Physical conditions for the r-process I. radioactive energy sources of kilonovae

TL;DR

This study models radioactive energy sources from neutron star merger ejecta to better understand kilonova light curves, highlighting the importance of specific isotopic decay chains and the production of both light trans-iron and r-process elements.

Contribution

It introduces a new modeling approach using reference abundance distributions and free-expansion models to accurately reproduce kilonova light curves, emphasizing the role of specific isotopes.

Findings

The model with A >= 69 matches GW170817 light curve well.

Identifies key beta-decay chains influencing heating rates.

Late-time luminosity suggests contribution from spontaneous fission of heavy isotopes.

Abstract

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., ~ t^-1.3) have frequently been used, which may be inadequate if the ejecta are dominated by nuclei other than the A ~ 130 region. We consider, therefore, two reference abundance distributions that match the r-process residuals to the solar abundances for A >= 69 (light trans-iron plus r-process elements) and A >= 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 >= 69) with an ejecta mass 0.06 M_sun reproduces the light curve remarkably well including its steepening at > 7 days, in which the mass of r-process elements is ~ 0.01 M_sun. Two beta-decay chains are identified: 66Ni -> 66Cu -> 66Zn and 72Zn -> 72Ga -> 72Ge with similar halflives of parent isotopes (~ 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 254Cf 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.