The Loneliest O Stars

We observe apparently isolated OB stars in the Small Magellanic Cloud (SMC) to probe the sparsest clusters containing high-mass stars. With the ACS camera aboard the Hubble Space Telescope, we obtained F555W and F814W imaging of eight SMC field OB stars, to spatially resolve the stellar population in their immediate vicinities. We find evidence of small scale clustering around four of our targets, which in each case amount to ≤ 10 companion stars in the range 1−4M⊙. Given the importance of stochastic processes in small clusters, we use two separate Monte Carlo models to simulate the cluster populations to which these objects belong. In the first model, we generate a parent cluster population based on a stellar clustering law with the form n(N∗) dN∗ ∝ N −β ∗ dN∗, where N∗ is the number of stars per cluster, and n(N∗) is the number of clusters in the range N∗ to N∗ + dN∗. We adopt a Kroupa (2001, MNRAS, 322, 231) IMF to randomly draw the stellar masses. The second model is the same, but drawing the stars according to a cluster mass function, n(Mcl) dMcl ∝ M −β cl dMcl, where Mcl is the mass of the cluster. We find the best agreement with statistics of SMC and Galactic massive field stars for the cluster simulations having a power-law index β = 2. For the simulations based on the N ∗ clustering law, it is also necessary to apply a lower cutoff around N∗,min ∼ 40. Our findings are consistent with clusters comprising a random sampling from a universal IMF. In particular, our observations exist in a region of cluster parameter space disallowed by a fixed relation between cluster mass and maximum stellar mass. Since such a relation is predicted by competetive accretion models, our results are in better agreement with the core collapse star formation models. Our findings also have implications for the form of the integrated galaxial initial mass function (IGIMF).