Exploring the Lower Mass Gap and Unequal Mass Regime in Compact Binary Evolution

TL;DR

This paper investigates the formation of asymmetric compact binary systems like GW190814 through isolated binary evolution, highlighting challenges in modeling their rates and implications for the neutron star-black hole mass gap.

Contribution

It updates mass loss modeling during collapse and explores the conditions needed to produce GW190814-like systems, suggesting the mass gap may be narrower or absent.

Findings

Population models struggle to match GW190814-like system rates.

Longer supernova engine timescales may enable formation of such systems.

The neutron star-black hole mass gap might be narrower or nonexistent.

Abstract

On August 14, 2019, the LIGO and Virgo detectors observed GW190814, a gravitational-wave signal originating from the merger of a $\simeq 23 M_\odot$ black hole with a $\simeq 2.6 M_\odot$ compact object. GW190814's compact-binary source is atypical both in its highly asymmetric masses and in its lower-mass component lying between the heaviest known neutron star and lightest known black hole in a compact-object binary. If formed through isolated binary evolution, the mass of the secondary is indicative of its mass at birth. We examine the formation of such systems through isolated binary evolution across a suite of assumptions encapsulating many physical uncertainties in massive-star binary evolution. We update how mass loss is implemented for the neutronization process during the collapse of the proto-compact object to eliminate artificial gaps in the mass spectrum at the transition between neutron stars and black holes. We find it challenging for population modeling to match the empirical rate of GW190814-like systems whilst simultaneously being consistent with the rates of other compact binary populations inferred by gravitational-wave observations. Nonetheless, the formation of GW190814-like systems at any measurable rate requires a supernova engine model that acts on longer timescales such that the proto-compact object can undergo substantial accretion immediately prior to explosion, hinting that if GW190814 is the result of massive-star binary evolution, the mass gap between neutron stars and black holes may be narrower or nonexistent.