Binary Progenitors of Electron-capture Supernovae

Abstract

Electron-capture supernovae (ECSNe) arise from a narrow progenitor parameter space between oxygen-neon white dwarf formation and iron-core collapse supernovae, governed by the competition between core growth and envelope removal. Binary interactions modify this balance through mass transfer and mergers. Using the detailed binary population synthesis framework POSYDON, we model ECSN progenitors across metallicities from Z/Z⊙ = 2 to 10−4, focusing on the initial parameter space, pre-supernova stripping, envelope-based classification, surviving companions, post-supernova orbits, and cosmic event rates. The lower edge of the binary ECSN initial primary mass lies near 7 M⊙, owing to efficient convective overshooting in the POSYDON grids, whereas the upper edge increases with metallicity because stronger line-driven winds require larger initial masses to build carbon-oxygen cores within the ECSN range. Stable mass transfer and mergers occupy distinct regions in initial mass ratio and orbital period space. ECSNe occurring in bound binaries are divided into hydrogen-rich Type II (∼30%), IIb (∼10%), and Ib (∼60%) outcomes according to residual hydrogen and helium envelopes. When all progenitor channels are considered, ECSNe occurring in binaries that remain bound at the time of explosion account for approximately one-third of the total ECSN population, whereas the remaining two-thirds arise from progenitors without a bound companion at explosion. Among these bound systems, approximately 90% of the companions are main-sequence stars at explosion. These main-sequence companions exhibit a bimodal spin distribution, including a rapidly rotating component associated with prior mass transfer. Evolved-star and compact-object companions each account for only a few percent. Combining the formation efficiencies with cosmic star formation and metallicity evolution models yields an intrinsic ECSN rate peaking at z ∼ 2 − 3 and declining at higher redshifts. Over cosmic history, ECSNe contribute approximately 1% − 4% of all core-collapse supernovae.