Massive Star Formation in the Sparsest Environments

There is growing evidence that massive stars sometimes form in extremely sparse environments. The RIOTS4 survey presents a variety of evidence supporting this scenario, including a sample of 14 OB stars in the Small Magellanic Cloud (SMC) that appear to have formed in situ as field stars. This is based on the presence of dense, symmetric HII regions hosting apparent non-runaway stars. We also present a spatially complete IMF of SMC field OB stars for masses > 7 M-circle dot, showing that the slope is much steeper than the Salpeter value. The binary fraction among field OB stars is also the same as in clusters, based on a RIOTS4 sub-sample. These results suggest a relative, but incomplete, suppression of massive star formation in the sparsest regimes.