Fingerprints of Heavy-Element Nucleosynthesis in the Late-Time Lightcurves of Kilonovae

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 ($\sim \mathrm{days}$) 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 possibly be dominated by a discrete number of $\alpha$ decays, ${}^{223}\mathrm{Ra}$ (half-life $t_{1/2}=11.43\mathrm{d}$), ${}^{225}\mathrm{Ac}$ ($t_{1/2}=10.0\mathrm{d}$, following the $\beta$ decay of ${}^{225}\mathrm{Ra}$ with $t_{1/2}=14.9\mathrm{d}$), and the fissioning isotope ${}^{254}\mathrm{Cf}$ ($t_{1/2}=60.5\mathrm{d}$), 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, multiepoch and wideband 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 AT2017gfo, we find that the yet-uncertain $r$ abundance of ${}^{72}\mathrm{Ge}$ 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 $A\sim 70$.

Figure 1

$L_{\mathrm{bol}}$ of the kilonova associated with GW170817 from Ref. [28] (filled black triangles), including uncertainties (gray band) derived from the range of values given in Refs. [12, 18, 28]. Also shown are lower limits (empty triangles) on the late-time luminosity as inferred from the Ks band with VLT/HAWK-I [29] (black) and the $4.5\mu \mathrm{m}$ detections by the Spitzer Space Telescope from Ref. [30] (green) and Ref. [31] (blue). Colored lines show the ejecta heating rate $\dot{Q}(t)$ for different models listed in Table 1. Their corresponding abundance distributions at $t=1$ d are shown in the inset. The black solid (dashed) horizontal lines in the lower right corner represent the approximate observation limits of the NIR (MIR) instruments on the JWST for a merger at 100 Mpc.

Figure 2

Lightcurve for the model $A1$ (thick solid blue line) showing the dominating contributions to the total radioactive heating: beta decays (thin maroon line) and individual $\alpha$ decays (thin blue lines).

Figure 3

The decay of ${}^{254}\mathrm{Cf}$ could produce a late-time plateau in the lightcurve. This figure is the same as Fig. 1, but showing both the model $A$ and a case with the ${}^{254}\mathrm{Cf}$ abundance artificially enhanced.

Figure 4

The radioactive decay heating rate powered by the solar-$r$ abundance distribution for nuclei between $A_{\min }$ and 205. We use two different abundance sets from [44, 45]. See text for discussions.