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 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 > ∼ 140. However, compelling evidence for the presence of even heavier third-peak (A ≈ 195) rprocess 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 β-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 possibly be dominated by a discrete number of α-decays, 223 Ra (half-life t 1/2 = 11.43 d), 225 Ac (t 1/2 = 10.0 d, following the β-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, by comparing the nuclear heating rate obtained with an abundance distribution that follows the Solar r abundance pattern, to the bolometric lightcurve of AT 2017gfo, we find that the yet-uncertain r abundance of 72 Ge 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 mass number A ∼ 70.

Kilonovae

“…8), is constant to within a factor of a few for (Lippuner and Roberts 2015; Wu et al. 2019b; see Fig. 10).…”

“…The radioactive decay power is sensitive to various uncertainties in the assumed nuclear physics (nuclear masses, cross sections, and fission fragment distribution) at the factor of a few level (e.g., Wu et al. 2019b)11, a point we shall return to when discussing GW170817 (Sect. 5).…”

“…2018; Wu et al. 2019b), much in the way that the half-life of Co is imprinted in the decay of Type Ia supernovae. The ability in practice to identify individual isotopes through this method will depend on accurate models for the ejecta thermalization entering the nebular phase (Kasen and Barnes 2019; Waxman et al.…”

“…Figure 10 from Wu et al. (2019b) shows the bolometric luminosity compiled from observations in the literature (Smartt et al. 2017; Cowperthwaite et al.…”

Colloquium : Superheavy elements: Oganesson and beyond

“…This suggests that the production of heavier nuclei could be enhanced if the r -process path proceeds closer to the neutron drip line, where neutron-induced fission is hindered. While superheavy nuclei produced in the r process are expected to be short-lived, they may impact the observed r -process abundances and electromagnetic transients produced by the radioactive decay of r -process nuclei (kilonova) (Wanajo, 2018;Wu et al, 2018;Zhu et al, 2018).…”