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

TL;DR

This study investigates whether OB stars form in isolation or within tiny clusters in the Small Magellanic Cloud, finding that most are likely runaway stars with few forming in isolation, thus constraining theories of massive star formation.

Contribution

The paper provides the first statistical analysis of tiny cluster environments around field OB stars in the SMC, showing a low occurrence of in-situ formation and supporting the runaway star hypothesis.

Findings

Few tiny clusters around OB stars (~4-5%)

Most OB stars are likely runaways or walkaways

Constraints on isolated massive star formation

Abstract

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 from the Runaways and Isolated O-Type Star Spectroscopic Survey of the SMC (RIOTS4), using friends-of-friends (FOF) and nearest neighbors (NN) 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 datasets, and we also compare the known runaways to non-runaways. We find that the local environments of non-runaways show higher aggregate central densities than for runaways, implying the presence of some "tips-of-iceberg" (TIB) clusters. We find that the frequency of these tiny clusters is low, $\sim 4-5\%$ 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 ($m_{\rm max}$) and the mass of the cluster ($M_{\rm cl}$). 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.