421 research outputs found
C+O detonations in thermonuclear supernovae: Interaction with previously burned material
In the context of explosion models for Type Ia Supernovae, we present one-
and two-dimensional simulations of fully resolved detonation fronts in
degenerate C+O White Dwarf matter including clumps of previously burned
material. The ability of detonations to survive the passage through sheets of
nuclear ashes is tested as a function of the width and composition of the ash
region. We show that detonation fronts are quenched by microscopically thin
obstacles with little sensitivity to the exact ash composition. Front-tracking
models for detonations in macroscopic explosion simulations need to include
this effect in order to predict the amount of unburned material in delayed
detonation scenarios.Comment: 6 pages, 9 figures, uses isotope.sty, accepted for publication in A&
Proton-Rich Nuclear Statistical Equilibrium
Proton-rich material in a state of nuclear statistical equilibrium (NSE) is
one of the least studied regimes of nucleosynthesis. One reason for this is
that after hydrogen burning, stellar evolution proceeds at conditions of equal
number of neutrons and protons or at a slight degree of neutron-richness.
Proton-rich nucleosynthesis in stars tends to occur only when hydrogen-rich
material that accretes onto a white dwarf or neutron star explodes, or when
neutrino interactions in the winds from a nascent proto-neutron star or
collapsar-disk drive the matter proton-rich prior to or during the
nucleosynthesis. In this paper we solve the NSE equations for a range of
proton-rich thermodynamic conditions. We show that cold proton-rich NSE is
qualitatively different from neutron-rich NSE. Instead of being dominated by
the Fe-peak nuclei with the largest binding energy per nucleon that have a
proton to nucleon ratio close to the prescribed electron fraction, NSE for
proton-rich material near freeze-out temperature is mainly composed of Ni56 and
free protons. Previous results of nuclear reaction network calculations rely on
this non-intuitive high proton abundance, which this paper will explain. We
show how the differences and especially the large fraction of free protons
arises from the minimization of the free energy as a result of a delicate
competition between the entropy and the nuclear binding energy.Comment: 4 pages, 7 figure
Neutrinos from beta processes in a presupernova: probing the isotopic evolution of a massive star
We present a new calculation of the neutrino flux received at Earth from a
massive star in the hours of evolution prior to its explosion as a
supernova (presupernova). Using the stellar evolution code MESA, the neutrino
emissivity in each flavor is calculated at many radial zones and time steps. In
addition to thermal processes, neutrino production via beta processes is
modeled in detail, using a network of 204 isotopes. We find that the total
produced flux has a high energy spectrum tail, at
MeV, which is mostly due to decay and electron capture on isotopes with . In a tentative window of observability of MeV and hours pre-collapse, the contribution of beta processes to the flux
is at the level of . For a star at kpc distance, a 17 kt
liquid scintillator detector would typically observe several tens of events
from a presupernova, of which up to due to beta processes. These
processes dominate the signal at a liquid argon detector, thus greatly
enhancing its sensitivity to a presupernova.Comment: 14 pages, 5 figure
Constraints on explosive silicon burning in core-collapse supernovae from measured Ni/Fe ratios
Measurements of explosive nucleosynthesis yields in core-collapse supernovae
provide tests for explosion models. We investigate constraints on explosive
conditions derivable from measured amounts of nickel and iron after radioactive
decays using nucleosynthesis networks with parameterized thermodynamic
trajectories. The Ni/Fe ratio is for most regimes dominated by the production
ratio of 58Ni/(54Fe + 56Ni), which tends to grow with higher neutron excess and
with higher entropy. For SN 2012ec, a supernova that produced a Ni/Fe ratio of
times solar, we find that burning of a fuel with neutron excess
is required. Unless the progenitor metallicity
is over 5 times solar, the only layer in the progenitor with such a neutron
excess is the silicon shell. Supernovae producing large amounts of stable
nickel thus suggest that this deep-lying layer can be, at least partially,
ejected in the explosion. We find that common spherically symmetric models of
Msun stars exploding with a delay time of less than
one second ( Msun) are able to achieve such silicon-shell
ejection. Supernovae that produce solar or sub-solar Ni/Fe ratios, such as SN
1987A, must instead have burnt and ejected only oxygen-shell material, which
allows a lower limit to the mass cut to be set. Finally, we find that the
extreme Ni/Fe value of 60-75 times solar derived for the Crab cannot be
reproduced by any realistic-entropy burning outside the iron core, and
neutrino-neutronization obtained in electron-capture models remains the only
viable explanation.Comment: 13 pages, 9 figures, accepted for publication in Ap
Low Mach Number Modeling of Type Ia Supernovae. IV. White Dwarf Convection
We present the first three-dimensional, full-star simulations of convection
in a white dwarf preceding a Type Ia supernova, specifically the last few hours
before ignition. For these long-time calculations we use our low Mach number
hydrodynamics code, MAESTRO, which we have further developed to treat spherical
stars centered in a three-dimensional Cartesian geometry. The main change
required is a procedure to map the one-dimensional radial base state to and
from the Cartesian grid. Our models recover the dipole structure of the flow
seen in previous calculations, but our long-time integration shows that the
orientation of the dipole changes with time. Furthermore, we show the
development of gravity waves in the outer, stable portion of the star. Finally,
we evolve several calculations to the point of ignition and discuss the range
of ignition radii.Comment: 42 pages, some figures degraded to conserve space. Accepted to The
Astrophysical Journal (http://journals.iop.org/
Turbulent Chemical Diffusion in Convectively Bounded Carbon Flames
It has been proposed that mixing induced by convective overshoot can disrupt
the inward propagation of carbon deflagrations in super-asymptotic giant branch
stars. To test this theory, we study an idealized model of convectively bounded
carbon flames with 3D hydrodynamic simulations of the Boussinesq equations
using the pseudospectral code Dedalus. Because the flame propagation timescale
is much longer than the convection timescale, we approximate the flame as fixed
in space, and only consider its effects on the buoyancy of the fluid. By
evolving a passive scalar field, we derive a {\it turbulent} chemical
diffusivity produced by the convection as a function of height, .
Convection can stall a flame if the chemical mixing timescale, set by the
turbulent chemical diffusivity, , is shorter than the flame
propagation timescale, set by the thermal diffusivity, , i.e., when
. However, we find for most of the flame
because convective plumes are not dense enough to penetrate into the flame.
Extrapolating to realistic stellar conditions, this implies that convective
mixing cannot stall a carbon flame and that "hybrid carbon-oxygen-neon" white
dwarfs are not a typical product of stellar evolution.Comment: Accepted to Ap
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