Using SMC OB Field Stars and Field Binaries to Study the Origin and Evolution of Massive Star Populations

Abstract

Massive field stars are a seemingly isolated population that can be used to probe massive star formation, binary interactions and massive star evolution. Field stars typically comprise about 30% of the total massive star population and arrive in these isolated environments either through in situ star formation or by being ejected from the parent cluster through binary interactions. In order to study these objects, I use the sample and observations from the Runaways and Isolated O-Type Star Spectroscopic Survey of the SMC (RIOTS4) survey, which identifies a uniform, statistically complete sample of 397 SMC field OB stars. I first look at the origins of field OB stars and determine the fraction of field stars that originate from in situ star formation, by searching for tiny host clusters around the target stars. Through cluster-finding algorithms applied to The Optical Gravitational Lensing Experiment (OGLE) survey data, I find that at most 5% of these massive stars form in the field and instead the majority are ejected from their parent clusters primarily through binary interactions. Due to massive star multiplicity, some of these field stars may themselves be ejected as binaries, thus their properties can help us understand the binary statistics and ejection mechanisms of the population. I identify new binaries in the SMC Wing field stars, a subset of the RIOTS4 survey, and combine this with previous identifications to obtain a binary fraction for field stars in this population. Moreover, I use these observations to set constraints on the binary properties of these targets, including companion mass, periods, eccentricities and inclinations. I then compare these observational properties against a model population generated by Binary Population and Spectral Synthesis (BPASS). I find that 1) our BPASS models underpredict the number of OBe stars, an indication that there are inconsistencies when modeling OBe stars; 2) our observations and BPASS models suggest that post-supernova OBe binary parameters depend on whether the companion is a neutron star or black hole; and 3) our mass estimates and BPASS models indicate the possibility of a high incidence of black hole companions for post-supernova OB binaries. Finally, we compare the number of dynamical ejections and supernova ejections in our sample and find that neither mechanism clearly dominates the ejected field binaries, contrary to previous results.