Testing Late Stellar Expansion with Populations of Stars Stripped in Binaries

Detailed evolutionary models predict that massive stars (M > 20 M_☉) at low metallicity remain small throughout most of their lifetimes and do not expand significantly until just before core collapse. If this is true, interaction should occur late for a large fraction of binaries and may also proceed differently due to the evolved stage of the donor. Envelope-stripping, which is thought to be an important step in the pathway to binary black hole mergers, could instead lead to nuclear timescale mass transfer or even a stellar merger. This should result in features in the mass distribution of envelope-stripped helium stars. To test the late expansion hypothesis, we use a stellar population synthesis code created specifically for mass-transferring binaries and based on detailed single and binary stellar evolution models. We create mass distributions of stripped stars for a range of metallicities. We find that for all the metallicities with Z≤0.006, the mass distribution exhibits a sharp drop in the number of stripped stars above a certain mass. At Z=0.006, the drop occurs at a stripped star mass of ~9-10 M_☉, while at Z=0.002, the drop is sharper and occurs at 8 M_☉. By interpreting these results using the metallicity and star-formation history of the Small Magellanic Cloud (SMC), we find that only a handful of stripped stars more massive than 8 M_☉ should exist in the SMC. As the population of stripped stars is revealed by an ongoing survey of the Magellanic Clouds, their mass distribution could provide a test of the hypothesis that massive stars expand late at low metallicity.