3,132 research outputs found
Electron-capture supernovae as sources of 60Fe
We investigate the nucleosynthesis of the radionuclide 60Fe in
electron-capture supernovae (ECSNe). The nucleosynthetic results are based on a
self-consistent, two-dimensional simulation of an ECSN as well as models in
which the densities are systematically increased by some factors (low-entropy
models). 60Fe is found to be appreciably made in neutron-rich ejecta during the
nuclear quasi-equilibrium phase with greater amounts being produced in the
lower-entropy models. Our results, combining them with the yields of
core-collapse supernovae (CCSNe) in the literature, suggest that ECSNe account
for at least 4-30% of live 60Fe in the Milky Way. ECSNe co-produce neutron-rich
isotopes, 48Ca, 50Ti, 54Cr, some light trans-iron elements, and possibly weak
r-process elements including some radionuclides such as 93Zr, 99Tc, and 107Pd,
whose association with 60Fe might have been imprinted in primitive meteorites
or in the deep ocean crust on the Earth.Comment: 6 pages, 2 figures, accepted for publication in ApJ
Electron-capture supernovae as origin of 48Ca
We report that electron-capture supernovae (ECSNe), arising from collapsing
oxygen-neon-magnesium cores, are a possible source of 48Ca, whose origin has
remained a long-standing puzzle. Our two-dimensional, self-consistent explosion
model of an ECSN predicts ejection of neutron-rich matter with electron
fractions Ye = 0.40-0.42 and relatively low entropies, s = 13-15 kB per nucleon
(kB is the Boltzmann constant). Post-processing nucleosynthesis calculations
result in appreciable production of 48Ca in such neutron-rich and low-entropy
matter during the quasi-nuclear equilibrium and subsequent freezeout phases.
The amount of ejected 48Ca can account for that in the solar inventory when we
consider possible uncertainties in the entropies or ejecta-mass distribution.
ECSNe could thus be a site of 48Ca production in addition to a hypothetical,
rare class of high-density Type Ia supernovae.Comment: 6 pages, 5 figures, accepted for publication in ApJ
Characterizing SASI- and Convection-Dominated Core-Collapse Supernova Explosions in Two Dimensions
The success of the neutrino mechanism of core-collapse supernovae relies on
the supporting action of two hydrodynamic instabilities: neutrino-driven
convection and the Standing Accretion Shock Instability (SASI). Depending on
the structure of the stellar progenitor, each of these instabilities can
dominate the evolution of the gain region prior to the onset of explosion, with
implications for the ensuing asymmetries. Here we examine the flow dynamics in
the neighborhood of explosion by means of parametric two-dimensional,
time-dependent hydrodynamic simulations for which the linear stability
properties are well understood. We find that systems for which the convection
parameter is sub-critical (SASI-dominated) develop explosions once large-scale,
high-entropy bubbles are able to survive for several SASI oscillation cycles.
These long-lived structures are seeded by the SASI during shock expansions.
Finite-amplitude initial perturbations do not alter this outcome qualitatively,
though they can lead to significant differences in explosion times.
