Explodability fluctuations of massive stellar cores enable asymmetric compact object mergers such as GW190814

TL;DR

This paper proposes that fluctuations in stellar core explodability can produce asymmetric compact object mergers like GW190814, challenging standard formation models and explaining the observed diversity in black hole mergers.

Contribution

It introduces a new formation channel based on explodability fluctuations that accounts for asymmetric mergers and populates the lower mass gap with compact objects.

Findings

A significant fraction of massive stars can produce objects in the lower mass gap.

This formation channel can explain GW190814-like asymmetric mergers.

Estimated merger-rate density is about 5% of total BBH mergers.

Abstract

The first three observing runs with Advanced LIGO and Virgo have resulted in the detection of binary black hole mergers (BBH) 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$_{\odot}$ black hole and a 2.6 M$_{\odot}$ compact object. Here, we explore recent results suggesting that a sizeable fraction of stars with pre-collapse carbon-oxygen core masses above 10 M$_{\odot}$, and extending up to at least 30 M$_{\odot}$, 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$_{\odot}$ merger discovered during the O3b observing run. We estimate the merger-rate density of these events in our scenario to be $\mathcal{O}$(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