Bare collapse, formation of neutron star binaries and fast optical transients

‘Bare collapse’, the collapse of a bare stellar core into a neutron star with a very small mass ejection links two seemingly unrelated phenomena: the formation of binary neutron star (BNS) systems and the observations of fast and luminous optical transients. We carried out calculations of the collapse due to electron-capture of both evolutionary and synthetic isentropic bare stellar cores. We find that the collapse results in the formation of a light ∼1.3M ⊙ neutron star and an ejection of ∼0.1M ⊙ at ∼0.1c. The outer shell of the ejecta is composed of ⁵⁶ Ni that can power an ultra-stripped supernova. The models we explored can explain most of the observed fast optical flares but not the brightest ones. Collapse of cores surrounded by somewhat more massive envelopes can produce larger amounts of ⁵⁶ Ni and explain brighter flares. Alternatively, those events can arise due to interaction of the very energetic ejecta with winds that were ejected from the progenitor a few days before the collapse.There are no data bases used.