Bare Collapse, Formation of Neutron Star Binaries and Fast Optical Transients

TL;DR

This paper investigates how the collapse of bare stellar cores can produce neutron stars and fast optical transients, linking binary neutron star formation with luminous transient observations through detailed collapse modeling.

Contribution

It presents new calculations of electron-capture collapse in bare stellar cores, explaining the origin of certain fast optical transients and their connection to neutron star binary formation.

Findings

Collapse results in a ~1.3 solar mass neutron star and ~0.1 solar mass ejection.

Ejecta contains Ni56 capable of powering ultra-stripped supernovae.

Models explain most fast optical flares but not the brightest ones.

Abstract

{"Bare collapse", the collapse of a bare stellar core to 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 ~ 1.3 solar mass neutron star and {an} ejection of ~0.1 solar mass at ~0.1c. The outer shell of the ejecta is composed of Ni56 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 Ni56 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.