The Binary Origins of OBe Stars in the Small Magellanic Cloud

Classical OBe stars are rapidly rotating OB stars with emission lines produced in a decretion disk. The origins of the fast rotation and subsequent disk are not yet well understood. In this work we add to the growing evidence that OBe stars form from massive binary systems. The model proposes that OBe stars gain angular momentum if their massive binary companion evolves to fill its Roche lobe. These close binary interactions should result in OBe stars residing farther into the field on average than OB stars, because mass transfer can prolong the life of the OBe star, allowing it to drift longer in the field; or, the companion & subsequent supernova can impart a runaway velocity on the OBe star. To test for this effect, we analyse the distances between OBe stars and their nearest O stars in the Small Magellanic Cloud. These distances serve as an effective measure of how far into the field each star resides. We find that OBe stars reside at a median distance of 36 ± 3.3 pc into the field vs 22 ±1.8 pc for OB stars, consistent with the expectation that post-mass-transfer objects are more isolated. Furthermore, the Oe and Be populations themselves are equally isolated in the field, with Oe stars residing at 34 ± 9.4 pc into the field and 39 ± 5.0 for Be stars. This is to be expected if their birth masses are obscured by mass transfer. We note that OBe stars and high-mass X-ray binaries (HMXBs) are closely related, with 39/49 HMXBs in the sample being OBe stars. These HMXBs have a median distance of 48 ± 6.5 pc, which further supports the scenario that most OBe stars are post-binary supernova objects released into the field. Finally, we show that supergiant OBe stars, which are known to not have formed via binary interactions, have a spatial distribution consistent with OB supergiants rather than non-supergiant OBe stars. Our analysis therefore finds multiple lines of evidence supporting the binary formation model of OBe stars, which has implications ranging from binary population models to gravitational wave astronomy.