Radioactive energies from unstable nuclei made in the ejecta of neutron star mergers play principal roles in powering kilonovae .
In previous studies power-law-type heating rates ( e.g. , \propto t ^ { -1.3 } ) have frequently been used , which may be inadequate if the ejecta are dominated by nuclei other than the A \sim 130 region .
We consider , therefore , two reference abundance distributions that match the r -process residuals to the solar abundances for A \geq 69 ( light trans-iron plus r -process elements ) and A \geq 90 ( r -process elements ) .
Nucleosynthetic abundances are obtained by using free-expansion models with three parameters : expansion velocity , entropy , and electron fraction .
Radioactive energies are calculated as an ensemble of weighted free-expansion models that reproduce the reference abundance patterns .
The results are compared with the bolometric luminosity ( > a few days since merger ) of the kilonova associated with GW170817 .
We find that the former case ( fitted for A \geq 69 ) with an ejecta mass 0.06 M _ { \odot } reproduces the light curve remarkably well including its steepening at \gtrsim 7 days , in which the mass of r -process elements is \approx 0.01 M _ { \odot } .
Two \beta -decay chains are identified : ^ { 66 } Ni \rightarrow ^ { 66 } Cu \rightarrow ^ { 66 } Zn and ^ { 72 } Zn \rightarrow ^ { 72 } Ga \rightarrow ^ { 72 } Ge with similar halflives of parent isotopes ( \approx 2 days ) , which leads to an exponential-like evolution of heating rates during 1–15 days .
The light curve at late times ( > 40 days ) is consistent with additional contributions from the spontaneous fission of ^ { 254 } Cf and a few Fm isotopes .
If this is the case , the event GW170817 is best explained by the production of both light trans-iron and r -process elements that originate from dynamical ejecta and subsequent disk outflows from the neutron star merger .