35 research outputs found
Silicon Burning II: Quasi-Equilibrium and Explosive Burning
Having examined the application of quasi-equilibrium to hydrostatic silicon
burning in Paper I of this series, Hix & Thielemann (1996), we now turn our
attention to explosive silicon burning. Previous authors have shown that for
material which is heated to high temperature by a passing shock and then cooled
by adiabatic expansion, the results can be divided into three broad categories;
\emph{incomplete burning}, \emph{normal freezeout} and \emph{-rich
freezeout}, with the outcome depending on the temperature, density and cooling
timescale. In all three cases, we find that the important abundances obey
quasi-equilibrium for temperatures greater than approximately 3 GK, with
relatively little nucleosynthesis occurring following the breakdown of
quasi-equilibrium. We will show that quasi-equilibrium provides better
abundance estimates than global nuclear statistical equilibrium, even for
normal freezeout and particularly for -rich freezeout. We will also
examine the accuracy with which the final nuclear abundances can be estimated
from quasi-equilibrium.Comment: 27 pages, including 15 inline figures. LaTeX 2e with aaspp4 and
graphicx packages. Accepted to Ap
Type Ia supernovae: Can Coriolis force break the symmetry of the gravitational confined detonation explosion mechanism?
© 2016. The American Astronomical Society. All rights reserved.. Currently the number of models aimed at explaining the phenomena of type Ia supernovae is high and distinguishing between them is a must. In this work we explore the influence of rotation on the evolution of the nuclear flame that drives the explosion in the so-called gravitational confined detonation models. Assuming that the flame starts in a pointlike region slightly above the center of the white dwarf (WD) and adding a moderate amount of angular velocity to the star we follow the evolution of the deflagration using a smoothed particle hydrodynamics code. We find that the results are very dependent on the angle between the rotational axis and the line connecting the initial bubble of burned material with the center of the WD at the moment of ignition. The impact of rotation is larger for angles close to 90° because the Coriolis force on a floating element of fluid is maximum and its principal effect is to break the symmetry of the deflagration. Such symmetry breaking weakens the convergence of the nuclear flame at the antipodes of the initial ignition volume, changing the environmental conditions around the convergence region with respect to non-rotating models. These changes seem to disfavor the emergence of a detonation in the compressed volume at the antipodes and may compromise the viability of the so-called gravitational confined detonation mechanism.Postprint (author's final draft
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
Sub-luminous type Ia supernovae from the mergers of equal-mass white dwarfs with M~0.9 M_sun
Type Ia supernovae (SNe Ia) are thought to result from thermonuclear
explosions of carbon-oxygen white dwarf stars. Existing models generally
explain the observed properties, with the exception of the sub-luminous
1991-bg-like supernovae. It has long been suspected that the merger of two
white dwarfs could give rise to a type Ia event, but hitherto simulations have
failed to produce an explosion. Here we report a simulation of the merger of
two equal-mass white dwarfs that leads to an underluminous explosion, though at
the expense of requiring a single common-envelope phase, and component masses
of ~0.9 M_sun. The light curve is too broad, but the synthesized spectra, red
colour and low expansion velocities are all close to what is observed for
sub-luminous 1991bg-like events. While mass ratios can be slightly less than
one and still produce an underluminous event, the masses have to be in the
range 0.83-0.9 M_sun.Comment: Accepted to Natur
Nucleosynthesis in Type II supernovae and the abundances in metal-poor stars
We explore the effects on nucleosynthesis in Type II supernovae of various
parameters (mass cut, neutron excess, explosion energy, progenitor mass) in
order to explain the observed trends of the iron-peak element abundance ratios
([Cr/Fe], [Mn/Fe], [Co/Fe] and [Ni/Fe]) in halo stars as a function of
metallicity for the range [Fe/H] . [Cr/Fe] and [Mn/Fe]
decrease with decreasing [Fe/H], while [Co/Fe] behaves the opposite way and
increases. We show that such a behavior can be explained by a variation of mass
cuts in Type II supernovae as a function of progenitor mass, which provides a
changing mix of nucleosynthesis from an alpha-rich freeze-out of Si-burning and
incomplete Si-burning. This explanation is consistent with the amount of
ejected Ni determined from modeling the early light curves of individual
supernovae. We also suggest that the ratio [H/Fe] of halo stars is mainly
determined by the mass of interstellar hydrogen mixed with the ejecta of a
single supernova which is larger for larger explosion energy and the larger
