Lower-mass-gap Black Holes in Dense Star Clusters

The existence of compact stellar remnants in the mass range 2-5\,M_ has long been debated. This so-called lower mass gap was initially suggested by the lack of low-mass X-ray binary observations with accretors in this mass range, but it has recently been called into question following newer observations, including a lower-mass-gap candidate with a millisecond pulsar companion in the dense globular cluster NGC~1851. Here we model NGC~1851 with a grid of similar dense star clusters utilizing the state-of-the-art Monte Carlo N-body code CMC, and we specifically study the formation of lower-mass-gap black holes. We demonstrate that both massive star evolution and dynamical interactions can contribute to forming lower-mass-gap black holes. In general, the collapse of massive remnants formed through mergers of neutron stars or massive white dwarfs produces the largest number of lower-mass-gap black holes among all formation channels. However, in more massive clusters, supernova core collapse can contribute comparable numbers. Our NGC~1851-like models can reproduce millisecond pulsar -- lower-mass-gap black hole binaries similar to the observed system. Additionally, the lower-mass-gap black holes can also become components of dynamically assembled binaries, and some will be in merging black hole--neutron star systems similar to the recently detected gravitational wave source GW230529. However, the corresponding merger rate is probably 1~ Gpc⁻³\,yr⁻¹.