Production of very light elements and strontium in the early ejecta of neutron star mergers

TL;DR

This study investigates the production of very light elements like hydrogen, helium, and strontium in neutron star merger ejecta, combining simulations and nucleosynthesis calculations to assess detectability and match observed kilonova features.

Contribution

It provides detailed predictions of light element yields from neutron star mergers and evaluates their potential observational signatures in kilonova spectra.

Findings

Hydrogen and helium are produced in small quantities, mainly from free neutron decay.

Detection of hydrogen and helium features in kilonova spectra is unlikely with current models.

Strontium production matches the amounts needed to explain early spectral features in GW170817.

Abstract

We study the production of very light elements ($Z < 20$) in the dynamical and spiral-wave wind ejecta of binary neutron star mergers by combining detailed nucleosynthesis calculations with the outcome of numerical relativity merger simulations. All our models are targeted to GW170817 and include neutrino radiation. We explore different finite-temperature, composition dependent nuclear equations of state and binary mass ratios, and find that hydrogen and helium are the most abundant light elements. For both elements, the decay of free neutrons is the driving nuclear reaction. In particular, $\sim 0.5-2 \times 10^{-6} M_{\odot}$ of hydrogen are produced in the fast expanding tail of the dynamical ejecta, while $\sim 1.5-11 \times 10^{-6} M_{\odot}$ of Helium are synthesized in the bulk of the dynamical ejecta, usually in association with heavy r-process elements. By computing synthetic spectra, we find that the possibility of detecting hydrogen and helium features in kilonova spectra is very unlikely for fiducial masses and luminosities, even when including non local thermodynamics equilibrium effects. The latter could be crucial to observe He lines a few days after merger for faint kilonovae or for luminous kilonovae ejecting large masses of helium. Finally, we compute the amount of strontium synthesized in the dynamical and spiral-wave wind ejecta, and find that it is consistent with (or even larger than, in the case of a long lived remnant) the one required to explain early spectral features in the kilonova of GW170817.