Abstract

Following the collapse of their cores, some of the massive binary stars that populate our Universe are expected to form merging binaries composed of black holes and neutron stars. Gravitational-wave observations of the resulting compact binaries can reveal precious details on the inner workings of the supernova mechanism and the subsequent formation of compact objects. Within the framework of the population-synthesis code mobse, we present the implementation of a new supernova model that relies on the compactness of the collapsing star. The model has two free parameters, namely the compactness threshold that separates the formation of black holes and that of neutron stars, and the fraction of the envelope that falls back onto the newly formed black holes. We compare this model extensively against other prescriptions that are commonly used in binary population synthesis. We find that the cleanest signatures of the role of the pre-supernova stellar compactness are (1) the relative formation rates of the different kinds of compact binaries, which mainly depend on the compactness threshold parameter, and (2) the location of the upper edge of the mass gap between the lightest black holes and the heaviest neutron stars, which mainly depends on the fallback fraction.

Highlights

  • Gravitational wave (GW) observations of merging compact binaries offer unprecedented insights into the life of massive stars

  • While in the rapid model the mass gap ranges from ∼ 2 to ∼ 5M⊙ independent of the metallicity, in the compactness model the size of the mass gap strongly depends on both Z and fH

  • We have investigated the impact of the pre-SN stellar compactness (O’Connor and Ott 2011) on the mass spectrum of compact objects and the resulting population of GW sources

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Summary

Introduction

Gravitational wave (GW) observations of merging compact binaries offer unprecedented insights into the life of massive stars. The black holes (BHs) and neutron stars (NSs) observed by LIGO and Virgo (Abbott et al 2019, 2021) constitute the end product of stellar collapse—the same cosmic events that are well understood to be behind supernova (SN) explosions. Lighter stars might instead explode as electroncapture SN In those cases, the degenerate oxygen–neon core reaches the critical mass of 1.38 M⊙ and electroncapture reactions destabilize the inner region (Miyaji et al 1980; Nomoto 1984). For progenitors heavier than 70 M⊙ , the collapsing core becomes unstable to pair production. This removes radiation pressure support from the star which, in turn, ignites explosive carbon-oxygen (CO) burning. The core is either partially (pulsational pair-instability SN; Woosley et al 2007) or entirely (pair-instability SN; Heger et al 2003) disrupted, introducing a characteristic upper limit of 50M⊙ (Farmer et al 2019; Woosley and Heger 2021) to the

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