Signatures of r-process elements in kilonova spectra

TL;DR

This study systematically models kilonova spectra from neutron star mergers to identify specific r-process elements, revealing that GW170817's ejecta was likely lanthanide-poor and providing insights into nucleosynthesis conditions.

Contribution

It introduces detailed line strength calculations and radiative transfer simulations for element identification in kilonova spectra, advancing understanding of r-process nucleosynthesis in neutron star mergers.

Findings

Sr II lines observed in GW170817 spectrum.

Ejecta was likely lanthanide-poor based on spectral features.

Constraints on Ca/Sr ratio and nucleosynthesis conditions.

Abstract

Binary neutron star (NS) mergers have been expected to synthesize r-process elements and emit radioactively powered radiation, called kilonova. Although r-process nucleosynthesis was confirmed by the observations of GW170817/AT2017gfo, no trace of individual elements has been identified except for strontium. In this paper, we perform systematic calculations of line strength for bound-bound transitions and radiative transfer simulations in NS merger ejecta toward element identification in kilonova spectra. We find that Sr II triplet lines appear in the spectrum of a lanthanide-poor model, which is consistent with the absorption feature observed in GW170817/AT2017gfo. The synthetic spectrum also shows the strong Ca II triplet lines. This is natural because Ca and Sr are co-produced in the material with relatively high electron fraction and their ions have similar atomic structures with only one s-electron in the outermost shell. The line strength, however, highly depends on the abundance distribution and temperature in the ejecta. For our lanthanide-rich model, the spectra show the features of doubly ionized heavy elements, such as Ce, Tb and Th. Our results suggest that the line forming region of GW170817/AT2017gfo was lanthanide-poor. We show that the Sr II and Ca II lines can be used as a probe of physical conditions in NS merger ejecta. Absence of the Ca II line features in GW170817/AT2017gfo implies that the Ca/Sr ratio is < 0.002 in mass fraction, which is consistent with nucleosynthesis for electron fraction >= 0.40 and entropy per nucleon (in units of Boltzmann constant) >= 25.