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 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.
