Abstract
Abstract A mass paucity of compact objects in the range of ∼2–5 M ⊙ has been suggested by X-ray binary observations, namely, the “lower mass gap.” Gravitational wave detections have unlocked another mass measurement method, and aLIGO/Virgo has observed some candidates in the gap. We revisit the numerical simulations on the core-collapse supernovae (CCSNe) for ∼20–40 M ⊙ progenitor stars with different initial explosion energies. As a result, the lower explosion energy naturally causes more efficient fallback accretion for low-metallicity progenitors, and then the newborn black holes (BHs) in the center of the CCSNe can escape from the gap, but neutron stars cannot easily collapse into BHs in the gap; nevertheless, the final remnants of the solar-metallicity progenitors stick to the gap. If we consider that only drastic CCSNe can be observed and that those with lower explosion energies are universal, the lower mass gap can be reasonably built. The width and depth of the gap are mainly determined by the typical CCSN initial explosion energy and metallicity. One can expect that the future multimessenger observations of compact objects delineate the shape of the gap, which might constrain the properties of the CCSNe and their progenitors.
Highlights
Discoveries of stellar-mass black holes (BHs) and neutron stars (NSs) are resulting from the observations of compact X-ray binaries
The lower explosion energy naturally causes more efficient fallback accretion for low-metallicity progenitors, and the newborn black holes (BHs) in the center of the core-collapse supernovae (CCSNe) can escape from the gap, but neutron stars cannot collapse into BHs in the gap; the final remnants of the solar-metallicity progenitors stick to the gap
We propose the possibility of the formation of the lower mass gap by considering the fallback mechanism in the CCSN scenario
Summary
Discoveries of stellar-mass black holes (BHs) and neutron stars (NSs) are resulting from the observations of compact X-ray binaries. Three massive NSs, PSR J1614-2230, PSR J0348+0432, and MSP J0740+6620, were detected during the last decade (Demorest et al 2010; Antoniadis et al 2013; Cromartie et al 2020). Their masses can be measured by the “Shapiro delay” effects caused by their white dwarf companions. In the aLIGO/Virgo detections, the compact remnants in two NS-NS merger events, GW170817 (Abbott et al 2017) and GW190425 (Abbott et al 2020a), and one of the objects in a binary merger event, GW190814 (Abbott et al 2020b), are definitely in the gap. The questions we must answer are whether the gap exists and how large the gap is in the mass distribution of the compact objects
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