Str\"omgren radius of the progenitor.Comment: 17 pages, LaTeX, Accepted for publication in the Astrophysical
Journal, more discussion on the Galactic chemical evolutio
Conformally Flat Smoothed Particle Hydrodynamics: Application to Neutron Star Mergers
We present a new 3D SPH code which solves the general relativistic field +
hydrodynamics equations in the conformally flat approximation. Several test
cases are considered to test different aspects of the code. We finally apply
then the code to the coalescence of a neutron star binary system. The neutron
stars are modeled by a polytropic equation of state (EoS) with adiabatic
indices , and . We calculate the
gravitational wave signals, luminosities and frequency spectra by employing the
quadrupole approximation for emission and back reaction in the slow motion
limit. In addition, we consider the amount of ejected mass.Comment: 23 pages, 12 figures. Accepted for publication in Phys. Rev. D. v3:
Final Versio
Nuclear Level Density and the Determination of Thermonuclear Rates for Astrophysics
The prediction of cross sections for nuclei far off stability is crucial in
the field of nuclear astrophysics. We discuss the model mostly employed for
such calculations: the statistical model (Hauser-Feshbach). Special emphasis is
put on the uncertainties arising from nuclear level density descriptions and an
improved global description is presented. Furthermore, criteria for the
applicability of the statistical model are investigated and a "map" for the
applicability of the model to reactions of stable and unstable nuclei with
neutral and charged particles is given.Comment: REVTeX paper + 7 B/W figures + 2 color figures; PRC, in press. Also
available at http://quasar.physik.unibas.ch/preps.htm
A New Approach to Determine the Initial Mass Function in the Solar Neighborhood
Oxygen to iron abundance ratios of metal-poor stars provide information on
nucleosynthesis yields from massive stars which end in Type II supernova
explosions. Using a standard model of chemical evolution of the Galaxy we have
reproduced the solar neighborhood abundance data and estimated the oxygen and
iron yields of genuine SN II origin. The estimated yields are compared with the
theoretical yields to derive the relation between the lower and upper mass
limits in each generation of stars and the IMF slope. Independently of this
relation, we furthermore derive the relation between the lower mass limit and
the IMF slope from the stellar mass to light ratio in the solar neighborhood.
These independent relations unambiguously determine the upper mass limit of
and the IMF slope index of 1.3 - 1.6 above 1 M_sun. This
upper mass limit corresponds to the mass beyond which stars end as black holes
without ejecting processed matter into the interstellar medium. We also find
that the IMF slope index below 0.5 M_sun cannot be much shallower than 0.8.Comment: 13 pages LaTex, 4 PostScript figures, to appear in ApJ (July 1,
Vol.483
Solar abundance of manganese: a case for near Chandrasekhar-mass Type Ia supernova progenitors
Context: Manganese is predominantly synthesised in Type Ia supernova (SN Ia)
explosions. Owing to the entropy dependence of the Mn yield in explosive
thermonuclear burning, SNe Ia involving near Chandrasekhar-mass white dwarfs
(WDs) are predicted to produce Mn to Fe ratios significantly exceeding those of
SN Ia explosions involving sub-Chandrasekhar mass primary WDs. Of all current
supernova explosion models, only SN Ia models involving near-Chandrasekhar mass
WDs produce [Mn/Fe] > 0.0. Aims: Using the specific yields for competing SN Ia
scenarios, we aim to constrain the relative fractions of exploding
near-Chandrasekhar mass to sub-Chandrasekhar mass primary WDs in the Galaxy.
Methods: We extract the Mn yields from three-dimensional thermonuclear
supernova simulations referring to different initial setups and progenitor
channels. We then compute the chemical evolution of Mn in the Solar
neighborhood, assuming SNe Ia are made up of different relative fractions of
the considered explosion models. Results: We find that due to the entropy
dependence of freeze-out yields from nuclear statistical equilibrium, [Mn/Fe]
strongly depends on the mass of the exploding WD, with near-Chandraskher mass
WDs producing substantially higher [Mn/Fe] than sub-Chandrasekhar mass WDs. Of
all nucleosynthetic sources potentially influencing the chemical evolution of
Mn, only explosion models involving the thermonuclear incineration of
near-Chandrasekhar mass WDs predict solar or super-solar [Mn/Fe]. Consequently,
we find in our chemical evolution calculations that the observed [Mn/Fe] in the
Solar neighborhood at [Fe/H] > 0.0 cannot be reproduced without
near-Chandrasekhar mass SN Ia primaries. Assuming that 50 per cent of all SNe
Ia stem from explosive thermonuclear burning in near-Chandrasekhar mass WDs
results in a good match to data.Comment: Added author R. Pakmor, who mistakenly was forgotten in the first
version. Sorry Ruediger! 5 pages, 1 figure, 1 table, submitted as a letter to
Astronomy & Astrophysic