Are High-Mass Stripped Stars Missing in the Small Magellanic Cloud?

The evolution leading to binary black hole (BBH) mergers is called into question following recent gravitational wave detections of merging BBH more massive than ~30 M_☉. A binary tight enough for such an event to occur is predicted to result from a common envelope phase and the subsequent ejection of the common envelope, leaving behind a stellar core stripped of its hydrogen-rich envelope (a stripped star) and a black hole. Most BBH mergers are thought to come from low metallicity environments. However, using the stellar evolution code MESA, we predict that in low-metallicity environments such as the Small Magellanic Cloud (SMC), the majority of stars with initial masses between 20-50 M_☉ will not expand, and therefore cannot be stripped of their envelopes, until late in evolution. This directly impacts the common envelope phase thought to be necessary for the creation of a BBH. Since stars in the SMC should be affected, we can test this process that occurs deep in the stars’ interiors by probing the mass distribution of stars stripped in binaries. Specifically, we expect a gap in that mass distribution between 7-22 M_☉, which corresponds to stripped stars expected to be created by stars that expand just prior to supernova. Accurate tests can be made by comparing this theory with recently created photometric catalogs of stripped star candidates in the Magellanic Clouds.