998 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&
On Type Ia Supernovae From The Collisions of Two White Dwarfs
We explore collisions between two white dwarfs as a pathway for making Type
Ia Supernovae (SNIa). White dwarf number densities in globular clusters allow
10-100 redshift <1 collisions per year, and observations by (Chomiuk et al.
2008) of globular clusters in the nearby S0 galaxy NGC 7457 have detected what
is likely to be a SNIa remnant. We carry out simulations of the collision
between two 0.6 solar mass white dwarfs at various impact parameters and mass
resolutions. For impact parameters less than half the radius of the white
dwarf, we find such collisions produce approximately 0.4 solar masses of Ni56,
making such events potential candidates for underluminous SNIa or a new class
of transients between Novae and SNIa.Comment: 4 pages, 4 figures, 1 tabl
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
Surface Detonations in Double Degenerate Binary Systems Triggered by Accretion Stream Instabilities
We present three-dimensional simulations on a new mechanism for the
detonation of a sub-Chandrasekhar CO white dwarf in a dynamically unstable
system where the secondary is either a pure He white dwarf or a He/CO hybrid.
For dynamically unstable systems where the accretion stream directly impacts
the surface of the primary, the final tens of orbits can have mass accretion
rates that range from to s, leading to the
rapid accumulation of helium on the surface of the primary. After of helium has been accreted, the ram pressure of the hot
helium torus can deflect the accretion stream such that the stream no longer
directly impacts the surface. The velocity difference between the stream and
the torus produces shearing which seeds large-scale Kelvin-Helmholtz
instabilities along the interface between the two regions. These instabilities
eventually grow into dense knots of material that periodically strike the
surface of the primary, adiabatically compressing the underlying helium torus.
If the temperature of the compressed material is raised above a critical
temperature, the timescale for triple- reactions becomes comparable to
the dynamical timescale, leading to the detonation of the primary's helium
envelope. This detonation drives shockwaves into the primary which tend to
concentrate at one or more focal points within the primary's CO core. If a
relatively small amount of mass is raised above a critical temperature and
density at these focal points, the CO core may itself be detonated.Comment: 6 pages, 4 figures, 1 table. Submitted to ApJL. For a high-resolution
version, movies, and other supporting material see
http://www.ucolick.org/~jfg/projects/double-white-dwarf-accretion
Making Black Holes in Supernovae
The possibility of making stellar mass black holes in supernovae that
otherwise produce viable Type II and Ib supernova explosions is discussed and
estimates given of their number in the Milky Way Galaxy. Observational
diagnostics of stellar mass black hole formation are reviewed. While the
equation of state sets the critical mass, fall back during the explosion is an
equally important (and uncertain) element in determining if a black hole is
formed. SN 1987A may or may not harbor a black hole, but if the critical mass
for neutron stars is 1.5 - 1.6 M\sun, as Brown and Bethe suggest, it probably
does. Observations alone do not yet resolve the issue. Reasons for this state
of ambiguity are discussed and suggestions given as to how gamma-ray and x-ray
observations in the future might help.Comment: 14 pages, uuencoded gzipped postscript, Accepted Nuclear Physics A,
Gerry Brown Festschrift contributio
Laterally Propagating Detonations in Thin Helium Layers on Accreting White Dwarfs
Theoretical work has shown that intermediate mass (0.01Msun<M_He<0.1Msun)
Helium shells will unstably ignite on the accreting white dwarf (WD) in an AM
CVn binary. For more massive (M>0.8Msun) WDs, these helium shells can be dense
enough (5x10^5 g/cc) that the convectively burning region runs away on a
timescale comparable to the sound travel time across the shell; raising the
possibility for an explosive outcome. The nature of the explosion (i.e.
