On the formation of the Black Hole–Red Straggler binary in M10

The role of stellar dynamics in dense star clusters in forming merging binary black holes (BHs) detectable via gravitational wave detectors have created immense interest in BHs inside globular clusters (GCs). The BH-red straggler (RS) binary detected in M10, M10-VLA1, is among the best studied stellar-mass BHs in a GC. This source is particularly fascinating because RSs, as well as BHs are rare species in a GC.  We study the formation channel for M10-VLA1-like binaries with a large grid of detailed binary stellar evolution models created using MESA. We find that magnetically-inhibited convection inside a star can create RSs with properties very similar to the observed system. The required decrease in convective mixing-length to produce M10-VLA1-like RSs matches well with independent asteroseismic constraints. We further find that other, previously proposed RS formation channels, such as tidal heating of an initially eccentric BH-star binary, mass transfer from a red giant star to a BH primary, and tidal envelope stripping by the BH, while can produce RSs, but those RSs are too blue compared to the RS in M10-VLA1.