The kilonova emission observed following the binary neutron star merger event GW170817 provided the first direct evidence for the synthesis of heavy nuclei through the rapid neutron capture process ( r -process ) .
The late-time transition in the spectral energy distribution to near-infrared wavelengths was interpreted as indicating the production of lanthanide nuclei , with atomic mass number A \gtrsim 140 .
However , compelling evidence for the presence of even heavier third-peak ( A \approx 195 ) r -process elements ( e.g. , gold , platinum ) or translead nuclei remains elusive .
At early times ( \sim days ) most of the r -process heating arises from a large statistical ensemble of \beta -decays , which thermalize efficiently while the ejecta is still dense , generating a heating rate that is reasonably approximated by a single power-law .
However , at later times of weeks to months , the decay energy input can also be dominated by a discrete number of \alpha -decays , ^ { 223 } Ra ( half-life t _ { 1 / 2 } = 11.43 d ) , ^ { 225 } Ac ( t _ { 1 / 2 } = 10.0 d , following the \beta -decay of ^ { 225 } Ra with t _ { 1 / 2 } = 14.9 d ) , and the fissioning isotope ^ { 254 } Cf ( t _ { 1 / 2 } = 60.5 d ) , which liberate more energy per decay and thermalize with greater efficiency than beta-decay products .
Late-time nebular observations of kilonovae which constrain the radioactive power provide the potential to identify signatures of these individual isotopes , thus confirming the production of heavy nuclei .
In order to constrain the bolometric light to the required accuracy , multi-epoch and wide-band observations are required with sensitive instruments like the James Webb Space Telescope .
In addition , we show how a precise determination of the r -process contribution to the ^ { 72 } Ge abundance in the Solar System sheds light on whether neutron star mergers can account for the full range of Solar r -process abundances .