Contributions from Λ hyperons to nucleosynthesis in kilonovae

We study the expansion of neutron star (NS) matter containing hyperons $\mathrm{\ensuremath{\Lambda}}$ from high densities up to the end of nucleosynthesis. This process should occur in NS-NS or NS--black hole collisions. Hyperons $\mathrm{\ensuremath{\Lambda}}$ decay by mesonic and nonmesonic reactions on a very short timescale to form proton and neutron matter. When material becomes diluted enough, the formation of helium and heavier nuclei releases energy and increases its temperature to reach nuclear statistical equilibrium (NSE) conditions with high entropy per baryon. From then on, nucleosynthesis proceeds and the final composition is largely determined by the proton per baryon fraction ${Y}_{p}$. We consider a recent equation of state that considers the presence of $\mathrm{\ensuremath{\Lambda}}$ hyperons [D. Logoteta, I. Vida\~na, and I. Bombaci, Eur. Phys. J. A 55, 207 (2019)] and simultaneously allows for the existence of NSs with masses $M\ensuremath{\gtrsim}2{M}_{\ensuremath{\bigodot}}$. We assumed the composition of NS matter at densities above the threshold for the occurrence of $\mathrm{\ensuremath{\Lambda}}$ hyperons and computed its decay, finding that it appreciably increases ${Y}_{p}$. Then, we computed the subsequent nucleosynthesis starting from NSE assuming that the density falls exponentially. We find that, depending on the initial composition of the ejected material, the increase in ${Y}_{p}$ due to hyperon decay is sufficient to sizeably affect the final isotopic composition of ejected matter (for example, lanthanide production may be strongly inhibited). These results indicate that $\mathrm{\ensuremath{\Lambda}}$ hyperons may affect the final composition of matter ejected in kilonova events. We also discuss the case of pure $\mathrm{\ensuremath{\Lambda}}$ matter as a possible scenario for the collision by strange stars, where we have obtained values ${Y}_{p}\ensuremath{\approx}0.4\text{--}0.5$, which leads to final isotopes far lighter than in the former case.