Binary neutron star merger in common envelope jets supernovae

I explore a triple-star scenario where a tight neutron star (NS)-NS binary system enters the envelope of a red supergiant (RSG) star and spirals-in towards its core. The two NSs accrete mass through accretion discs and launch jets that power a very luminous and long transient event, a common envelope jets supernova (CEJSN) event. Dynamical friction brings the two NSs to merge either in the RSG envelope or core. The total energy of the event, radiation and kinetic, is > 1052 erg. The light curve stays luminous for months to years and a signal of gravitational waves might be detected. The ejecta contain freshly synthesized r-process elements not only from the NS-NS merger as in kilonova events, but possibly also from the pre-merger jets that the NSs launch inside the core, as in the r-process CEJSN scenario. This scenario shortens the time to NS-NS merger compared with that of kilonovae, and might somewhat ease the problem of the NS-NS r-process scenario to account for r-process nucleosynthesis in the early Universe. I estimate the ratio of NS-NS merger in CEJSN events to core collapse supernova (CCSN) events to be ≤10-6 - 2 × 10-5. However, because they are much more luminous I expect their detection fraction to that of CCSNe to be much larger than this number. This study calls for considering this and similar CEJSN scenarios in binary and in triple-star systems when explaining peculiar and puzzling superluminous supernovae.