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Abstract
The first three observing runs with Advanced LIGO and Virgo have resulted in the detection of binary black hole (BBH) mergers with highly unequal mass components, which are difficult to reconcile with standard formation paradigms. The most representative of these is GW190814, a highly asymmetric merger between a 23 M⊙ black hole (BH) and a 2.6 M⊙ compact object. Here, we explore recent results, suggesting that a sizable fraction of stars with pre-collapse carbon-oxygen core masses above 10 M⊙, and extending up to at least 30 M⊙, may produce objects inside the so-called lower mass gap that bridges the division between massive pulsars and BHs in Galactic X-ray binaries. We demonstrate that such an explosion landscape would naturally cause a fraction of massive binaries to produce GW190814-like systems instead of symmetric-mass BBHs. We present examples of specific evolutionary channels leading to the formation of GW190814 and GW200210, a 24 + 2.8 M⊙ merger discovered during the O3b observing run. We estimate the merger-rate density of these events in our scenario to be 𝒪(5%) of the total BBH merger rate. Finally, we discuss the broader implications of this formation channel for compact object populations, and its possible relevance to less asymmetric merger events such as GW200105 and GW200115.
Highlights
Following the detection of GW150914 – the first binary black hole (BBH) merger observed in gravitational waves (GWs; Abbott et al 2016a) – the three Advanced LIGO–Virgo Collaboration ( LVC) observing runs (O1–3; Aasi et al 2015; Acernese et al 2015) have uncovered a diverse collection of GW signals originating in collisions between black holes (BHs) and neutron stars (NSs; Abbott et al 2016b,c, 2017b, 2021b)
In this work we investigated the potential role of explodability islands in the formation of highly asymmetric-mass mergers such as GW190814 and the recently announced GW200210
We used a dense grid of SN progenitor models at different metallicities to demonstrate that both NSs and lower mass gap’ (LMG) objects – which are assumed to be BHs here – may be produced from a broad range of massive stars, with MCO masses extending up to ∼30 M (Sect. 3; Fig. 1)
Summary
Following the detection of GW150914 – the first binary black hole (BBH) merger observed in gravitational waves (GWs; Abbott et al 2016a) – the three Advanced LIGO–Virgo Collaboration ( LVC) observing runs (O1–3; Aasi et al 2015; Acernese et al 2015) have uncovered a diverse collection of GW signals originating in collisions between black holes (BHs) and neutron stars (NSs; Abbott et al 2016b,c, 2017b, 2021b). While the massive primary is undoubtedly a BH, the nature of the secondary is uncertain and not constrained by the GW signal itself: its mass securely places it inside the observed ‘lower mass gap’ (LMG) that separates the most massive NSs in binary pulsar systems ( 2.1 M ; Özel et al 2012; Antoniadis et al 2013, 2016; Fonseca et al 2021) from the lightest BHs in X-ray binaries (XRBs; 5 M ; Özel et al 2010; Farr et al 2011)
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