deflagration or detonation) remains ambiguous. In the case of detonation, this
causes a laterally propagating front whose properties in these geometrically
thin and low density shells we begin to study here. Our calculations show that
the radial expansion time of <0.1 s leads to incomplete helium burning, in
agreement with recent work by Sim and collaborators, but that the nuclear
energy released is still adequate to realize a self-sustaining detonation
propagating laterally at slower than the Chapman-Jouguet speed. Our simulations
resolve the subsonic region behind the front and are consistent with a direct
computation of the reaction structure from the shock strength. The ashes are
typically He rich, and consist of predominantly Ti-44, Cr-48, along with a
small amount of Fe-52, with very little Ni-56 and with significant Ca-40 in
carbon-enriched layers. If this helium detonation results in a Type Ia
Supernova, its spectral signatures would appear for the first few days after
explosion. (abridged)Comment: 7 pages, 5 figure, accepted to the Astrophysical Journa
The light curve of SN 1987A revisited: constraining production masses of radioactive nuclides
We revisit the evidence for the contribution of the long-lived radioactive
nuclides 44Ti, 55Fe, 56Co, 57Co, and 60Co to the UVOIR light curve of SN 1987A.
We show that the V-band luminosity constitutes a roughly constant fraction of
the bolometric luminosity between 900 and 1900 days, and we obtain an
approximate bolometric light curve out to 4334 days by scaling the late time
V-band data by a constant factor where no bolometric light curve data is
available. Considering the five most relevant decay chains starting at 44Ti,
55Co, 56Ni, 57Ni, and 60Co, we perform a least squares fit to the constructed
composite bolometric light curve. For the nickel isotopes, we obtain best fit
values of M(56Ni) = (7.1 +- 0.3) x 10^{-2} Msun and M(57Ni) = (4.1 +- 1.8) x
10^{-3} Msun. Our best fit 44Ti mass is M(44Ti) = (0.55 +- 0.17) x 10^{-4}
Msun, which is in disagreement with the much higher (3.1 +- 0.8) x 10^{-4} Msun
recently derived from INTEGRAL observations. The associated uncertainties far
exceed the best fit values for 55Co and 60Co and, as a result, we only give
upper limits on the production masses of M(55Co) < 7.2 x 10^{-3} Msun and
M(60Co) < 1.7 x 10^{-4} Msun. Furthermore, we find that the leptonic channels
in the decay of 57Co (internal conversion and Auger electrons) are a
significant contribution and constitute up to 15.5% of the total luminosity.
Consideration of the kinetic energy of these electrons is essential in lowering
our best fit nickel isotope production ratio to [57Ni/56Ni]=2.5+-1.1, which is
still somewhat high but is in agreement with gamma-ray observations and model
predictions.Comment: 7 pages, 6 pages, 2 table
On Carbon Burning in Super Asymptotic Giant Branch Stars
We explore the detailed and broad properties of carbon burning in Super
Asymptotic Giant Branch (SAGB) stars with 2755 MESA stellar evolution models.
The location of first carbon ignition, quenching location of the carbon burning
flames and flashes, angular frequency of the carbon core, and carbon core mass
are studied as a function of the ZAMS mass, initial rotation rate, and mixing
parameters such as convective overshoot, semiconvection, thermohaline and
angular momentum transport. In general terms, we find these properties of
carbon burning in SAGB models are not a strong function of the initial rotation
profile, but are a sensitive function of the overshoot parameter. We
quasi-analytically derive an approximate ignition density, g cm, to predict the location of first carbon ignition
in models that ignite carbon off-center. We also find that overshoot moves the
ZAMS mass boundaries where off-center carbon ignition occurs at a nearly
uniform rate of / 1.6
. For zero overshoot, =0.0, our models in the ZAMS mass
range 8.9 to 11 show off-center carbon ignition. For
canonical amounts of overshooting, =0.016, the off-center carbon
ignition range shifts to 7.2 to 8.8 . Only systems with
and ZAMS mass 7.2-8.0 show
carbon burning is quenched a significant distance from the center. These
results suggest a careful assessment of overshoot modeling approximations on
claims that carbon burning quenches an appreciable distance from the center of
the carbon core.Comment: Accepted ApJ; 23 pages, 21 figures, 5 table
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