60 research outputs found
The r-process nucleosynthesis in the various jet-like explosions of magnetorotational core-collapse supernovae
The r-process nucleosynthesis in core-collapse supernovae (CC-SNe) is
studied, with a focus on the explosion scenario induced by rotation and strong
magnetic fields. Nucleosynthesis calculations are conducted based on
magneto-hydrodynamical explosion models with a wide range of parameters for
initial rotation and magnetic fields. The explosion models are classified in
two different types: i.e., prompt-magnetic-jet and delayed-magnetic-jet, for
which the magnetic fields of proto-neutron stars (PNSs) during collapse and the
core-bounce are strong and comparatively moderate, respectively. Following the
hydrodynamical trajectories of each explosion model, we confirmed that
r-processes successfully occur in the prompt-magnetic-jets, which produce heavy
nuclei including actinides. On the other hand, the r-process in the
delayed-magnetic-jet is suppressed, which synthesizes only nuclei up to the
second peak (). Thus, the r-process in the delayed-magnetic-jets
could explain only "weak r-process" patterns observed in metal-poor stars
rather than the "main r-process", represented by the solar abundances. Our
results imply that core-collapse supernovae are possible astronomical sources
of heavy r-process elements if their magnetic fields are strong enough, while
weaker magnetic explosions may produce "weak r-process" patterns (). We show the potential importance and necessity of magneto-rotational
supernovae for explaining the galactic chemical evolution, as well as
abundances of r-process enhanced metal-poor stars. We also examine the effects
of the remaining uncertainties in the nature of PNSs due to weak interactions
that determine the final neutron-richness of ejecta. Additionally, we briefly
discuss radioactive isotope yields in primary jets (e.g., Ni), with
relation to several optical observation of SNe and relevant high-energy
astronomical phenomena.Comment: 25 pages, 17 figures, 6 tables, ApJ in press, numerical data are
available at http://www.astro.keele.ac.uk/~nobuya/mrsn and
https://github.com/nnobuya/mrs
Impact of the first-forbidden decay on the production of r-process peak
We investigated the effects of first-forbidden transitions in decays
on the production of the r-process peak. The theoretical
calculated -decay rates with -delayed neutron emission were
examined using several astrophysical conditions. As the first-borbidden decay
is dominant in neutron-rich nuclei, their inclusion shortens
-decay lifetimes and shifts the abundance peak towards higher masses.
Additionally, the inclusion of the -delayed neutron emission results in
a wider abundance peak, and smoothens the mass distribution by removing the
odd-even mass staggering. The effects are commonly seen in the results of all
adopted astrophysical models. Nevertheless there are quantitative differences,
indicating that remaining uncertainty in the determination of half-lives for
nuclei is still significant in order to determine the production of the
r-process peak.Comment: 6 pages, 4 figures, 1 table, Phys. Lett. B, in pres
Mass Ejection from the Remnant of a Binary Neutron Star Merger: Viscous-Radiation Hydrodynamics Study
We perform long-term general relativistic neutrino radiation hydrodynamics
simulations (in axisymmetry) for a massive neutron star (MNS) surrounded by a
torus, which is a canonical remnant formed after the binary neutron star
merger. We take into account the effects of viscosity, which is likely to arise
in the merger remnant due to magnetohydrodynamical turbulence. As the initial
condition, we employ the azimuthally averaged data of the MNS-torus system
derived in a three-dimensional, numerical-relativity simulation for the binary
neutron star merger. The viscous effect plays key roles for the remnant
evolution and mass ejection from it in two phases of the evolution. In the
first ms, a differential rotation state of the MNS is changed to
a rigidly rotating state, and as a result, a sound wave, which subsequently
becomes a shock wave, is formed in the vicinity of the MNS due to the variation
of the quasi-equilibrium state of the MNS. The shock wave induces significant
mass ejection of mass for the alpha
viscosity parameter of . For the longer-term evolution with s, a significant fraction of the torus material is ejected. The ejecta
mass is likely to be of order , so that the total mass of the
viscosity-driven ejecta could dominate that of the dynamical ejecta of mass
. The electron fraction, , of the ejecta is
always high enough () that this post-merger ejecta is
lanthanide-poor; hence, the opacity of the ejecta is likely to be
times lower than that of the dynamical ejecta. This indicates that the
electromagnetic signal from the ejecta would be rapidly evolving, bright, and
blue if it is observed from a small viewing angle () for
which the effect of the dynamical ejecta is minor.Comment: 21 pages, 18 figures, accepted for publication in Ap
Production of all the r-process nuclides in the dynamical ejecta of neutron star mergers
Recent studies suggest that binary neutron star (NS-NS) mergers robustly
produce the heavy r-process nuclei above the atomic mass number A ~ 130 because
of their ejecta consisting of almost pure neutrons (electron fraction of Y_e <
0.1). However, little production of the lighter r-process nuclei (A = 90-120)
conflicts with the spectroscopic results of r-process-enhanced Galactic halo
stars. We present, for the first time, the result of nucleosynthesis
calculations based on the fully general-relativistic simulation of a NS-NS
merger with approximate neutrino transport. It is found that the bulk of the
dynamical ejecta are appreciably shock-heated and neutrino-processed, resulting
in a wide range of Y_e (= 0.09-0.45). The mass-averaged abundance distribution
of calculated nucleosynthesis yields is in reasonable agreement with the
full-mass range (A = 90-240) of the solar r-process curve. This implies, if our
model is representative of such events, that the dynamical ejecta of NS-NS
mergers can be the origin of the Galactic r-process nuclei. Our result also
shows that the radioactive heating after ~ 1 day from the merging, giving rise
to r-process-powered transient emission, is dominated by the beta-decays of
several species close to stability with precisely measured half-lives. This
implies that the total radioactive heating rate for such an event can be well
constrained within about a factor of two if the ejected material has a
solar-like r-process pattern.Comment: 6 pages, 5 figures, accepted for publication in ApJL. Section 2 was
significantly changed according to the referee's advices. Our result was
unchange
Impact of New beta-decay Half-lives on r-process Nucleosynthesis
We investigate the effects of newly measured beta-decay half-lives on
r-process nucleosynthesis. These new rates were determined by recent
experiments at the radioactive isotope beam factory facility in the RIKEN
Nishina Center. We adopt an r-process nucleosynthesis environment based on a
magnetohydrodynamic supernova explosion model that includes strong magnetic
fields and rapid rotation of the progenitor. A number of the new beta-decay
rates are for nuclei on or near the r-process path, and hence they affect the
nucleosynthesis yields and time scale of the r-process. The main effect of the
newly measured beta-decay half-lives is an enhancement in the calculated
abundance of isotopes with mass number A = 110 -- 120 relative to calculated
abundances based upon beta-decay rates estimated with the finite-range droplet
mass model. This effect slightly alleviates, but does not fully explain, the
tendency of r-process models to underproduce isotopes with A = 110 -- 120
compared to the solar-system r-process abundances.Comment: 9 pages, 3 figures, 1 table, published in PR
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