From the detection of gravitational waves in GW190814, the merging of a 2.6 solar mass compact object with a heavier object has been inferred. The lighter object is in the "mass gap" between the heaviest neutron stars and the lightest stellar mass black holes, making astrophysicists wonder what kind of an object this was.
The theoretical lower bound of the gap is probably not much higher than 2.16 solar masses (Using Gravitational-wave Observations and Quasi-universal Relations to Constrain the Maximum Mass of Neutron Stars, In between neutron stars and black holes). This would exclude a neutron star as the 2.6 solar mass object.
Regarding the upper bound of the mass gap, no black hole candidates have (indirectly) been "observed" below 5 solar masses. However theoretically, much lighter black holes could exist, e.g. primordial black holes.
My question: why this 5 solar mass upper bound of the mass gap? does the evolution of massive stars preclude the formation of a 2.6 solar mass black hole?