96 research outputs found
On the astrophysical robustness of neutron star merger r-process
In this study we explore the nucleosynthesis in the dynamic ejecta of compact
binary mergers. We are particularly interested in the question how sensitive
the resulting abundance patterns are to the parameters of the merging system.
Therefore, we systematically investigate combinations of neutron star masses in
the range from 1.0 to 2.0 \Msun and, for completeness, we compare the results
with those from two simulations of a neutron star black hole merger. The ejecta
masses vary by a factor of five for the studied systems, but all amounts are
(within the uncertainties of the merger rates) compatible with being a major
source of cosmic r-process. The ejecta undergo a robust r-process
nucleosynthesis which produces all the elements from the second to the third
peak in close-to-solar ratios. Most strikingly, this r-process is extremely
robust, all 23 investigated binary systems yield practically identical
abundance patterns. This is mainly the result of the ejecta being extremely
neutron rich (\ye ) and the r-process path meandering along the
neutron drip line so that the abundances are determined entirely by nuclear
rather than by astrophysical properties. This robustness together with the ease
with which both the second and third peak are reproduced make compact binary
mergers the prime candidate for the source of the observed unique heavy
r-process component.Comment: accepted for publication in MNRA
Boron depletion in 9 to 15 M(circle dot) stars with rotation
The treatment of mixing is still one of the major uncertainties in stellar evolution models. One open question is how well the prescriptions for rotational mixing describe the real effects. We tested the mixing prescriptions included in the Geneva stellar evolution code (GENEC) by following the evolution of surface abundances of light isotopes in massive stars, such as boron and nitrogen. We followed 9, 12 and 15 M(O) models with rotation from the zero age main sequence up to the end of He burning. The calculations show the expected behaviour with faster depletion of boton for faster rotating stars and more massive stars. The mixing at the surface is more efficient, than predicted by prescriptions used in other codes and reproduces the majority of observations very well However two observed stars with strong boron depletion but, no nitrogen enhancement still can not be explained and let the question open whether additional mixing processes are acting in these massive star
Boron depletion in 9 to 15 M stars with rotation
The treatment of mixing is still one of the major uncertainties in stellar evolution models. One open question is how well the prescriptions for rotational mixing describe the real effects. We tested the mixing prescriptions included in the Geneva stellar evolution code (GENEC) by following the evolution of surface abundances of light isotopes in massive stars, such as boron and nitrogen. We followed 9, 12 and 15 M models with rotation from the zero age main sequence up to the end of He burning. The calculations show the expected behaviour with faster depletion of boron for faster rotating stars and more massive stars. The mixing at the surface is more efficient than predicted by prescriptions used in other codes and reproduces the majority of observations very well. However two observed stars with strong boron depletion but no nitrogen enrichment still can not be explained and let the question open whether additional mixing processes are acting in these massive star
The Role of Fission in Neutron Star Mergers and Its Impact on the r-Process Peaks
Comparing observational abundance features with nucleosynthesis predictions of stellar evolution or explosion simulations, we can scrutinize two aspects: (a) the conditions in the astrophysical production site and (b) the quality of the nuclear physics input utilized. We test the abundance features of r-process nucleosynthesis calculations for the dynamical ejecta of neutron star merger simulations based on three different nuclear mass models: The Finite Range Droplet Model, the (quenched version of the) Extended Thomas Fermi Model with Strutinsky Integral, and the Hartree-Fock-Bogoliubov mass model. We make use of corresponding fission barrier heights and compare the impact of four different fission fragment distribution models on the final r-process abundance distribution. In particular, we explore the abundance distribution in the second r-process peak and the rare-earth sub-peak as a function of mass models and fission fragment distributions, as well as the origin of a shift in the third r-process peak position. The latter has been noticed in a number of merger nucleosynthesis predictions. We show that the shift occurs during the r-process freeze-out when neutron captures and β-decays compete and an (n,γ)-(γ,n) equilibrium is no longer maintained. During this phase neutrons originate mainly from fission of material above A = 240. We also investigate the role of β-decay half-lives from recent theoretical advances, which lead either to a smaller amount of fissioning nuclei during freeze-out or a faster (and thus earlier) release of fission neutrons, which can (partially) prevent this shift and has an impact on the second and rare-earth peak as well.Peer reviewe
The Impact of Fission on R-Process Calculations
