484 research outputs found
High resolution simulations of the head-on collision of white dwarfs
The direct impact of white dwarfs has been suggested as a plausible channel
for type Ia supernovae. In spite of their (a priori) rareness, in highly
populated globular clusters and in galactic centers, where the amount of white
dwarfs is considerable, the rate of violent collisions between two of them
might be non-negligible. Even more, there are indications that binary white
dwarf systems orbited by a third stellar-mass body have an important chance to
induce a clean head-on collision. Therefore, this scenario represents a source
of contamination for the supernova light-curves sample that it is used as
standard candles in cosmology, and it deserves further investigation. Some
groups have conducted numerical simulations of this scenario, but their results
show several differences. In this paper we address some of the possible sources
of these differences, presenting the results of high resolution hydrodynamical
simulations jointly with a detailed nuclear post-processing of the nuclear
abundances, to check the viability of white dwarf collisions to produce
significant amounts of 56Ni. To that purpose, we use a 2D-axial symmetric
smoothed particle hydrodynamic code to obtain a resolution considerably higher
than in previous studies. In this work, we also study how the initial mass and
nuclear composition affect the results. The gravitational wave emission is also
calculated, as this is a unique signature of this kind of events. All
calculated models produce a significant amount of 56Ni, ranging from 0.1 Msun
to 1.1 Msun, compatible not only with normal-Branch type Ia supernova but also
with the subluminous and super-Chandrasekhar subset. Nevertheless, the
distribution mass-function of white dwarfs favors collisions among 0.6-0.7 Msun
objects, leading to subluminous events.Comment: 24 pages, 12 figures, accepted for publication in MNRA
Nucleosynthesis in O-Ne-Mg Supernovae
We have studied detailed nucleosynthesis in the shocked surface layers of an
Oxygen-Neon-Magnesium core collapse supernova with an eye to determining if the
conditions are suitable for r process nucleosynthesis. We find no such
conditions in an unmodified model, but do find overproduction of N=50 nuclei
(previously seen in early neutron-rich neutrino winds) in amounts that, if
ejected, would pose serious problems for galactic chemical evolution.Comment: 12 pages, 1 figure, to be published in Astrophysical Journal Letter
Parity-Dependence in the Nuclear Level Density
Astrophysical reaction rates are sensitive to the parity distribution at low
excitation energies. We combine a formula for the energy-dependent parity
distribution with a microscopic-macroscopic nuclear level density. This
approach describes well the transition from low excitation energies, where a
single parity dominates, to high excitations where the two densities are equal.Comment: 4 pages, 3 figures; contribution to Nuclei In The Cosmos VIII, to
appear in Nucl. Phys.
Unbound states of 32Cl and the 31S(p,\gamma)32Cl reaction rate
The 31S(p,\gamma)32Cl reaction is expected to provide the dominant break-out
path from the SiP cycle in novae and is important for understanding enrichments
of sulfur observed in some nova ejecta. We studied the 32S(3He,t)32Cl
charge-exchange reaction to determine properties of proton-unbound levels in
32Cl that have previously contributed significant uncertainties to the
31S(p,\gamma)32Cl reaction rate. Measured triton magnetic rigidities were used
to determine excitation energies in 32Cl. Proton-branching ratios were obtained
by detecting decay protons from unbound 32Cl states in coincidence with
tritons. An improved 31S(p,\gamma)32Cl reaction rate was calculated including
robust statistical and systematic uncertainties
On three topical aspects of the N=28 isotonic chain
The evolution of single-particle orbits along the N=28 isotonic chain is
studied within the framework of a relativistic mean-field approximation. We
focus on three topical aspects of the N=28 chain: (a) the emergence of a new
magic number at Z=14; (b) the possible erosion of the N=28 shell; and (c) the
weakening of the spin-orbit splitting among low-j neutron orbits. The present
model supports the emergence of a robust Z=14 subshell gap in 48Ca, that
persists as one reaches the neutron-rich isotone 42Si. Yet the proton removal
from 48Ca results in a significant erosion of the N=28 shell in 42Si. Finally,
the removal of s1/2 protons from 48Ca causes a ~50% reduction of the spin-orbit
splitting among neutron p-orbitals in 42Si.Comment: 12 pages with 5 color figure
Thermodynamics of the Spin Luttinger-Liquid in a Model Ladder Material
The phase diagram in temperature and magnetic field of the metal-organic,
two-leg, spin-ladder compound (C5H12N)2CuBr4 is studied by measurements of the
specific heat and the magnetocaloric effect. We demonstrate the presence of an
extended spin Luttinger-liquid phase between two field-induced quantum critical
points and over a broad range of temperature. Based on an ideal spin-ladder
Hamiltonian, comprehensive numerical modelling of the ladder specific heat
yields excellent quantitative agreement with the experimental data across the
complete phase diagram.Comment: 4 pages, 4 figures, updated refs and minor changes to the text,
version accepted for publication in Phys. Rev. Let
Astrophysical S-factors for fusion reactions involving C, O, Ne and Mg isotopes
Using the Sao Paulo potential and the barrier penetration formalism we have
calculated the astrophysical factor S(E) for 946 fusion reactions involving
stable and neutron-rich isotopes of C, O, Ne, and Mg for center-of-mass
energies E varying from 2 MeV to 18-30 MeV (covering the range below and above
the Coulomb barrier). We have parameterized the energy dependence S(E) by an
accurate universal 9-parameter analytic expression and present tables of fit
parameters for all the reactions. We also discuss the reduced 3-parameter
version of our fit which is highly accurate at energies below the Coulomb
barrier, and outline the procedure for calculating the reaction rates. The
results can be easily converted to thermonuclear or pycnonuclear reaction rates
to simulate various nuclear burning phenomena, in particular, stellar burning
at high temperatures and nucleosynthesis in high density environments.Comment: 30 pages including 11 tables, 4 figures, ADNDT, accepte
Large-scale prediction of the parity distribution in the nuclear level density and application to astrophysical reaction rates
A generalized method to calculate the excitation-energy dependent parity
ratio in the nuclear level density is presented, using the assumption of
Poisson distributed independent quasi particles combined with BCS occupation
numbers. It is found that it is crucial to employ a sufficiently large model
space to allow excitations both from low-lying shells and to higher shells
beyond a single major shell. Parity ratios are only found to equilibrate above
at least 5-10 MeV of excitation energy. Furthermore, an overshooting effect
close to major shells is found where the parity opposite to the ground state
parity may dominate across a range of several MeV before the parity ratio
finally equilibrates. The method is suited for large-scale calculations as
needed, for example, in astrophysical applications. Parity distributions were
computed for all nuclei from the proton dripline to the neutron dripline and
from Ne up to Bi. These results were then used to recalculate astrophysical
reaction rates in a Hauser-Feshbach statistical model. Although certain
transitions can be considerably enhanced or suppressed, the impact on
astrophysically relevant reactions remains limited, mainly due to the thermal
population of target states in stellar reaction rates.Comment: 15 pages, 17 figures; corrected/updated references in v2; additional
material can be found at http://nucastro.org/adndt.html#parit
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