Fingerprints of heavy element nucleosynthesis in the late-time lightcurves of kilonovae

Abstract: 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 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>140. However, compelling evidence for the presence of heavier third-peak r-process elements (e.g., gold, platinum) or translead nuclei remains elusive. At early times (~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 (weeks to months), the decay energy input can possibly be dominated by a discrete number of alpha-decays, 223Ra (half-life t_{1/2}=11.43d), 225Ac (t_{1/2}=10.0d, following the beta-decay of 225Ra with t_{1/2}=14.9d), and the fissioning isotope 254Cf (t_{1/2}=60.5d), 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 & wide-band observations are required with sensitive instruments like the James Webb Space Telescope. In addition, by comparing the nuclear heating rate obtained with an abundance distribution following the Solar r abundance pattern, to the bolometric lightcurve of AT2017gfo, we find that the yet-uncertain r abundance of 72Ge plays a decisive role in powering the lightcurve, if one assumes that GW170817 has produced a full range of the Solar r abundances down to A~70.