We have performed r-process calculations in neutron star mergers (NSM) and jets of magnetohydrodynamically driven (MHD) supernovae. In these very neutron-rich environments the fission model of heavy nuclei has an impact on the shape of the final abundance distribution and the second r-process peak in particular. We have studied the effect of different fission fragment mass distribution models in calculations of low-Ye ejecta, ranging from a simple parametrization to extensive statistical treatments (ABLA07). The r-process path ends when it reaches an area in the nuclear chart where fission dominates over further neutron captures. The position of this point is determined by the fission barriers and the neutron separation energies of the nuclei involved. As these values both depend on the choice of the nuclear mass model, so does the r-process path. Here we present calculations using the FRDM (Finite Range Droplet Model) and the ETFSI (Extended Thomas Fermi with Strutinsky Integral) mass model with the related TF and ETFSI fission barrier predictions. Utilizing sophisticated fission fragment distribution leads to a highly improved abundance distribution.Peer reviewe
The Nuclear Reaction Network WinNet
© 2023. The Author(s). Published by the American Astronomical Society. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/We present the state-of-the-art single-zone nuclear reaction network WinNet, which is capable of calculating the nucleosynthetic yields of a large variety of astrophysical environments and conditions. This ranges from the calculation of the primordial nucleosynthesis, where only a few nuclei are considered, to the ejecta of neutron star mergers with several thousands of involved nuclei. Here we describe the underlying physics and implementation details of the reaction network. We additionally present the numerical implementation of two different integration methods, the implicit Euler method and Gears method, along with their advantages and disadvantages. We furthermore describe basic example cases of thermodynamic conditions that we provide together with the network and demonstrate the reliability of the code by using simple test cases. With this publication, WinNet will be publicly available and open source at GitHub and Zenodo.Peer reviewe
Chemistry of heavy elements in the Dark Ages
Primordial molecules were formed during the Dark Ages, i.e. the time between
recombination and reionization in the early Universe. The purpose of this
article is to analyze the formation of primordial molecules based on heavy
elements during the Dark Ages, with elemental abundances taken from different
nucleosynthesis models. We present calculations of the full non-linear equation
set governing the primordial chemistry. We consider the evolution of 45
chemical species and use an implicit multistep method of variable order of
precision with an adaptive stepsize control. We find that the most abundant
Dark Ages molecules based on heavy elements are CH and OH. Non-standard
nucleosynthesis can lead to higher heavy element abundances while still
satisfying the observed primordial light abundances. In that case, we show that
the abundances of molecular species based on C, N, O and F can be enhanced by
two orders of magnitude compared to the standard case, leading to a CH relative
abundance higher than that of HD+ or H2D+.Comment: 14 pages, accepted by Astronomy and Astrophysic
Explosive Nucleosynthesis: What we learned and what we still do not understand
This review touches on historical aspects, going back to the early days of
nuclear astrophysics, initiated by BFH and Cameron, discusses (i) the
required nuclear input from reaction rates and decay properties up to the
nuclear equation of state, continues (ii) with the tools to perform
nucleosynthesis calculations and (iii) early parametrized nucleosynthesis
studies, before (iv) reliable stellar models became available for the late
stages of stellar evolution. It passes then through (v) explosive environments
from core-collapse supernovae to explosive events in binary systems (including
type Ia supernovae and compact binary mergers), and finally (vi) discusses the
role of all these nucleosynthesis production sites in the evolution of
galaxies. The focus is put on the comparison of early ideas and present, very
recent, understanding.Comment: 11 pages, to appear in Springer Proceedings in Physics (Proc. of
Intl. Conf. "Nuclei in the Cosmos XV", LNGS Assergi, Italy, June 2018
An r-process enhanced star in the dwarf galaxy Tucana III*
Chemically peculiar stars in dwarf galaxies provide a window for exploring the birth environment of stars with varying chemical enrichment. We present a chemical abundance analysis of the brightest star in the newly discovered ultra-faint dwarf galaxy candidate Tucana III. Because it is particularly bright for a star in an ultra-faint Milky Way (MW) satellite, we are able to measure the abundance of 28 elements, including 13 neutron-capture species. This star, DES J235532.66−593114.9 (DES J235532), shows a mild enhancement in neutron-capture elements associated with the r-process and can be classified as an r-I star. DES J235532 is the first r-I star to be discovered in an ultra-faint satellite, and Tuc III is the second extremely low-luminosity system found to contain r-process enriched material, after Reticulum II. Comparison of the abundance pattern of DES J235532 with r-I and r-II stars found in other dwarf galaxies and in the MW halo suggests a common astrophysical origin for the neutron-capture elements seen in all r-process enhanced stars. We explore both internal and external scenarios for the r-process enrichment of Tuc III and show that with abundance patterns for additional stars, it should be possible to distinguish between them
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