The impact of isomers on a kilonova associated with neutron star mergers

Abstract

We investigate the significance of isomers on a kilonova associated with neutron star mergers (NSMs) for the first time. We calculate the evolution of abundances and energy generation rates (ǫ nuc ) of ejecta from NSMs, taking into account β − decay through isomers. We find that for ejecta with electron fraction (Y e ) of 0.2 − 0.3,ǫ nuc is appreciably changed from those without isomers, due to the large change in timing of β − decay through the isomeric states of 123,125,127 Sn and 128 Sb. In particular, the effects of the isomers onǫ nuc are prominent for ejecta of Y e ∼ 0.25, which could emit a fraction of an early, blue component of a kilonova observed in GW170817. When the excitation from a ground state to its isomeric state is important, the isomers of 129 Sb and 129,131 Te also cause additional and appreciable change inǫ nuc . Furthermore, we demonstrate that larger amounts of lanthanide-free ejecta result in a better fit of an observed light curve of the kilonova in GW170817 if the isomers are taken into account.

Sensitivity of Neutron-Rich Nuclear Isomer Behavior to Uncertainties in Direct Transitions

“…Despite being known in the contexts discussed above, isomers have only recently been included in larger networks, such as those that describe the r process that is believed to occur in explosive environments [46]. Fujimoto and Hashimoto [47] included the direct population of nuclear isomers in the r process by replacing GS properties with isomer properties. In their study, several hand-picked isomers in the second r-process peak were shown to impact the radioactive heating of a kilonova.…”

Neutron transfer reactions on the ground state and isomeric state of a 130Sn beam

“…Both situations provide a clean tag of the reaction channel involving a 8 Be ejectile. The 12 C ejectiles can be partially separated from the 13 C target nuclei although it is less clean than the 9 Be case. The Doppler correction was made according to the ring of BareBall where the emergent α particles, or 12 C nucleus, were detected, which relates directly to the angle and nominal β =v/c of the recoiling 131 Sn.…”

“…However, this tendency is moderated by the weak binding of 9 Be (S n = 1.66 MeV). In contrast, ( 13 C, 12 C) preferentially populates states with j > = + 1/2, owing to the dominantly p 1/2 nature of the last neutron in 13 C. The relative cross sections for these states were measured using the γ-ray intensities (shown in Figure 4) normalized to the number of elastically scattered beam particles detected in the innermost ring of BareBall. The figure does not show the optimal Doppler correction for all peaks, as one value of β had to be selected for all the lines, resulting in some peaks being centered away from their true energy, and counts being dispersed across more channels.…”

“…For an isomer to be an astromer it has to behave in a significantly different way to the ground state and either survive long enough to undergo a reaction, or undergo β decay at a different rate to the ground state. Theorists have started working on identifying and treating astromers in the r-process as well as other nucleosynthetic processes [12,13]. With the occurrence of many low-lying isomers around 132 Sn, as highlighted in [13] and [14], and displayed in Figure 1, the influence of these long-lived states could be profound.…”

“…Theorists have started working on identifying and treating astromers in the r-process as well as other nucleosynthetic processes [12,13]. With the occurrence of many low-lying isomers around 132 Sn, as highlighted in [13] and [14], and displayed in Figure 1, the influence of these long-lived states could be profound. The isomeric 7 − state at E x = 1.95 MeV in 130 Sn has a t 1/2 = 1.7 minutes, long enough to be considered stable on an r-process time scale.…”