Mach number stellar
hydrodynamics code that can be used to simulate long-time, low-speed flows that
would be prohibitively expensive to model using traditional compressible codes.
MAESTRO is based on an equation set derived using low Mach number asymptotics;
this equation set does not explicitly track acoustic waves and thus allows a
significant increase in the time step. MAESTRO is suitable for two- and
three-dimensional local atmospheric flows as well as three-dimensional
full-star flows. Here, we continue the development of MAESTRO by incorporating
adaptive mesh refinement (AMR). The primary difference between MAESTRO and
other structured grid AMR approaches for incompressible and low Mach number
flows is the presence of the time-dependent base state, whose evolution is
coupled to the evolution of the full solution. We also describe how to
incorporate the expansion of the base state for full-star flows, which involves
a novel mapping technique between the one-dimensional base state and the
Cartesian grid, as well as a number of overall improvements to the algorithm.
We examine the efficiency and accuracy of our adaptive code, and demonstrate
that it is suitable for further study of our initial scientific application,
the convective phase of Type Ia supernovae.Comment: Accepted to Astrophysical Journal Suppliment (http://iop.org). 56
pages, 15 figures
The Reduction of the Electron Abundance during the Pre-explosion Simmering in White Dwarf Supernovae
Prior to the explosion of a carbon-oxygen white dwarf in a Type Ia supernova
there is a long "simmering," during which the 12C + 12C reaction gradually
heats the white dwarf on a long (~ 1000 yr) timescale. Piro & Bildsten showed
that weak reactions during this simmering set a maximum electron abundance Ye
at the time of the explosion. We investigate the nuclear reactions during this
simmering with a series of self-heating, at constant pressure, reaction network
calculations. Unlike in AGB stars, proton captures onto 22Ne and heavier trace
nuclei do not play a significant role. The 12C abundance is sufficiently high
that the neutrons preferentially capture onto 12C, rather than iron group
nuclei. As an aid to hydrodynamical simulations of the simmering phase, we
present fits to the rates of heating, electron capture, change in mean atomic
mass, and consumption of 12C in terms of the screened thermally averaged cross
section for 12C + 12C. Our evaluation of the net heating rate includes
contributions from electron captures into the 3.68 MeV excited state of 13C.
This results in a slightly larger energy release, per 12C consumed, than that
found by Piro & Bildsten, but less than that released for a burn to only 20Ne
and 23Na. We compare our one-zone results to more accurate integrations over
the white dwarf structure to estimate the amount of 12C that must be consumed
to raise the white dwarf temperature, and hence to determine the net reduction
of Ye during simmering.Comment: Accepted for publication in The Astrophysical Journal, 9 pages, 6
figure
Study of the Detonation Phase in the Gravitationally Confined Detonation Model of Type Ia Supernovae
We study the gravitationally confined detonation (GCD) model of Type Ia
supernovae through the detonation phase and into homologous expansion. In the
GCD model, a detonation is triggered by the surface flow due to single point,
off-center flame ignition in carbon-oxygen white dwarfs. The simulations are
unique in terms of the degree to which non-idealized physics is used to treat
the reactive flow, including weak reaction rates and a time dependent treatment
of material in nuclear statistical equilibrium (NSE). Careful attention is paid
to accurately calculating the final composition of material which is burned to
NSE and frozen out in the rapid expansion following the passage of a detonation
wave over the high density core of the white dwarf; and an efficient method for
nucleosynthesis post-processing is developed which obviates the need for costly
network calculations along tracer particle thermodynamic trajectories.
Observational diagnostics are presented for the explosion models, including
abundance stratifications and integrated yields. We find that for all of the
ignition conditions studied here, a self regulating process comprised of
neutronization and stellar expansion results in final \iso{Ni}{56} masses of
1.1\msun. But, more energetic models result in larger total NSE and
stable Fe peak yields. The total yield of intermediate mass elements is
\msun and the explosion energies are all around 1.5
ergs. The explosion models are briefly compared to the inferred properties of
recent Type Ia supernova observations. The potential for surface detonation
models to produce lower luminosity (lower \iso{Ni}{56} mass) supernovae is
discussed.Comment: 43 pages, 4 tables, 20 figures -- submitted to Ap
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