259 research outputs found
A Subgrid-scale Model for Deflagration-to-Detonation Transitions in Type Ia Supernova Explosion Simulations - Numerical implementation
A promising model for normal Type Ia supernova (SN Ia) explosions are delayed
detonations of Chandrasekhar-mass white dwarfs, in which the burning starts out
as a subsonic deflagration and turns at a later phase of the explosion into a
supersonic detonation. The mechanism of the underlying
deflagration-to-detonation transition (DDT) is unknown in detail, but necessary
conditions have been determined recently. The region of detonation initiation
cannot be spatially resolved in multi-dimensional full-star simulations of the
explosion. We develop a subgrid-scale (SGS) model for DDTs in thermonuclear
supernova simulations that is consistent with the currently known constraints.
The probability for a DDT to occur is calculated from the distribution of
turbulent velocities measured on the grid scale in the vicinity of the flame
and the fractal flame surface area that satisfies further physical constraints,
such as fuel fraction and fuel density. The implementation of our DDT criterion
provides a solid basis for simulations of thermonuclear supernova explosions in
the delayed detonation scenario. It accounts for the currently known necessary
conditions for the transition and avoids the inclusion of resolution-dependent
quantities in the model. The functionality of our DDT criterion is demonstrated
on the example of one three-dimensional thermonuclear supernova explosion
simulation.Comment: accepted for publication in Astronomy and Astrophysic
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
[Fe XIV] and [Fe XI] reveal the forward shock in SNR 1E0102.2-7219
Aims. We study the forward shock in the oxygen-rich young supernova remnant
(SNR) 1E0102.2-7219 (1E0102 in short) via optical coronal emission from [Fe
XIV] and [Fe XI]: emission lines which offer an alternative method to X-rays to
do so.
Methods. We have used the Multi-Unit Spectroscopic Explorer (MUSE) optical
integral field spectrograph at the Very Large Telescope (VLT) on Cerro Paranal
to obtain deep observations of SNR 1E0102 in the Small Magellanic Cloud. Our
observations cover the entire extent of the remnant with a seeing limited
spatial resolution of 0.7" = 0.2 pc at the distance of 1E 0102.
Results. Our MUSE observations unambiguously reveal the presence of [Fe XIV]
and [Fe XI] emission in 1E0102. The emission largely arises from a thin,
partial ring of filaments surrounding the fast moving O-rich ejecta in the
system. The brightest [Fe XIV] and [Fe XI] emission is found along the eastern
and north-western sides of 1E0102, where shocks are driven into denser ISM
material, while fainter emission along the northern edge reveals the location
of the forward shock in lower density gas, possibly the relic stellar wind
cavity. Modeling of the eastern shocks and the photoionization precursor
surrounding 1E0102, we derive a pre-shock density = (7.4 +-1.5)
cm, and a shock velocity 330 km/s < < 350 km/s.Comment: 4 pages, 4 figures, accepted for publications in A&A as a Letter to
the Edito
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
Nucleosynthesis in thermonuclear supernovae with tracers: convergence and variable mass particles
Nucleosynthetic yield predictions for multi-dimensional simulations of
thermonuclear supernovae generally rely on the tracer particle method to obtain
isotopic information of the ejected material for a given supernova simulation.
We investigate how many tracer particles are required to determine converged
integrated total nucleosynthetic yields. For this purpose, we conduct a
resolution study in the number of tracer particles for different hydrodynamical
explosion models at fixed spatial resolution. We perform hydrodynamic
simulations on a co-expanding Eulerian grid in two dimensions assuming
rotational symmetry for both pure deflagration and delayed detonation Type Ia
supernova explosions. Within a given explosion model, we vary the number of
tracer particles to determine the minimum needed for the method to give a
robust prediction of the integrated yields of the most abundant nuclides. For
the first time, we relax the usual assumption of constant tracer particle mass
and introduce a radially vary- ing distribution of tracer particle masses. We
find that the nucleosynthetic yields of the most abundant species (mass
fraction > 10E-5) are reasonably well predicted for a tracer number as small as
32 per axis and direction - more or less independent of the explosion model. We
conclude that the number of tracer particles that were used in extant published
works appear to have been sufficient as far as integrated yields are concerned
for the most copiously produced nuclides. Additionally we find that a suitably
chosen tracer mass distribution can improve convergence for nuclei produced in
the outer layer of the supernova where the constant tracer mass prescription
suffers from poor spatial resolution.Comment: 9 pages, 5 figures, accepted for publication in MNRA
Deflagrations in hybrid CONe white dwarfs: a route to explain the faint Type Iax supernova 2008ha
Stellar evolution models predict the existence of hybrid white dwarfs (WDs)
with a carbon-oxygen core surrounded by an oxygen-neon mantle. Being born with
masses ~1.1 Msun, hybrid WDs in a binary system may easily approach the
Chandrasekhar mass (MCh) by accretion and give rise to a thermonuclear
explosion. Here, we investigate an off-centre deflagration in a near-MCh hybrid
WD under the assumption that nuclear burning only occurs in carbon-rich
material. Performing hydrodynamics simulations of the explosion and detailed
nucleosynthesis post-processing calculations, we find that only 0.014 Msun of
material is ejected while the remainder of the mass stays bound. The ejecta
consist predominantly of iron-group elements, O, C, Si and S. We also calculate
synthetic observables for our model and find reasonable agreement with the
faint Type Iax SN 2008ha. This shows for the first time that deflagrations in
near-MCh WDs can in principle explain the observed diversity of Type Iax
supernovae. Leaving behind a near-MCh bound remnant opens the possibility for
recurrent explosions or a subsequent accretion-induced collapse in faint Type
Iax SNe, if further accretion episodes occur. From binary population synthesis
calculations, we find the rate of hybrid WDs approaching MCh to be on the order
of 1 percent of the Galactic SN Ia rate.Comment: 9 pages, 7 figures, 2 tables, accepted for publication in MNRA
LIN 358: A symbiotic binary accreting above the steady hydrogen fusion limit
Symbiotic binaries are long period interacting binaries consisting of a white
dwarf (WD) accreting material from a cool evolved giant star via stellar winds.
In this paper we study the symbiotic binary LIN 358 located in the SMC. We have
observed LIN 358 with the integral field spectrograph WiFeS and obtained its
line emission spectrum. With the help of the plasma simulation and spectral
synthesis code Cloudy, we have constructed a 2D photo-ionisation model of LIN
358. From comparison with the observations, we have determined the colour
temperature of the WD in LIN 358 to be 19 eV, its bolometric luminosity erg s, and the mass-loss rate from the
donor star to be M yr. Assuming a solar
H to He ratio in the wind material, a lower limit to the accreted mass fraction
in LIN 358 is 0.31. The high mass-accretion efficiency of a wind Roche lobe
overflow implies that the WD is accreting above the upper boundary of stable
hydrogen fusion and thus growing in mass with the maximal rate of M yr. This causes the WD photosphere to
expand, which explains its low colour temperature. Our calculations show that
the circumstellar material in LIN 358 is nearly completely ionized except for a
narrow cone around the donor star, and that the WD emission is freely escaping
the system. However, due to its low colour temperature, this emission can be
easily attenuated by even moderate amounts of neutral ISM. We speculate that
other symbiotic systems may be operating in a similar regime, thus explaining
the paucity of observed systems.Comment: 14 pages, 13 figures. Accepted for publication in MNRA
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