Nebular spectra of kilonovae with detailed recombination rates -- I. Light r-process composition

TL;DR

This study calculates detailed dielectronic recombination rates for light r-process elements and demonstrates their significant impact on kilonova spectral models, revealing new spectral features and the importance of microphysics in late-time spectra.

Contribution

The paper provides new atomic data for recombination rates of light r-process elements and integrates this data into kilonova spectral models, showing substantial effects on predicted spectra.

Findings

Zr's role in spectra is more prominent with new rates.

Mid-infrared [Se I] line becomes a key signature.

Optical Rb lines weaken in updated models.

Abstract

To investigate spectra of kilonovae in the NLTE phase (t>= 1 week), we perform atomic calculations for dielectronic recombination (DR) rates for the light r-process elements Se (Z = 34), Rb (Z = 37), Sr (Z = 38), Y (Z = 39), and Zr (Z = 40) using the HULLAC code. For the different elements, our results for the DR rate coefficients for recombining from the ionization states of II to I, III to II, and IV to III vary between 2x10^{-12} - 5x10^{-11} cm^3/s, 10^{-13} - 5x10^{-11} cm^3/s and 2x10^{-15} - 10^{-11} cm^3/s, respectively, at a temperature of T = 10,000 K. Using this new atomic data (DR), we study the impact on kilonova model spectra at phases of t = 10 days and t = 25 days after the merger using the spectral synthesis code SUMO. Compared to models using the previous treatment of recombination as a constant rate, the new models show significant changes in ionization and temperature, and correspondingly, in emergent spectra. With the new rates, we find that Zr (Z = 40) plays a yet more dominant role in kilonova spectra for light r-process compositions. Further, we show that previously predicted mid-infrared (e.g. [Se III] 4.55 mum) and optical (e.g. Rb I 7802, 7949 {\AA}) lines weaken in the new model. Instead [Se I] 5.03 mum emerges as a signature. These results demonstrate the importance of considering the detailed microphysics for modelling and interpreting the late-time kilonova spectra.