A Search for In Situ Field OB Star Formation in the Small Magellanic Cloud

Whether any OB stars form in isolation is a question central to theories of massive star formation. To address this, we search for tiny, sparse clusters around 210 field OB stars in the Small Magellanic Cloud (SMC) from the Runaways and Isolated O-Type Star Spectroscopic Survey of the SMC (RIOTS4), using friends-of-friends and nearest neighbors algorithms. We also stack the target fields to evaluate the presence of an aggregate density enhancement. Using several statistical tests, we compare these observations with three random-field data sets, and we also compare the known runaways to nonrunaways. We find that the local environments of nonrunaways show higher aggregate central densities than for runaways, implying the presence of some "tips of icebergs" (TIB) clusters. We find that the frequency of these tiny clusters is low, ∼4%-5% of our sample. This fraction is much lower than some previous estimates, but is consistent with field OB stars being almost entirely runaway and walkaway stars. The lack of TIB clusters implies that such objects either evaporate on short timescales or do not form, implying a higher cluster lower-mass limit and consistent with a relationship between maximum stellar mass (m_max) and the mass of the cluster (M_cl). On the other hand, we also cannot rule out that some OB stars may form in highly isolated conditions. Our results set strong constraints on the formation of massive stars in relative isolation.