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Register a new userIsochronic evolution and the radioactive decay of r-process nuclei
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We report on the creation and application of a novel decay network that uses the latest data from experiment and evaluation. We use the network to simulate the late-time phase of the rapid neutron capture (r) process. In this epoch, the bulk of nuclear reactions, such as radiative capture, have ceased and nuclear decays are the dominant transmutation channels. We find that the decay from short-lived to long-lived species naturally leads to an isochronic evolution in which nuclei with similar half-lives are populated at the same time. We consider random perturbations along each isobaric chain to initial solar-like r-process compositions to demonstrate the isochronic nature of the late-time phase of the r-process. Our analysis shows that detailed knowledge of the final isotopic composition allows for the prediction of late-time evolution with a high degree of confidence despite uncertainties that exist in astrophysical conditions and the nuclear physics properties of the most neutron-rich nuclei. We provide the time-dependent nuclear composition in the Appendix as supplemental material.
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We study the impact of astrophysically relevant nuclear isomers (astromers) in the context of the rapid neutron capture process (r-process) nucleosynthesis. We compute thermally mediated transition rates between long-lived isomers and the corresponding ground states in neutron-rich nuclei. We calculate the temperature-dependent beta-decay feeding factors which represent the fraction of material going to each of the isomer and ground state daughter species from the beta-decay parent species. We simulate nucleosynthesis by including as separate species nuclear excited states with measured terrestrial half-lives greater than 100 microseconds. We find a variety of isomers throughout the chart of nuclides are populated, and we identify those most likely to be influential. We comment on the capacity of isomer production to alter radioactive heating in an r-process environment.
We study the neutrino-induced production of nuclides in explosive supernova nucleosynthesis for progenitor stars with solar metallicity and initial main sequence masses between 15 M$_odot$ and 40 M$_odot$. We improve previous investigations i) by using a global set of partial differential cross sections for neutrino-induced charged- and neutral-current reactions on nuclei with charge numbers $Z < 76 $ and ii) by considering modern supernova neutrino spectra which have substantially lower average energies compared to those previously adopted in neutrino nucleosynthesis studies. We confirm the production of $^7$Li, $^{11}$B, $^{138}$La, and $^{180}$Ta by neutrino nucleosynthesis, albeit at slightly smaller abundances due to the changed neutrino spectra. We find that for stars with a mass smaller than 20 M$_odot$, $^{19}$F is produced mainly by explosive nucleosynthesis while for higher mass stars it is produced by the $ u$ process. We also find that neutrino-induced reactions, either directly or indirectly by providing an enhanced abundance of light particles, noticeably contribute to the production of the radioactive nuclides $^{22}$Na and $^{26}$Al. Both nuclei are prime candidates for gamma-ray astronomy. Other prime targets, $^{44}$Ti and $^{60}$Fe, however, are insignificantly produced by neutrino-induced reactions. We also find a large increase in the production of the long-lived nuclei $^{92}$Nb and $^{98}$Tc due to charged-current neutrino capture.
Any simulation of the r-process is affected by uncertainties in our present knowledge of nuclear physics quantities and astrophysical conditions. It is common to quantify the impact of these uncertainties through a global sensitivity metric, which is then used to identify specific nuclides that would be most worthwhile to measure experimentally. Using descriptive statistics, we assess a set of metrics used in previous sensitivity studies, as well as a new logarithmic measure. For certain neutron-rich nuclides lying near the r-process path for the typical hot-wind scenario, we find opposing conclusions on their relative sensitivity implied by different metrics, although they all generally agree which ones are the most sensitive nuclei. The underlying reason is that sensitivity metrics which simply sum over variations in the r-process distribution depend on the scaling used in the baseline, which often varies between simulations. We show that normalization of the abundances causes changes in the reported sensitivity factors and recommend reporting a minimized F statistic in addition to a scale estimation for rough calibration to be used when comparing tables of sensitivity factors from different studies.
Of the variations in the elemental abundance patterns of stars enhanced with $r$-process elements, the variation in the relative actinide-to-lanthanide ratio is among the most significant. We investigate the source of these actinide differences in order to determine whether these variations are due to natural differences in astrophysical sites, or due to the uncertain nuclear properties that are accessed in $r$-process sites. We find that variations between relative stellar actinide abundances is most likely astrophysical in nature, owing to how neutron-rich the ejecta from an $r$-process event may be. Furthermore, if an $r$-process site is capable of generating variations in the neutron-richness of its ejected material, then only one type of $r$-process site is needed to explain all levels of observed relative actinide enhancements.
We have performed r-process calculations for matter ejected dynamically in neutron star mergers based on a complete set of trajectories from a three-dimensional relativistic smoothed particle hydrodynamic simulation. Our calculations consider an extended nuclear network, including spontaneous, $beta$- and neutron-induced fission and adopting fission yield distributions from the ABLA code. We have studied the sensitivity of the r-process abundances to nuclear masses by using different models. Most of the trajectories, corresponding to 90% of the ejected mass, follow a relatively slow expansion allowing for all neutrons to be captured. The resulting abundances are very similar to each other and reproduce the general features of the observed r-process abundance (the second and third peaks, the rare-earth peak and the lead peak) for all mass models as they are mainly determined by the fission yields. We find distinct differences in the abundance yields at and just above the third peak, which can be traced back to different predictions of neutron separation energies for r-process nuclei around neutron number $N=130$. The remaining trajectories, which contribute 10% by mass to the total integrated abundances, follow such a fast expansion that the r process does not use all the neutrons. This also leads to a larger variation of abundances among trajectories as fission does not dominate the r-process dynamics. The total integrated abundances are dominated by contributions from the slow abundances and hence reproduce the general features of the observed r-process abundances. We find that at timescales of weeks relevant for kilonova light curve calculations, the abundance of actinides is larger than the one of lanthanides. Hence actinides can be even more important than lanthanides to determine the photon opacities under kilonova conditions. (Abridged)
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