Supercritical systems (convection-dominated) also explode by developing
large-scale bubbles, though the formation of these structures is due to buoyant
activity. Non-exploding systems achieve a quasi-steady state in which the
time-averaged flow adjusts itself to be convectively sub-critical. We
characterize the turbulent flow using a spherical Fourier-Bessel decomposition,
identifying the relevant scalings and connecting temporal and spatial
components. Finally, we verify the applicability of these principles on the
general relativistic, radiation-hydrodynamic simulations of Mueller, Janka, &
Heger (2012), and discuss implications for the three-dimensional case.Comment: accepted by MNRAS with minor change
Supernova deleptonization asymmetry: Impact on self-induced flavor conversion
During the accretion phase of a core-collapse supernova (SN), the
deleptonization flux has recently been found to develop a global dipole pattern
(LESA---Lepton Emission Self-sustained Asymmetry). The minus
flux essentially vanishes in one direction, potentially
facilitating self-induced flavor conversion. On the other hand, below the
stalled shock wave, self-induced flavor conversion is typically suppressed by
multi-angle matter effects, preventing any impact of flavor conversion on SN
explosion dynamics. In a schematic model of SN neutrino fluxes, we study the
impact of modified - flux asymmetries on collective flavor
conversion. In the parameter space consisting of matter density and effective
neutrino density, the region of instability with regard to self-induced flavor
conversion is much larger for a vanishing lepton number flux, yet this
modification does not intersect a realistic SN profile. Therefore, it appears
that, even in the presence of LESA, self-induced flavor conversion remains
suppressed below the shock front.Comment: 14 pages, 6 figures; v2: significant change in presentation, results
and conclusion unchanged, appendix adde
Exploring the relativistic regime with Newtonian hydrodynamics: II. An effective gravitational potential for rapid rotation
We present the generalization of a recently introduced modified gravitational
potential for self-gravitating fluids. The use of this potential allows for an
accurate approximation of general relativistic effects in an otherwise
Newtonian hydrodynamics code also in cases of rapid rotation. We test this
approach in numerical simulations of astrophysical scenarios related to compact
stars, like supernova core collapse with both a simplified and detailed
microphysical description of matter, and rotating neutron stars in equilibrium.
We assess the quality of the new potential, and demonstrate that it provides a
significant improvement compared to previous formulations for such potentials.
Newtonian simulations of compact objects employing such an effective
relativistic potential predict inaccurate pulsation frequencies despite the
excellent agreement of the collapse dynamics and structure of the compact
objects with general relativistic results. We analyze and discuss the reason
for this behavior.Comment: 15 pages, 12 figures, minor modification
3D Simulations of Magnetoconvection in a Rapidly Rotating Supernova Progenitor
We present a first 3D magnetohydrodynamic (MHD) simulation of oxygen, neon
and carbon shell burning in a rapidly rotating 16 M_sun core-collapse supernova
progenitor. We also run a purely hydrodynamic simulation for comparison. After
180s (15 and 7 convective turnovers respectively), the magnetic fields in the
oxygen and neon shells achieve saturation at 10^{11}G and 5 x 10^{10}G. The
strong Maxwell stresses become comparable to the radial Reynolds stresses and
eventually suppress convection. The suppression of mixing by convection and
shear instabilities results in the depletion of fuel at the base of the burning
regions, so that the burning shell eventually move outward to cooler regions,
thus reducing the energy generation rate. The strong magnetic fields
efficiently transport angular momentum outwards, quickly spinning down the
rapidly rotating convective oxygen and neon shells and forcing them into rigid
rotation. The hydrodynamic model shows complicated redistribution of angular
momentum and develops regions of retrograde rotation at the base of the
convective shells. We discuss implications of our results for stellar evolution
and for the subsequent core-collapse supernova. The rapid redistribution of
angular momentum in the MHD model casts some doubt on the possibility of
retaining significant core angular momentum for explosions driven by
millisecond magnetars. However, findings from multi-D models remain tentative
until stellar evolution calculations can provide more consistent rotation
profiles and estimates of magnetic field strengths to initialise multi-D
simulations without substantial numerical transients. We also stress the need
for longer simulations, resolution studies, and an investigation of non-ideal
effects.Comment: Submitted to MNRAS (14 pages, 11 Figures
High-resolution supernova neutrino spectra represented by a simple fit
To study the capabilities of supernova neutrino detectors, the instantaneous
spectra are often represented by a quasi-thermal distribution of the form f(E)
= E^alpha e^{-(alpha+1)E/E_{av}} where E_{av} is the average energy and alpha a
numerical parameter. Based on a spherically symmetric supernova model with full
Boltzmann neutrino transport we have, at a few representative post-bounce
times, re-converged the models with vastly increased energy resolution to test
the fit quality. For our examples, the spectra are well represented by such a
fit in the sense that the counting rates for a broad range of target nuclei,
sensitive to different parts of the spectrum, are reproduced very well.
Therefore, the mean energy and root-mean-square energy of numerical spectra
hold enough information to provide the correct alpha and to forecast the
response of multi-channel supernova neutrino detection.Comment: 6 pages, including 4 figures and 2 tables. Clarifying paragraphs
added; results unchanged. Matches published version in PR
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